Physical Processes Employed in the
• 1. Incandescence - Thermo luminescence is by definition radiation
at high temperature. The sources employing this process are
Incandescent Lamp, Gas Lamp, (flames and in oil Lamps and wax
candles). They lead to a continuous spectrum of radiation.
• 2. Luminescence -Luminescence Electro luminescence by
definition Chemical or Electrical Action on gases or vapour
radiation. Here colour of radiation depends on the material
employed. Usually this process leads to Line or Band Spectrum.
• 3. Fluorescence - Fluorescence is a process in which radiation is
absorbed at one wavelength and radiated at another wavelength
e.g.UV impinging on Uranium – Fluorescent oils. This re radiation
makes the light radiated visible.
• 4. Phosphorescence - Phosphorescence is a process when energy is
absorbed at some time and radiated later as glow. Examples of this
process are Luminous paints that contain calcium sulphide that lead to
Phosphorescence. They produce light Radiation after exposure to
In practice good efficient lighting is obtained by combining
Luminescence and Fluorescence. Fluorescent lamp is Luminescent
source of low luminous value activating Fluorescent surfaces which
lead to visible radiation. Here intensity depends on gas or vapour
involved and phosphor material. However, the temperature of the
material play a role in radiation.
Visible light is light that the human eye can perceive. When you look
at the sun's visible light, it appears to be colourless, which we call
white. Although we can see this light, white isn't considered part of
the visible spectrum. That's because white light isn't the light of a
single colour but instead many colours.
• The visual system carries out a number of complex
tasks, including the reception of light and the formation of
• The build up of a binocular perception from a pair of two
• The identification and categorization of visual objects.
• Assessing distances to and between objects and guiding body
movements in relation to visual objects.
• The psychological process of visual information is known
as visual perception, a lack of which is called blindness.
• Non-image forming visual functions, independent of visual
perception, include the pupillary light reflex(PLR) and
• Eye acquires > 80% information acquired by human.
• An Eye comprises of Iris, Focusing Lens and Retina. It
Resembles – a Camera in general structure and action.
A structure located behind the iris (rarely visible) which produces
aqueous fluid that fills the front part of the eye and thus maintains
the eye pressure. It also allows focusing of the lens.
A thin lining over the sclera, or white part of the eye. This also
lines the inside of the eyelids. Cell in the conjunctiva produce
mucous, which helps to lubricate the eye.
The transparent, outer "window" and primary focusing element of
the eye. The outer layer of the cornea is known as epithelium. Its
main job is to protect the eye. The epithelium is made up of
transparent cells that have the ability to regenerate quickly. The
inner layer of the cornea is also made up of transparent tissue,
which allows light to pass.
Inside the anterior chamber is the iris. This is the part of the eye
which is responsible for one‘s eye colour. It acts like the diaphragm
of a camera, dilating and constricting the pupil to allow more or less
light into the eye.
The dark opening in the centre of the colored iris that controls how
much light enters the eye. The colored iris functions like the iris of
a camera, opening and closing, to control the amount of light
entering through the pupil.
The part of the eye immediately behind the iris that performs
delicate focusing of light rays upon the retina. In persons under
40, the lens is soft and pliable, allowing for fine focusing from a
wide variety of distances. For individuals over 40, the lens begins
to become less pliable, making focusing upon objects near to the
eye more difficult. This is known as presbyopia.
The membrane lining the back of the eye that contains
photoreceptor cells. These photoreceptor nerve cells react to
the presence and intensity of light by sending an impulse to the
brain via the optic nerve. In the brain, the multitude of nerve
impulses received from the photoreceptor cells in the retina are
assimilated into an image.
The optic nerve is the structure which takes the information
from the retina as electrical signals and delivers it to the brain
where this information is interpreted as a visual image. The
optic nerve consists of a bundle of about one million nerve
• Newton thought that colours were a sensory experience,
rather than a property of light.
• Newton’s theory of colour was that the sun’s light, or any
other white light, was a mixture of rays of light, each with its
own refrangibility, by which he meant characteristic angle of
refraction in a prism.
• What is a colour: when we look at a scene, our visual nerves
register colour in terms of the attributes of colour the
amount of green-or-red; the amount of blue-or-yellow; and
Colour nerves sense green or red — but never both; and
Blue or yellow — but never both.
Thus, we never see bluish-yellows or reddish-greens. The
opposition of these colours forms the basis of colour vision.
Ewald Hering (1834-1918) devised the first accurate
theory of colour vision.
