Jwn student notes

FIRESPEC ENGINEERING (M) SDN BHD

Industrial Standard and Glossary
Lighting Design

Hj Azlan Abass
11/7/2011

Proprietary and Confidential Information

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Contents

Basic Lighting Knowledge .................................................................................................. 2
1). Lumen(lm) .................................................................................................................. 2
2). Candela(cd) ................................................................................................................. 2
3). Beam Angle [Unit:degree] ........................................................................................... 2
4). LUX(lx) ........................................................................................................................ 3
5). Color Temperature / Correlated Color Temperature(CCT) ........................................ 3
6). Color Rendering Index(CR) ........................................................................................ 3
7). Efficacy(lm/W) ........................................................................................................... 4
8). Design Life Time ........................................................................................................ 4
9). Dimmability ............................................................................................................... 4
Why LED Lighting Solution ?............................................................................................. 5
Industry Standards and Glossary ...................................................................................... 6
LumaStream embraces the industry standards evolving on both federal and state
levels. ........................................................................................................................... 6
Lumen Method Calculations ............................................................................................ 29

Firespec Engineering (M) Sdn Bhd By: Hj Azlan Abass

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Basic Lighting Knowledge

1). Lumen(lm)

Lumen is the international (SL) unit of luminous flux, a measure of overall light emitted by a light
source.

2). Candela(cd)

Candela is the international(sl) unit of luminuous intensity,a measure of light emitted by a light source
in a particular direction.

Any given light source will have different luminous intensities in different directions considered.

[candela = lumen/solid angle]

3). Beam Angle [Unit:degree]

Beam angle is the angle between two directions for which the luminous intensity is 50% of the
maxmum intensity as measured in a plan through the nominal beam centerline.Beam angle of a bulb
gives an idea of how the light beam spreads away from the beam center.


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4). LUX(lx)

Lux is the international(SL)unit of illuminance, a measure of light arriving at a surface, 1 lumen per
square foot equals 1 footcandle, while 1 lumen per square meter equals 1 lux .

[lux = lumen/m2]

5). Color Temperature / Correlated Color Temperature(CCT)

A black-Body radiator will have a different color whent it is heated up to different temperatures(in
kevin).The Temperature at which the heated black-body radiator matches the color of the light source
is the color temperature of the light source.

6). Color Rendering Index(CR)

Color rendering index ranges from 0-100 ,measures a light source's ability to reproduce the color of
various objects being lit by a reference light source, black body radiator(CRI=100)


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7). Efficacy(lm/W)

Eifficacy is the effciency of a light source to concert electricity energy to light energy which is expressed
in lumen output per unit power input (watt)

[Efficacy = lumen / wattage]

8). Design Life Time

Design life time is the average life of a lighting product when it is operated at nominal lamp
voltage,current and controlled working environment. Actual lamp life varies because of any different
from those rating condition.

9). Dimmability

Not all lighting prodcuts is designed for dimming function. Dimming a non-dimmable LED lighting
product will cause un-predictable result and may affect the life time of the products.The below figure
shows the typical input current variation while adjusting the DC voltage input to dimmable and non-
dimmable product.


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Why LED Lighting Solution ?

Technology is changing lives at a pace never before witnessed in all of human history, and
with it, the way we see the world. More than a hundred years ago the incandescent filament
brought illumination to the night, multiplying human productivity at an exponential rate.
Later, fluorescent light rose up as a power-saving alternative to the incandescent.

And now there is a new source of light, once again on the cusp of transformational change -
OPTILED has seen the future, and it will no doubt be lit by Light Emitting Diodes, more
commonly known as LED. A low-power, high-efficiency technology created with non-toxic
materials, LED lamps are the next generation of lighting, leading the way into the green age
of illumination. Because LED lighting by OPTILED is safe, long-lasting and increasingly
cost-effective, it will play an important role in creating a more environmentally sustainable
future for many years to come.

OPTILED has touted the benefits of LED since long before it became cool to do so and
continues to work hard to change perceptions of LED technology. LED is no longer just a
niche product, and we have the expertise, technical knowhow, R&D and the people needed
to create the best general functional LED lighting solutions on the market.

There are a number of important reasons why OPTILED LED lighting is better than other
lighting technologies:

Longer life spans: OPTILED LED lamps last 35,000 hours - 30 times longer than
incandescent, 25 times longer than halogen and 10 times longer than compact fluorescent
lamps. Not only will you be helping the environment, you'll be saving money, too.

High Efficiency: Saves users up to 70% on energy usage compared to standard dichroic
low-wattage halogen and incandescent lamps, lowering electricity bills. An 5W LED MR16
bulb generates as much light as an 9W CFL, an 35W halogen, and 80% energy saving
compared to CFL downlight - A 15W LED downlight generates as much light as a 2x13W
CFL downlight and 75W incandescent bulb. Energy usage translates directly into significant
cost savings, especially in commercial applications where large quantities of lamps are
used.

Green lighting technology: Lower energy usage makes LED lighting better for the
environment, but it's a very green technology in other ways as well. Since no heat is
produced, it can reduce interior temperatures by 1-2 degrees, lowering air-conditioning
costs and CO2 emissions. LED lighting is also much safer for the environment because it is
mercury-free and doesn't produce IR or UV rays, which can be harmful to humans. In other
words, LED lamps are the greenest and most environmentally-friendly source of light on the
market.

Superior color, increased comfort: Clear light and high marks on the Color Rendering
Index (CRI), which indicates accurate true color reproduction, means LED lights produce
light that is more pleasing and relaxing, increasing comfort and mood.


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Industry Standards and Glossary

Like traditional lighting products, LED-based luminaires sold in the US are subject to
industry standards governing safety and performance. To accommodate LEDs, some
existing standards and test procedures are being modified and new standards have been
developed.

LumaStream embraces the industry standards evolving on both federal and state
levels.

Within this rapidly evolving industry, the "language" for certain concepts, components,
systems, applications, etc. is not well established and consistent throughout the industry.
This fact has created confusion between vendors, engineers, consumers, manufacturers,
and others. LumaStream believes it is important to define the language we use within our
company and throughout our website. As naming conventions become standardized we will
update these definitions.

Binning and labeling

is the process of separating LEDs based on their various characteristics and assigning them
with a specific bin code. LEDs with the same characteristics are placed on a reel. White
LEDs are sorted by color and brightness. Color LEDs are sorted by dominant wavelength
and brightness. LumaStream relies on Philips, an industry partner, for precise binning
specs of the Philips Rebel chip.

CALiPER

is Department of Energy's (DOE) SSL Commercially Available LED Product Evaluation and
Reporting (CALiPER) program. CALiPER independently tests and provides unbiased
information on the performance of commercially available SSL products. Working in the
public's interest, the DOE publishes test reports, technical information, and summaries of
manufacturer's products and is intended solely for the benefit of the public, in order to help
buyers, specifiers, testing laboratories, energy experts and others make informed decisions
about SSL products and related technologies.

CALiPER Program

Department of Energy's (DOE) SSL Commercially Available LED Product Evaluation and
Reporting (CALiPER) program independently tests and provides unbiased information on
the performance of commercially available SSL products. Working in the public interest, the
DOE publishes test reports, technical information, and summaries of manufacturer's
products and is intended solely for the benefit of the public, in order to help buyers,
specifiers, testing laboratories, energy experts and others make informed decisions about
SSL products and related technologies. Manufacturers' products are selected randomly for
CALiPER program testing.
CALiPER's main tests measure and assess (1) energy efficiency of SSL products, as specified
in IES LM-79 under luminaire efficacy, via this formula: Total lumens emitted by the
luminaire, divided by the total watts drawn by the luminaire's power supply; (2) thermal
management and the effect of temperature on lumen depreciation.


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Correlated Color Temperature (CCT)

is the measure used to describe the relative color appearance of a white light source. CCT
indicates whether a light source appears more yellow/gold/orange or more blue, in terms of
the range of available shades of "white." CCT is measured in degrees Kelvin (unit of absolute
temperature). Cool White targets a typical CCT between 10,000K and 4500K; Neutral White,
between 4,500K and 3,500K; Warm White, between 3,500K and 2,540K.

Color Rendering Index (CRI)

indicates how well a light source renders colors of people and objects, compared to a
reference source of like color temperature.

Integration

is the "marrying" of lighting and lighting control components. Integration can set proper
light levels and atmosphere for office video-conferencing, or create comfortable dynamic
scenes within a residential setting.

Interface

is the human mechanism for interacting with the lighting system; typically consisting of a
computer or a wall mounted control panel.

LED Light Engine

is a subsystem of an LED luminaire that includes one or more LED packages, an LED
array, or an LED module; an LED driver, electrical and mechanical interfaces; and an
integral heat sink to provide thermal dissipation. An LED light engine may be designed to
accept additional components that provide additional aesthetic, optical, and environmental
controls. An LED light engine is designed to connect to the branch circuit.

LM-79

IESNA approved method for the electrical and photometric measurements of SSL. LM-79
specifies procedures for measuring total luminous flux, electrical power, luminous efficacy,
and chromaticity of SSL luminaires and replacement lamp products.

LM-80

IESNA approved method for measuring lumen maintenance of LED light sources. LM-80
specifies procedures for determining lumen maintenance of LEDs and LED modules (but
not luminaires) related to effective useful life of the product.

Lumen depreciation

is the decrease in lumen output that occurs as a lamp is operated. For most light sources,
the lamp fails before significant lumen depreciation occurs; for example, a typical
incandescent bulb's lumen output will only decrease 10-15% over 1,000 hours of use, but
the bulb will fail at this time.* Because LEDs will continue to operate at very low light


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levels, LED life is usually defined as lumen depreciation to a particular point, such as 70%
of initial lumens

Luminous efficacy

is the most commonly used measure of the energy efficiency of a light source. It is stated in
lumens per watt (lm/w), indicating the amount of light a light source produces for each watt
of electricity consumed.

NEMA SSL-1 Power Supply

National Electrical Manufacturer's Association (NEMA) standard that specifies operational
characteristics and electrical safety of SSL power supplies and drivers.

Retrofit

means to substitute new or modernized systems for older lighting equipment. LumaStream
design-build systems can incorporate partial or complete retrofit and upgrade solutions.

Scalability

is a desirable property and benefit of a solid-state lighting system whereby capabilities can
be put in place to successfully accommodate future lighting needs. LumaStream systems
allow for expanding lighting needs and system control.

Scalable System

is a system designed to accommodate scalability, i.e. it can grow as the client's lighting or
control needs grow and change. LumaStream specializes in identifying whether our client's
systems need to be designed to accommodate scalability.

Solid-State Lighting (SSL)

refers to a type of lighting that uses light-emitting diodes (LEDs) as its source of
illumination.

TM-21

Lumen Depreciation Lifetime Estimation Method for LED Light Sources. TM-21 will provide
a method for determining an LED luminaire or integral replacement lamp's expected
operating life, based on initial performance data collected per IES-LM-80.

UL1598/8750A

UL standard for Luminaires - specifies the minimum safety requirements for luminaires.
The requirements in this document may be referenced in other documents such as UL8750
or separately used as part of the requirements for SSL products.


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UL1310/8750

UL standard for Class 2 Power Units - specifies the minimum safety requirements for Class
2 power supplies. (as defined by the National Fire Protection Association (NFPA), 70-2005)

UL-8750

Underwriters Laboratory (UL) has developed this safety standard for "Light Emitting Diode
(LED) Light Sources for Use in Lighting Products." Currently, UL has in place an "Outline of
Investigation" that references all existing UL standards applicable to LED lighting products.
The purpose of the outline is to provide a comprehensive approach and listing of applicable
standards for UL treatment of lighting products based on LEDs. The outline will be used
until the full LED specific document is completed.

White Light

comprises light from all the frequency bands in the visible spectral range. Hence, it is a
combination of red, orange, yellow, green, blue, indigo, and violet.


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Achromatic

Describing the property of having no colour (or hue). For example a neutral white, grey or
black colour.

Additive Primary Colour

The additive primary colours are Red, Green & Blue. When equal amounts of light of the
three additive primary colours is combined, white light is generated. Combining two additive
primaries in equal amounts creates a subtractive primary colour. See also: Subtractive
Primary Colour.

Angstrøm (Å)

A unit of wavelength of light. One Angstrøm (Å) is 0.1nm (or 1 x 10 -10 m).

Average (Luminous) Intensity

Applies to LEDs. The near-field luminous intensity for a non point source. Defined in CIE
document 127 as being the intensity (luminous flux per unit solid angle) measured by a
photodetector with an input area of 100 mm2 positioned at either 316mm (condition A) or
100mm (condition B) from the tip of the LED source, measured on the mechanical axis of
the LED. Average luminous intensity has units of candela (cd). Not to be confused with
luminous intensity which is a far-field measurement of the luminous flux per unit solid
angle from a light source. See also: LED; Luminous Intensity; Luminous Flux; CIE; Far-
Field; Near-Field

Bandwidth

See Linewidth.

Barium Sulphate

A white pigment commonly used to coat the interior of integrating spheres and other
reflecting chambers. Exhibits diffuse reflectance. Forms the basis of the following Labsphere
diffuse relfectance coatings: Spectraflect; Duraflect; and 6080.