A modern representation of colour space. This is
conceptually similar to Hering’s circles, but the middle
fades to gray.
• Modern colour theory is based on three primary
colours red, green and blue.
• Also warm and cool colours has been important since at
least the late 18th century used in design practices today
• Colour theory has described perceptual and psychological
Warm colours are said to advance or appear more
active in a painting, warm colours are said to arouse or
stimulate the viewer.
while cool colours tend to recede used in interior
design or fashion, also cool colours calm and relax.
• Additive colour synthesis is the creation of colour by
mixing colours of light. Human vision relies on light
sensitive cells in the retina of the eye. There are two
basic kinds of sensors. These are rods and cones.
• Rods are cells which can work at very low intensity, but
cannot resolve sharp images or colour.
• Cones are cells that can resolve sharp images and
colour, but require much higher light levels to work.
• The combined information from these sensors is sent to
the brain and enables us to see.
• There are three types of cone. Red cones are sensitive to
red light, green cones are sensitive to green light, and blue
cones are sensitive to blue light. The perception of colour
depends on an imbalance between the stimulation level of
the different cell types.
• Additive colour processes, by having the capability to
generate an image composed of red, green, and blue light.
• Since the intensity information for each of the three
colours is preserved, the image colour is preserved as well.
• The spectral distribution of the image will probably be
wrong, but if the degree of intensity for each of the
primary colours is correct, the image will appear to be in
The three primaries in light are red, blue, and green, because
they correspond to the red, green, and blue cones in the eye.
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
Computer monitors and televisions are the most common
examples of additive colour.
• When learning basic colour theory, art students typically use
familiar colours like red, yellow, and blue.
• The most frequently used primary colours for subtractive
colour mixing are cyan, magenta and yellow.
• Subtractive colour mixing is employed with paints and
pigments, in contrast with additive colour mixing with
coloured lights for spotlighting and theatrical lighting.
• Subtractive colour processes work by blocking out parts of
the spectrum. The idea of subtractive colour is to reduce the
amount of undesired colour reaching the eye.
• The three primaries on the artist’s colour wheel
are red, blue, and yellow.
• Yellow light is observed when all blue light is removed from
white light, magenta forms when green is removed, and
cyan is produced when red is removed.
• If, for example, you had a yellow image, you would want to
have a dye that would let red and green reach the eye, and
block out blue. The additive secondaries become
subtractive primaries, because each of the additive
secondaries will reflect two of the additive primaries, and
absorb one of the additive primaries.
• White cannot be produced by any combination of the
primary subtractive colours, which is the main reason that
no mixture of coloured paints or inks can be used to print
When all three primary subtractive colors are
added, the three primary additive colors are
removed from white light, leaving black (the
absence of any color).
Innovation in lighting has brought new products and
classifications to the home, office and industrial.
Lighting classifications include indoor and outdoor lights
with entirely different purposes.
Street and highway lighting can be low-pressure sodium
that has poor colour rendition and that takes 10 minutes to
illuminate. Stadium lights, Emergency lighting,
Photographic lighting etc.,
Natural: Sunlight, candlelight and firelight; this is light
that moves and is sometimes referred to as kinetic.
High Intensity Discharge
Broad classification of light
• An incandescent bulb uses heat caused by an electrical
• When electrical current passes through a wire, it causes the
wire to heat. The wire, or filament, gets so hot that it glows
and gives off light.
• Everyday incandescent light bulbs have a filament made of
tungsten. Since the hot tungsten would quickly burn away
if it were exposed to oxygen, it must be placed in a sealed
glass bulb which is either evacuated or filled with a gas that
wont let it burn.
• Because of their inefficiency, incandescent light bulbs are
gradually being replaced in many applications by other
types of electric lights, such as fluorescent lamps, compact
fluorescent lamps (CFL) etc.,
• Fluorescent lamp produces white light.
• The average life is 7500 hours.
• The light output is 70 lumens per watt.
• They have poor rendering especially for red colour.
• Light-emitting diodes (LED) are semiconductors. As electrons
pass through this type of semiconductor, it turns into light.
• Compared to incandescent and CFL bulbs, LED lights are more
efficient at turning energy into light.
• Therefore, less of the energy radiates from the bulb as heat.
This is why LED bulbs are cooler during operation than
incandescent and CFL bulbs.
• As the light-emitting diodes create light, they warm up quite a
bit for their size. LEDs are heat sensitive, so it's important that
the heat move away so that it doesn't damage the
• In order to do this, these lights need a system to keep cool.