Black

The (near) complete absorption of light as a result of no (low) reflectance. In colorimetry,
describes a colour of low saturation and low luminance.

Blackbody Radiation

Radiation that is full or complete, containing all wavelengths. The spectral power
distribution of light emitted from a blackbody is a function of its temperature only and is
described by Planck‟s law. See also: Colour Temperature.

Blackbody Radiator

A source that emits blackbody radiation.


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Brightness

That attribute of visual perception that describes the degree to which an object emits or
reflects light. In colorimetry, brightness is used in the HSB colour model – Hue, Saturation
and Brightness. Describes the lightness of the colour, on a scale ranging from black to
white. Can be confused with saturation hence its use is discouraged. Can also be used
instead of luminance – again its use is not recommended. See also: Luminance.

Candela (cd)

The SI base unit used in photometry. The candela is the unit of luminous intensity. The
candela is one of the seven base units of the SI system. Since 1979, the candela has been
defined as: “the luminous intensity, in a given direction, of a source that emits
monochromatic radiation of frequency 540 x 10 12 Hertz and has a radiant intensity in that
direction of 1/683 Watts per steradian” (where the steradian is the unit of solid angle). One
candela equals one lumen per steradian. See also: Luminous Intensity; Steradian.

Candela per Square Meter (cd/m2)

The SI unit of luminance. One cd/m 2 equals one lumen per square meter per steradian. See
also: Luminance; Steradian.

Candle Power

The luminous intensity of a light source having units of candelas.

CCD

Acronym. Stands for Charge Coupled Device. A monolithic, two-dimensional semiconductor
(silicon-based) detector array. When illuminated by optical radiation in the range between
approximately 300 and 1100nm, produces a photocurrent, the magnitude of which is
proportional to the level of light received. Each detector in the array is called a pixel.

CFL

Acronym. Stands for Compact Fluorescent Lamp.

Chroma

The lightness of a colour. Denotes the degree of lightness or darkness. See also: Lightness.

Chromatic

Described as having colour (or hue) – not white, grey or black.

Chromaticity

In colorimetry, describes the intensity or level of saturation of a colour (hue), defined as the
distance in the particular colour space of a colour from the neutral grey colour with the
same value.


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Chromaticity Coordinates (CIE)

A numeric descriptor of colour. Defined as the ratio of the three tristimulus values XYZ in
relation to the sum of the three, designated by xyz respectively. It is normal to assume that
the chromaticity coordinates refer to the CIE 2° observer (1931). For reflected colour, it is
assumed to be calculated for standard illuminant C unless specified differently. See also:
(CIE) x, y; (CIE) u‟, v‟.

Chromaticity Diagram

The two-dimensional graph which plots the chromaticity coordinates. For the CIE 2°
observer (1931), x is plotted as the abscissa, y as the ordinate. The colour space plots the
spectrum locus of monochromatic radiation in the spectral range 380-770nm.

CIE

Acronym. From the French name "Commission Internationale de L'éclairage". In English
this is the International Commission on Illumination. CIE sets the standards and provides
a worldwide forum for the exchange of information on the science of light, lighting, colour
and vision. Click here to visit the CIE's web site

CIE 1976 L*u*v* Colour Space

A uniform colour space adopted by CIE in 1976 that is used when measuring colour as a
result of additive mixing from emitters of light.

CIELAB

A uniform colour space adopted by CIE in 1976 within which L*a*b* colour coordinates are
plotted using a Cartesian coordinate system. This colour space plots equal colour
differences at approximately equal distances. The L* value denotes the lightness, a*
represents the red/green axis, while b* represents the yellow/blue axis. CIELAB is that
colour space used in relation to the measurement of reflected or transmitted colour.

CIE Luminosity Function (Y)

See: Photopic Response.

CIE Standard Illuminant

Standard light source spectral power distributions as defined by CIE for several types of
light source. See Illuminant A, B, C, D, E & F.

CIE Standard Observer Function

See: Photopic Response; Scotopic Response; Tristmulus Response.

CMYK

Acronym. Stands for Cyan, Magenta, Yellow & blacK. See also: Subtractive Primary Colour.


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Colorimeter

An instrument that measures the colour of the light it receives. Applies to instruments that
measure reflected (transmitted) light as well as to those that analyse the light emitted by a
source. Applies to instruments that use RGB optical filters to mimic the tristimulus colour
response of the human eye. See also: Tristimulus.

Colorimetry

The science of measuring the colour of light (emitted, reflected or transmitted) from an
object with a spectral response matching that of the human eye. See also: Tristimulus.

Colour

Can be described as being one aspect of an object‟s appearance. In colorimetry, colour is a
visual perception and is defined in respect of its hue, saturation and lightness.

Colour Difference

Being the size of the difference between two colours in a defined colour space.

Colour Rendering Index (CRI)

A metric which defines how well colours are rendered by different sources of illumination
compared to a defined standard illuminant. There are fourteen special colour rendering
indices (Ri where i = 1-14) which define the colour rendering of the light source when used
to illuminate standard colours. The general colour rendering index (Ra) is the average of the
first eight special colour rendering indices (which correspond to non-saturated colours).

Colour Space

The three-dimensional solid enclosing all possible colours. The dimensions of the colour
space can be described in a number of different geometries, which leads to various spacings
within the colour space.

Colour Specification

Being the tristimulus values, chromaticity coordinates and luminance value (or other colour
scale) that are used to uniquely describe a colour numerically.

Colour Temperature

Colour temperature defines the colour of radiation emitted from a perfect blackbody
radiator held at a particular temperature. Colour temperature is reported in units of Kelvin
(K). The plot of the chromaticity coordinates of a blackbody radiator with temperatures from
1,000 to 20,000 Kelvin is called the Planckian locus. Colours on this locus in the range
from 2,500 to 20,000 K are considered to be white, with 2,500 K being reddish white and
20,000 K being bluish white (warm to cool white). See also: Black Body Radiation;
Correlated Colour Temperature.

Cone (Receptor)


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The visual receptor in the retina of the human eye responsible for daylight adapted, colour
vision. There are three types of cones, each sensitive to red, green or blue wavelengths. See
also: Retina; Rod.

Cornea

The transparent front part of the human eye. To use an engineering analogy, the cornea
functions as a "dust cap" and also performs primary focussing of the light rays into the eye.