Most LED lights have a heat sink plate that moves the heat
away from the light-emitting diodes through the heat sink
Classification of light in Decoration
• Ambient: A hidden source of light that washes a room
with a glow. It flattens an interior and creates very little
• Accent: Directional lighting or lighting that adds interest
or highlights a certain object or unusual architectural
feature in a room.
• Task: Task lighting is just that; lighting that's used to
perform daily activities such as
reading, cooking, shaving, putting on makeup, etc. It
needs to be glare-free.
• Aesthetic: Lighting itself can be a work of art. A neon
sculpture would be purely decorative and an example of
• Tungsten: Gives off a slightly yellow tinge. These are
ordinary light bulbs.
• Spotlight: Gives a focused burst of white light.
• Halogen: Gives the closest approximation of natural
daylight, known as "white light." colours appear sharper
under halogen light. The halogen bulb is also an energy
saver. Can be dimmed.
• Fluorescent: The typical fluorescent gives a flat, cold
light, often bluish and harsh. It is a daylight-equivalent
and cannot be put on a dimmer.
A spectral energy distribution (SED) is a plot of brightness
or flux density versus frequency or wavelength of light.
It is used in many branches of astronomy to characterize
Galaxies emit electromagnetic radiation over the full possible
frequency/wavelength range. Analysis of these radiations is the
main means through which astronomers study the distant
galaxies and thus learns about their formation and evolution. The
distribution of energy over wavelength/frequency is called the
spectral Energy Distribution.
Spectral Energy Distribution
• A spectral energy distribution (SED) is a graph of the energy
emitted by an object as a function of different wavelengths.
• The graph is a typical curve, called a blackbody curve. It
shows that the amount of energy emitted by the object at all
wavelengths varies with the temperature of the object.
• Hotter objects emit more light at shorter wavelengths than
cooler objects; therefore the hotter the object, the more the
peak wavelength is shifted toward the left of the graph.
• Stars aren’t really blackbodies but the emission from them is
very similar to blackbodies. We can "fit" a blackbody curve to
Luminous efficacy is a measure of how well a light source
produces visible light. It is the ratio of luminous flux to power.
Depending on context, the power can be either the radiant
flux of the source's output, or it can be the total power
(electric power, chemical energy, or others) consumed by the
• The effectiveness of illumination (luminous efficiency) is
defined by the ratio of the luminous flux emitted by a specific
source of light and the energy consumed by it in a time unit.
• The unit: lumen/watt (lm/W)
Examples of luminous efficiency of various light sources:
light bulb: 8-10 lm/W
halogen lamp: 16 lm/W
fluorescent lamp: 45-104 lm/W
white LED: 26-100 lm/W
metal halide lamp: 85-115 lm/W
high pressure sodium lamp: 150 lm/W
low pressure sodium lamp: 200 lm/W
The colour temperature of a light source is the
temperature of an ideal black body radiator that radiates light
of comparable hue to that of the light source. In practice,
colour temperature is only meaningful for light sources
that do in fact correspond somewhat closely to the radiation of
some black body.
Colour temperatures over 5,000K are called cool
colours (bluish white), while lower colour temperatures
(2,700–3,000 K) are called warm colours (yellowish white
Colour temperature applications
TV, video, and digital still cameras
Artistic application via control of colour temperature
Lighting: For lighting building interiors, it is often important to
take into account the colour temperature of illumination.
Aquaculture: In fishkeeping, colour temperature has different
functions and foci, for different branches.
In freshwater aquaria, colour temperature is generally of concern
only for producing a more attractive display. In a
saltwater/reef aquarium, colour temperature is an essential part of
Digital photography:In digital photography, colour
temperature is sometimes used interchangeably with white
balance, which allow a remapping of colour values to simulate
variations in ambient colour temperature
• Photographic film: Photographic emulsion film
sometimes appears to exaggerate the colour of the
light, as it does not adapt to lighting colour as human
visual perception does.
• Desktop publishing: In the desktop publishing industry,
it is important to know a monitor’s colour
temperature. Colour matching software, such
as coloursync will measure a monitor's colour
temperature and then adjust its settings accordingly.
Colour rendering index
• Colour rendering relates to the way objects
appear under a given light source. The measure is
called the "colour rendering index", or CRI.
• A low CRI indicates than objects may appear
unnatural under the source, while a light with a
high CRI rating will allow an object's colours to
appear more natural.
• For lights with a "warm" colour temperature the
reference point is an incandescent light. For lights
with a cool colour temperature the reference is