Correlated Colour Temperature (CCT)

Correlated Colour Temperature (CCT) describes the colour temperature of those white light
sources whose colours don‟t fall exactly on the Planckian locus (i.e. for non blackbody
emitters). The CCT of a non-Planckian source is the blackbody colour temperature that the
source resembles most closely. Correlated colour temperature is reported in units of Kelvin
(K). CCT can be considered to be a simplified metric for describing the colour of a white light
source. For coloured light sources, dominant wavelength can be used to express colour in a
single number. See also: Colour Temperature; Dominant Wavelength.

Cosine Diffuser

An object which exhibits Lambertian like reflectance or transmittance. Transmission
diffusers are used in photometry to impart a cosine response at the input to a detector to
correct for the effective illuminance of off-axis rays, or to more uniformly illuminate a multi-
element detector. Thin sheets of ground glass (quartz), PTFE as well as the input port of an
integrating sphere all function as effective transmission diffusers. See also: Cosine
Response.

Cosine Response

The Lambertian spatial response of a perfect reflecting or transmitting diffuser whereby the
intensity from or through the surface varies with the cosine of the angle subtended between
the direction of view and the normal to the surface. Applies to light meters (radiometers and
photometers) designed for measuring irradiance or illuminance which should apply a cosine
angular response to light rays received at angles other than normal incidence, typically
achieved using a transmission diffuser or integrating sphere. See also: Cosine Diffuser;
Lambertian.

CRT

Acronym. Stands for Cathode Ray Tube. The type of television display that was popular
before the introduction of flat panel displays (LCD, OLED, RPTV & plasma).

Diode Array

A detector that comprises a linear array of segmented photodiode detectors. See also:
Photodetector; Photodiode; Spectroradiometer.

Dominant Wavelength


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Dominant wavelength is a useful measure of the effective wavelength (in nm) of non-
monochromatic light sources, in particular LEDs. It is defined in relation to a specified
standard illuminant (usually Illuminant E, but this is arbitrary). It is a measure of the hue
(or colour sensation) produced by the light source. Not to be confused with peak
wavelength. Dominant wavelength is normally applied to coloured LEDs whereas correlated
colour temperature is applied to white LEDs.

Duraflect

Labsphere's proprietary, waterproof, diffuse white coating for the UV-VIS-NIR spectral
region (350-1200nm).

Electroluminescence

The emission of light as a result of the passage of an electrical current. Distinct from
incandescence which is the emission of light as a result of heat. See also: LED.

Electromagentic Radiation

Electromagnetic radiation is generated by the motion of electrically charged particles.
Optical radiation, light, X-rays, radio waves and microwaves are all examples of
electromagnetic radiation. See: Light; Optical Radiation.

EULUMDAT (File)

A standardised data file which expresses the light output of a luminaire as luminous
intensity versus angle. EULUMDAT files are assigned the file extension ".ldt". Used by
lighting designers to model the illumination performance of a lighting scheme using
commercial lighting design programmes. An EULUMDAT file is commonly referred to as
"photometric data" for a luminaire. See also: Goniophotometer; IES (file).

Far Field

In photometry, that distance from a lamp (called the photometric distance) where the
emitter is considered to be a point source, usually at a distance of between 5-10 times the
source diameter (although this is not the case for LEDs). Intensity measurements are
performed in the far field, in which the inverse squared law applies. See also: Near Field;
Intensity; Average Intensity; Inverse Squared Law; Point Source; Photometric Distance.

Fluorescence

The emission of light at a longer wavelength as a result of absorption of light at a shorter
wavelength in a fluorescent material. Applies for that case whereby the emission occurs at
the same time as the absorption. See also: Luminescence; Phosphorescence.

Fluorescent Lamp

A lamp comprising a glass tube filled with mercury gas and coated on its inner surface with
a phosphor. When the gas is charged with an electrical current, ultraviolet radiation is
produced. This in turn is absorbed in the phosphor, causing it to emit visible light by the
process of fluorescence. See also: CFL; Fluorescence.


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Foot-candle (fc)

The English unit of illuminance. One foot-candle (fc) equals one lumen per square foot,
which equals 10.76 lux. See also: Illuminance; Lux.

Foot-Lambert (fL)

The English unit of luminance. One foot-Lambert (fL) equals 1/π candelas per square foot,
which equals 3.426 candelas per sq. meter. See also: Luminance.

Fovia

That part of the eye‟s retina that has the greatest density of cones. See also: Retina, Cone.

Goniometer

A goniometer is an (electro) mechanical positioning device which allows one to change the
relative angle (φ, θ) at which a photometer (also a spectroradiometer or colorimeter) views a
light source. This can be accomplished by either moving the detector around a stationary
source or by tipping and tilting the source whilst the detector remains in a fixed position.
See also: Goniophotometer.

Goniophotometer

An instrument which performs measurements of the (far-field) luminous intensity or
illuminance or (near-field) luminance of a light source as a function of viewing angle. Can
also be used to measure the total luminous flux of a light source by measuring the
directional luminous intensity and integrating over 4π steradians. A goniophotometer is
commonly used to measure so-called "photometric data" for luminaires. Photometric data in
this context is the light output (luminous intensity) as a function of angle presented in
industry-standard file formats. See also: EULUMDAT; Goniometer; IES.

Hue

The perceived colour of a light source.

IES (File)

A standardised data file which expresses the light output of a luminaire as luminous
intensity versus angle. Derives from the acronym for the Illuminating Engineering Society of
North America (IESNA). IES files are assigned the file extension ".ies". Used by lighting
designers to model the illumination performance of a lighting scheme using commercial
lighting design programmes. An IES file is commonly referred to as "photometric data" for a
luminaire. See also: EULUMDAT (file); Goniophotometer.

Illuminance

Illuminance (Ev) is the luminous flux received per unit area. Illuminance is measured in lux
(lx) where 1 lux equals 1 lumen per square meter. See also: Foot-candle.

Illuminant A


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Illuminant A (CIE) represents the colour temperature of an incandescent lamp (2856K). See
also: Standard Illuminant; Colour Temperature.

Illuminant B

Illuminant B (CIE) represents the colour temperature of direct sunlight (4874K). See also:
Standard Illuminant; Colour Temperature.

Illuminant C

Illuminant C (CIE) represents the colour temperature of indirect sunlight (6774K). See also:
Standard Illuminant; Colour Temperature.

Illuminant D

Illuminant D (CIE) represents the colour temperature of daylight. There are several
standard illuminants in the D series including D50 which represents bright incandescent
light (5000K) and D65 which represents natural daylight (6504K). See also: Standard
Illuminant; Colour Temperature.

Illuminant E

Illuminant E (CIE) is the colour temperature of an artificial, normalising source (5500K).
Has chromaticity coordinates of x=0.3333 & y=0.3333. See also: Standard Illuminant;
Colour Temperature.

Illuminant F

Illuminant F (CIE) represents the correlated colour temperature of light from a fluorescent
lamp. There are several standard illuminants in the F series including F2 which represents
cool white light (4200K), F7 which represents broadband daylight (6500K) and F11 which
represents a narrow band white lamp (4000K). See also: Standard Illuminant; Colour
Temperature.

Imaging Photometer (Colorimeter)

A photometric instrument capable of spatially-resolved luminance, illuminance and colour
measurements. Sometimes referred to as a video photometer. Employs a CCD detector.

Incandescence

Light that is emitted by thermal radiation at a temperature high enough to be visible.

Incandescent Lamp

A lamp which emits light as a result of resistive heating when an electrical current passes
through a metal wire in a vacuum.

Infragold


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Labsphere's proprietary electro-plated gold coating for the infrared spectral region
(>800nm).

Infrared

Describing that part of the electromagnetic spectrum comprising optical radiation having
wavelengths longer than 780nm but shorter than 1mm. Infrared radiation is not visible to
the naked eye.

Integrating Sphere

A hollow, spherical chamber with a diffuse, high reflectance interior coating. Used in
photometry to measure the total luminous flux (power) of a light source.

Intensity

See: Luminous Intensity

Inverse Squared Law

Describes the relationship between the illuminance from a light source which varies in
inverse proportion to the square of the distance from the source. Applies to the case of a
point source of light in the photometric far-field. See also: Far-Field; Illuminance; Point
Source.

Iris

The variable, limiting aperture found in the human eye.

Irradiance

Irradiance (Ee) is the radiometric analogue of illuminance, the radiant flux received per unit
area. Irradiance is measured in Watts per square meter.

Isotropic

A light source is isotropic when it radiates with equal intensity (flux per unit solid angle) in
all directions. Not to be confused with Lambertian.

Kelvin (K)

The unit of measurement of colour temperature and correlated colour temperature. The
Kelvin scale starts at absolute zero (-273°C). See also: Colour Temperature; Correlated
Colour Temperature.

Lambert's Cosine Law

See Lambertian.

Lambertian


19 | P a g e

A Lambertian surface is one that reflects (or transmits) with equal radiance or luminance in
all directions. In other words, the reflected (transmitted) intensity (flux pew unit solid angle)
from a Lambertian surface varies in proportion to the cosine of the angle subtended
between the direction of view and the surface normal. See also: Cosine Response.

Laser

From the acronym for Light Amplification by the Stimulated Emission of Radiation. A light
source typically characterised by a combination of coherence, narrow spectral linewidth and
a collimated beam. The term "laser light" refers to electromagnetic radiation with
wavelengths between 150nm up to 11µm. A laser was first demonstrated in 1960 by
Theodore H Maiman working at the Hughes Corporation, although the term „laser‟ was first
coined by Gordon Gould of Columbia University.

LCD

Acronym. Stands for Liquid Crystal Display.

LED

Acronym. Stands for Light Emitting Diode. LEDs are solid state light sources which
generate light by electroluminescence when an electrical current is passed through a
semiconductor p-n junction. See also: Electroluminescence; OLED.

Light

Light is a term that we use to define that part of the electromagnetic radiation that humans
can see (“visible light”). Visible light has a wavelength bteween 380 and 780nm. More
accurately described as optical radiation, it forms part of the electromagnetic spectrum that
encompasses radio waves, microwaves, x-rays and gamma rays. Optical radiation is that
which obeys wave-like behaviour, as opposed to particle-like.

Lightness

See Chroma.

Linewidth

Denotes the spectral purity or width in wavelength of a light source. The spectral width of a
light source is normally defined as the full width half max (FWHM) linewidth. The term
bandwidth is often used interchangeably with linewidth.

LOR

Acronym. Stands for Light Output Ratio. Being the ratio of the luminous flux produced by a
luminaire to that produced by the lamp (or lamps) fitted to the luminaire. A measure of the
efficiency of a luminaire. See also: Luminaire; Luminous Flux.

Lumen (lm)

The SI unit of luminous flux (lm).


20 | P a g e

Luminaire

A light fitting which generally comprises a lamp in a housing together with a reflector
and/or lens. May include control gear (drive electronics).

Luminance

Luminance (Lv) is the luminous flux emitted per unit solid angle and per unit area. In the SI
system, luminance is measured in candelas per square meter where 1 cd/m2 equals 1
lumen per steradian per square meter. The photometric analogue of radiance. Luminance is
a near-field measurement of a light source; luminous intensity is the measurement made in
the photometric far-field. See also: Far-Field; Foot-Lambert; Luminous Intensity; Near-Field;
Nit; Steradian.

Luminescence

Luminescence can be taken to describe the process of fluorescence or phosphorescence.
See: Fluorescence, Phosphorescence.

Luminous Efficacy

The efficiency of a light source, the quotient of the luminous flux emitted divided by the
electrical power consumed. Reported in units of lumens per Watt (lm/W).

Luminous Exitance

Luminous exitance (Mv) is the luminous flux emitted from a surface per unit area, measured
in units of lumens per square meter. Geometrically equivalent to illuminance, luminous
exitance is not however reported in lux.

Luminous Flux

The luminous flux (Fv) or more correctly the total luminous flux of a light source is the total
photometric power emitted in all directions. Luminous flux is measured in lumens (lm),
which is the photometric analogue of the Watt.

Luminous Intensity

The luminous intensity (Iv) in a given direction (often abbreviated to intensity, sometimes
referred to as beam candela) is the photometric power from a point source emitted per unit
solid angle. Intensity is measured in the SI base unit of the candela (cd) where 1 candela
equals 1 lumen per steradian. Luminous intensity is a far-field measurement of a light
source; luminance is the measurement made in the photometric near-field. See also: Far-
Field; Luminance; Near-Field; Steradian.

Lux (lx)

The SI unit of illuminance (lx).

Mean Spherical Candle Power (MSCP)


21 | P a g e

Mean Spherical Candle Power (M.S.C.P.) is the luminous flux of a light source divided by 4π
steradians.

Mesopic

Photopic (cone) vision operates when the eye sees luminance levels of ≥ 3 candelas per
square meter, whereas scotopic (night time) vision operates when the luminance falls below
0.01 candelas per square meter. For luminance levels between 0.01 and 3 cd/m 2, both rods
and cones contribute to the vision process to varying degrees. This "in-between" region is
called mesopic, and research is ongoing into the development of a standard observer
function for mesopic vision. See also: Photopic; Scotopic.

Micrometer (µm)

A unit of wavelength, normally used to describe infrared radiation. Often abbreviated
to 'micron'. One micrometer or micron (µm) equals 1 x 10-6 meters. One micrometer also
equals 1000 nanometers (nm).

Monochromatic

Light that is composed of a single wavelength or colour (hue), having 100% purity and a
narrow linewidth.

Monochromator

See Spectrometer.

Nanometer (nm)

A unit of wavelength of light, normally used to describe visible light. One nanometer (nm)
equals 10-9 meters.

Near-Field

In photometry, that region close to a lamp where the emitter is not considered to be a point
source, usually at a distance of less than 5-10 times the source diameter (although this is
not the case for LEDs). Intensity measurements are performed in the far field, in which the
inverse squared law applies. See also: Far Field, Intensity, Average Intensity, Inverse
Squared Law, Point Source.

Nit

A historically significant unit of luminance. Derives from the Latin "to illuminate". One nit
equals 1 candela per square meter. See also: Luminance.

OLED

Acronym. Stands for Organic Light Emitting Diode. A type of light emiting diode (LED) with
an emissive electroluminescent layer made of organic compunds. Related devices are Light
Emitting Polymers (LEPs). See also: Electroluminescence; LED.


22 | P a g e

Optical Density (OD)

Describes the level of attenuation of light as it passes through an optical filter. The higher
the OD value, the higher the attenuation (and the higher the protection). OD increases with
increasing thickness of absorptive filters. Optical density is the logarithm (to the base ten)
of the reciprocal of the transmittance. For example, an OD of 1 means 10%, an OD of 2
means 1% and an OD of 3 means 0.1% transmittance (and so on)

Optical Radiation

Describing that part of the electromagnetic spectrum comprising rays that exhibit a wave-
like property (as opposed to particle-like). Includes ultraviolet, visible (light) and infrared
radiation.

Phosphorescence

The emission of light at a longer wavelength as a result of absorption of light at a shorter
wavelength in a phosphorescent material. Applies for that case whereby the emission
occurs after the absorption. See also: Fluorescence, Luminescence.

Photodetector

A generic name given to a device that detects optical radiation. See also: Photodiode;
PMT; Diode Array; CCD.

Photodiode

A discrete semiconductor photodetector that produces a photocurrent the magnitude of
which is proportional to the level of light received when illuminated by optical radiation. In
photometers, the detector is based on silicon with a photopic filter; the combined detector
plus filter is designed to provide a spectral responsivity scaled to match that of the photopic
response, the CIE standard observer for photopic vision.

Photometric Distance

That distance from a light source that defines the far field. In the photometric far-field,
illuminance (irradiance) varies in inverse proportion to the square of the distance form the
light source. See also: Inverse Squared Law; Far-Field; Near-Field.

Photometer

An optical instrument that measures a defined geometric property of visible light with a
photopic spectral response matching that of the human eye. See also: Photopic;
Radiometer; Spectroradiometer.

Photometry

The science of measuring optical radiation with a photopic spectral response matching that
of the human eye. See also: Photopic; Radiometry; Spectroradiometry.

Photomultiplier Tube (PMT)


23 | P a g e

A photomultiplier tube (PMT) is a photodetector that comprises a photocathode which is
held in vacuum and emits electrons when exposed to light. This charge is accelerated by a
high voltage field causing the electrons to hit a metal plate, whereupon more electrons are
emitted (the multiplier effect). Not normally used in photometers, PMTs are commonly used
in traditional scanning spectroradiometer (monochromator) systems.

Photopic (Response)

The daylight-adapted relative spectral response of the eye is called the spectral luminous
efficiency function for photopic vision, V(l). This is an empirical distribution that was first
adopted in 1924. Applies for luminance values above 3cd/m 2 to the spectral range between
380 and 780nm having a peak response at 555nm. The photopic response explains why the
human eye is more sensitive to green light than blue or red light. At luminance levels below
3cd/m2, the eye transitions through a mesopic response. At luminance levels at or below
0.01 cd/m2, the eye becomes dark adapted and possesses a scoptic response. See also:
Mesopic; Scotopic.

Pixel

The individual detector element in a two-dimensional CCD detector array. See also: CCD.

Plankian Locus

The locus of points on the CIE chromaticity diagram that plots the chromaticity of
blackbody radiators of differing colour temperatures.

Plankian Radiator

A synonym for blackbody radiator.

Point Source

A light source is considered to be a point source if it is observed at a position at or beyond
the photometric distance, that is, the far-field. See also: Intensity; Average Intensity; Far-
Field; Photometric Distance.

Polychromatic

Used to describe optical radiation that is comprised of multiple discrete wavelength or
having a continuous spectrum.

Primary Colour

In colorimetry, primary (additive) colours are red, green and blue, and their subtractive
opposites cyan, magenta and yellow. See also: Additive Primary Colours; Subtractive
Primary Colours.

Purity

Purity is a measure of the degree of saturation of a light source, reported as a percentage. It
is defined with respect to a specified standard illuminant (usually illuminant E, but this


24 | P a g e

arbitrary). As most (coloured) LEDs have a narrow spectral emission (that is, near-
monochromatic), they have a purity of close to 100% (in other words, the colour cannot be
distinguished from a true monochromatic light source). See also: Saturation.

Radiance

Radiance (Le) is the radiant flux emitted per unit solid angle and per unit area. Radiance is
measured in Watts per steradian per square meter. The radiometric analogue of luminance.

Radiant Exitance

Radiant exitance (Me) is the radiant flux emitted from a surface per unit area, measured in
units of Watts per square meter. Geometrically equivalent to irradiance and the radiometric
analogue of luminous exitance.

Radiant Flux

The radiant flux (Φe) - more correctly the total radiant flux - of a light source is the total
power emitted in all directions. Radiant flux is measured in Watts (W), which is the
radiometric analogue of the lumen.

Radiant Intensity

The radiant intensity (Ie) in a given direction (often abbreviated to intensity) is the radiant
flux from a point source emitted per unit solid angle. Intensity is measured in units of
Watts per steradian. The radiometric analogue of luminous intensity.

Radiometer

An optical instrument that measures the absolute amount of a defined geometric property
of light. A laser power meter is a type of radiometer. See also: Photometer;
Spectroradiometer.

Radiometry

The science of measuring the absolute amount of optical radiation.

Retina

The light-sensitive area on the rear of the human eye containing photo-receptors. The area
containing the highest concentration of photo-receptors is called the fovea. See also: Rod;
Cone; Fovea.

RGB

Meaning Red, Green & Blue which are the additive primary colours. Used in relation to the
human eye‟s tristimulus colour response. See also: Additive Primary Colour.

Rod


25 | P a g e

The visual receptor in the retina of the human eye responsible for scotopic (night-time,
monochromatic) vision. See also: Retina; Cone.

Saturation

The attribute of colour perception that describes the degree of departure of the colour from
the neutral grey of the same lightness. See also: Purity.

Scotopic

The dark-adapted relative spectral response of the eye is called the spectral luminous
efficiency function for scotopic vision, V‟(l). The scotopic response was first adopted by CIE
in 1951 and applies to the spectral region between 380 and 780nm, with a peak at 507nm.
The Scotopic response applied to luminance levels of 0.01 cd/m2 of less. See also: Mesopic;
Photopic.

SI

Systeme Internationale d‟Unites, the international metric system of measurement units.

Silicon Photodiode

See Photodiode.

Solid Angle

See Steradian.

Spectral Power Distribution (SPD)

The power distribution of a light source as a function of wavelength. Determines the colour,
colour temperature and colour rendering properties of a light source.

Spectraflect

Labsphere's proprietary white reflectance coating. The coating is barium sulphate based
and hydroscopic. Available as a coating service and as a liquid paint (6080) for user
application. Best applied by spray painting.

Spectralon

Labsphere's patented diffuse reflectance material. A solid thermoplastic which exhibits the
highest diffuser reflectance of any material in the 250-2500nm range. Integrating spheres
and reflectors are machined from a block of Spectralon. Spectralon is widely used as a
white reflectance standard (99%). Can be doped with inorganic materials to create grey
scale standards (2-80%), colour standards, wavelength standards and fluorescence
standards.

Spectrometer


26 | P a g e

An instrument that separates polychromatic light into its constituent wavelengths. Employs
a dispersive optical element, usually a diffraction grating. A spectroradiometer and a
spectrophotometer comprise a spectrometer combined with a detector, normally a CCD
array. A (scanning) monochromator is a form of spectrometer in which the diffraction
grating rotates to transmit one wavelength at a time, whereas a CCD spectrometer analyses
a wide range of wavelengths simultaneously.

Spectroradiometer

An instrument for measuring the radiant power from a light source as a function of
wavelength. Normally combines a spectrometer with a photodetector or CCD. Can be used
to more accurately calculate the photometric and colorimetric properties of a light source
than a photometer or colorimeter by multiplying the spectral radiant power by standard
photopic and tristimulus observer functions.

Spectrum

The spectral arrangement of electromagnetic energy in order of wavelength. See also:
Spectral Power Distribution (SPD).

Spectrum Locus

The plot of the chromaticity coordinates of monochromatic light in the CIE colour space
diagram. See also: Chromaticity Diagram (CIE).

Standard Illuminant

See CIE Standard Illuminant.

Standard Observer

See: CIE Standard Observer Function.

Steradian

The steradian is the (dimensionless) unit of solid angle. A sphere contains 4π steradians
(and by definition, a hemisphere comprises 2π steradians). A steradian is that solid angle
subtended from the centre of a sphere by a spherical surface area having an area equal to
the square of the radius. In other words, the steradian solid angle of a beam of light is equal
to the projected area divided by the square of the distance.

Subtractive Primary Colour

The subtractive primary colours are Cyan, Magenta & Yellow. When inks of these colours
are applied to white paper in equal amounts, these combine to produce black. Combining
two subtractive primaries in equal amounts creates an additive primary colour. See also:
CMYK; Additive Primary Colour.

Thermopile


27 | P a g e

A broadband detector (thermal rather than optical) that is commonly used in the radiometry
of lasers. Comprises an array of thermocouple junctions.

Tristimulus

Describing the RGB 2° colour matching functions, which CIE transformed into the XYZ
colour matching functions. The XYZ functions define the CIE 1931 standard colorimetric
observer. The Y channel (green) was chosen to match that of the photopic response, so that
colour measurement instruments could double as photometers. For observer fields of view
of greater than 2°, CIE published its 1964 supplementary standard colorimetric observer.

(CIE) u' Chromaticity Coordinate

Together with the CIE v' chromaticity coordinate provides a numeric description of colour.
The CIE u' chromaticity coordinate is plotted as the abscissa (horizontal axis) on the two-
dimensional colour space graph defined by CIE to communicate the colour of a light source.
The CIE v' chromaticity coordinate is plotted as the ordinate (vertical axis). Refers to the CIE
10° tristimulus observer function (1964) as plotted in the 1976 uniform colour space
chromaticity diagram. The intersection of u' and v' within the CIE colour space diagram
uniquely defines the colour of a light source. See also: Chromaticity Coordinates;
Chromaticity Diagram; Tristimulus; (CIE) x; (CIE) y.

Ultraviolet (UV)

wavelengths between 100 and 400nm. Ultraviolet (UV) radiation is absorbed in the lens or
cornea and is not visible to the naked human eye.

UVA

That part of the Ultraviolet spectral range with wavelengths between 315 and 400nm.

UVB


UVC


(CIE) v' Chromaticity Coordinate

Together with the CIE u' chromaticity coordinate provides a numeric description of colour.
The CIE v' chromaticity coordinate is plotted as the (vertical axis) on the two-dimensional
colour space graph defined by CIE to communicate the colour of a light source. The CIE u'
chromaticity coordinate is plotted as the abscissa (horizontal axis). Refers to the CIE 10°
tristimulus observer function (1964) as plotted in the 1976 uniform colour space
chromaticity diagram. The intersection of u' and v' within the CIE colour space diagram
uniquely defines the colour of a light source. See also: Chromaticity Coordinates;
Chromaticity Diagram; Tristimulus; (CIE) x; (CIE) y.


28 | P a g e

Visible (Light)

wavelengths between 380 and 780nm that can be detected by the retina in the human eye.
Wavelengths in this range combine to create the sensation of colour.

V(λ) Function

See Photopic.

Watt (W)

The SI unit of power and radiant flux.

Wavelength

The periodic peak-to-peak distance in an electromagnetic wave. For ultraviolet and visible
light, defined in nanometers (nm); for infrared light, defined in micrometers (µm).

White Light

Light that appears to be white. Can contain a continuous spectrum of light (as from the
sun) or comprise as few as two colours. White light comprising two colours exhibits poor
colour rendering. Light on the Planckian locus (blackbody curve) on the CIE chromaticity
diagram with colour temperatures between 2500 and 20000K is considered to be “white”.
See also: CIE Standard Illuminant.

(CIE) x Chromaticity Coordinate

Together with the CIE y chromaticity coordinate provides a numeric description of colour.
The CIE x chromaticity coordinate is plotted as the abscissa (horizontal axis) on the two-
dimensional colour space graph defined by CIE to communicate the colour of a light source.
The CIE y chromaticity coordinate is plotted as the ordinate (vertical axis). Refers to the CIE
2° tristimulus observer function (1931). The intersection of x and y within the CIE colour
space diagram uniquely defines the colour of a light source. See also: Chromaticity
Coordinates; Chromaticity Diagram; Tristimulus; (CIE) u'; (CIE) v'.

(CIE) y Chromaticity Coordinate

Together with the CIE x chromaticity coordinate provides a numeric description of colour.
The CIE y chromaticity coordinate is plotted as the (vertical axis) on the two-dimensional
colour space graph defined by CIE to communicate the colour of a light source. The CIE x
chromaticity coordinate is plotted as the abscissa (horizontal axis). Refers to the CIE 2°
tristimulus observer function (1931). The intersection of x and y within the CIE colour
space diagram uniquely defines the colour of a light source. See also: Chromaticity
Coordinates; Chromaticity Diagram; Tristimulus; (CIE) u'; (CIE) v'.


29 | P a g e

Lumen Method Calculations

The lumen method is based on fundamental lighting calculations. The lumen method
formula is easiest to appreciate in the following form.

(1)

where E = average illuminance over the horizontal working plane

n = number of lamps in each luminaire

N = number of luminaire

F = lighting design lumens per lamp, i.e. initial bare lamp luminous

flux

UF = utilisation factor for the horizontal working plane

LLF = light loss factor

A = area of the horizontal working plane

2.1 Light Loss Factor

Light loss factor (LLF) is the ratio of the illuminance produced by the lighting
installation at the some specified time to the illuminance produced by the same
installation when new. It allows for effects such as decrease in light output caused by

(a) the fall in lamp luminous flux with hours of use,

(b) the deposition of dirt on luminaire, and

(c) reflectances of room surfaces over time.

In fact, light loss factor is the product of three other factors:


30 | P a g e

(2)

where LLMF = lamp lumen maintenance factor

LMF = luminaire maintenance factor

RSMF = room surface maintenance factor

2.1.1 Lamp Lumen Maintenance Factor

Lamp lumen maintenance factor (LLMF) is the proportion of the initial light output of
a lamp produced after a set time to those produced when new. It allows for the decline
in lumen output from a lamp with age. Its value can be determined in two ways:

(a) by consulting a lamp manufacturer's catalog for a lumen depreciation chart, and

(b) by dividing the maintained lumens by the initial lamps.

2.1.2 Luminaire Maintenance Factor

Luminaire maintenance factor (LMF) is the proportion of the initial light output from
a luminaire after a set time to the initial light output from a lamp after a set time. It
constitutes the greatest loss in light output and is mainly due to the accumulation of
atmospheric dirt on luminaire. Three factors must be considered in its determination:

(a) the type of luminaire,

(b) atmospheric conditions, and

(c) maintenance interval.

2.1.3 Room Surface Maintenance Factor

Room surface maintenance factor (RSMF) is the proportion of the illuminance
provided by a lighting installation in a room after a set time compared with that
occurred when the room was clean. It takes into account that dirt accumulates on
room surfaces and reduces surface reflectance. Figure 4 shows the typical changes in
the illuminance from an installation that occur with time due to dirt deposition on the
room surfaces.


31 | P a g e

2.2 Utilisation Factor

Utilisation factor (UF) is the proportion of the luminous flux emitted by the lamps
which reaches the working plane. It is a measure of the effectiveness of the lighting
scheme. Factors that affect the value of UF are as follows:

(a) light output ratio of luminaire

(b) flux distribution of luminaire

(c) room proportions

(d) room reflectances

(e) spacing/mounting height ratio

2.2.1 Light Output Ratio of Luminaire

Light output ratio of luminaire (LOR) takes into account for the loss of light energy
both inside and by transmission through light fittings. It is given by the following
expression.

(3)

Example 1

The total, upward and downward lamp output from a lamp are 1000 lm, 300 lm and
500 lm respectively. Calculate upward light output ratio (ULOR), downward light
output ratio (DLOR), light output ratio (LOR) of luminaire and percentage of light
energy absorbed in luminaire.


32 | P a g e

Amount of light energy absorbed in luminaire = 100 - 80 = 20 %

A greater DLOR usually means a higher UF.

A simple classification of luminaires according to their distribution is based on flux
fractions, as shown in Figure 5. Upward flux fraction (UFF) and downward flux
fraction (DFF) are used as a basis of comparison.

Example 2

For data given in Example 1 determine upward flux fraction (UFF), downward flux
fraction (DFF) and flux fraction ratio (FRR).


33 | P a g e

Figure 5 Flux Fraction of Various Luminaires

2.2.2 Flux Distribution of Luminaire

Direct ratio is the proportion of the total downward luminous flux from a conventional
installation of luminaires which his directly incident on the working plane. It is used to
assess the flux distribution of luminaire. Since the intensity distribution pattern of the
light radiated from a luminaire in the lower hemisphere will affect:

(a) the quantity of the downward flux falls directly on the working plane and

(b) the quantity of flux available for reflection from the walls in a given room,

Direct ratio depends on both the room proportions and the luminaires. Direct ratio
has a low value with a narrow room (small room index) and a luminaire which emits
most of its light sideways (BZ 10), and on the contrary, a high value with a wide room
(large room index) and a luminaire which emits most of its light downwards (BZ 1).

2.2.3 Room Proportion

Room index (RI) is the ratio of room plan area to half the wall area between the
working and luminaire planes.

(4)

where L = length of room

W = width of room

Hm = mounting height, i.e. the vertical distance between the working plane and the
luminaire.

2.2.4 Room Reflectances

The room is considered to consist of three main surfaces:

(a) the ceiling cavity,

(b) the walls, and


34 | P a g e

(c) the floor cavity (or the horizontal working plane).

The effective reflectances of the above three surfaces affect the quantity of reflected
light received by the working plane.

2.2.5 Spacing to Height Ratio

Spacing to Height ratio (SHR or S/Hm) is defined as the ratio of the distance between
adjacent luminaires (centre to centre), to their height above the working plane. For a
rectangular arrangement of luminaires and by approximation,

(5)

where A = total floor area

N = number of luminaires

Hm = mounting height

Under a regular array of luminaires the illuminance on the working plane is not
uniform. The closer spaced the luminaires for a given mounting height, the higher the
uniformity; or the greater the mounting height for a given spacing, the greater the
uniformity. If uniformity of illuminance is to be acceptable for general lighting,

(a) SHR should not exceed maximum spacing to height ratio (SHR MAX) of the given
luminaire as quoted by the manufacturer, and

(b) geometric mean spacing to height ratio of the luminaire layout should be within the
range of nominal spacing to height ratio (SHR NOM) of the given luminaire as quoted
by the manufacturer, i.e.

(6)


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