measure light p5

1. MEASURE LIGHT
Ms. DIAH

2. What is the speed of light ?
● Do you know what is the fastest thing ever?
The cheetah? From the animal kingdom? Sure! Usain bolt? In
humans, yep!
● But do you know what is faster than the fastest? Look around
you!
Any guesses?
● It is… LIGHT!
● HOW FAST IS THE SPEED OF LIGHT ?
● mathematically, light travels at the speed of 300000000 meters
per second in vacuum OR 3 . 10^8 m/s. It is a constant quantity
which remains the same throughout the reaches of humans.
● This constant attribute, though changes when the medium
changes. In vacuum, there is no obstacle for the light waves and
hence, it travels unobstructed.

3. Speed of Light
● Vacuum: 3 x10^8 meters per second.
● Water: 2.26 x10^8 meters per second.
● Air: 2.9 x10^8 meters per second.
● Glass: 1.9 x10^8 meters per second.

4. ● Our eyes can tell the difference between bright light and dull
light.
● To measure light accurately , we can use a light meter
● The unit that we use to measure the brightness (or illuminance)
of light is the LUX (lx)
● This measures the intensity of the light as it appears to our
eyes.
● It does not measure the energy of the light source
Can we Measure Light ?

6. Light Intensity
● Light intensity (quantity) is measured in foot-candles in the
United States, or in lux in most other countries. Even in
the United States lux is replacing foot-candles.
● As we've noted, a foot-candle equals about 10.74 lux (or,
for a rough conversion, multiply foot-candles by 10 to get
lux).
● To provide some points of reference:
1. sunlight on an average day ranges from 32,000 to 100,000
lux
2. TV and film sets are lit at about 600 lux
3. a bright office has about 400 lux
4. moonlight represents about 1 lux
5. starlight measures a mere 0.00005 lux

7. Use information in the table
Time of day or place Lux (lx) measurement
Direct sunlight 32000 - 130000
Full daylight 10000 - 25000
Shady place on a cloudy day 11500
Overcast day 1000
Sunrise or sunset 400
Inside an office 400
Full moon on a clear night 1. 00
starlight 0,0011

8. Light Meter
● Light meters are used to
measure light intensity.

9. Light Meter
● As we've noted, our eyes are drawn to light areas in
a scene as illustrated in this painting.
● Therefore, you can use light to emphasize the
scene's center of interest and downplay secondary
or potentially distracting elements.
● Before you can use relative intensities creatively
(and avoid intensity related problems) you need to be
able to accurately measure light intensity.
● Because the eye is a rather unreliable judge in
setting up lighting, you need to use either a light
meter or a keen eye coupled with a high-quality color
monitor.
● Although the latter is preferred for making final
adjustments, when lights are first being put into
place it's much faster to use a light meter.
● There are two types of meters: reflected and
incident.

10. How to Measure Light Intensity
● how to measure light intensity,
the difference is between
lumens and illuminance (and what
they mean)

11. What is the Best Metric to Measure Light Intensity ?
● Illuminance is the metric that is used to
measure the light intensity within a space.
It is measured in footcandles or lux – it is
the amount of light (lumens) falling on a
surface (over any given square foot or
square meter). Therefore, light intensity
is measured in terms of lumens per square
foot (footcandles) or lumens per square
meter (lux). Measuring the amount of light
that falls on a surface allows us to
evaluate if we have sufficient light to
perform various visual tasks.

12. What are Lumens ?
● Lumens (lm) are the unit of measurement we use to quantify the
amount of visible light the human eye can see. The luminous flux
of a particular light source is measured in lumens. You many have
noticed when you buy light bulbs for your house that they
indicate lumen output. The higher the lumen output, the ‘brighter’
or higher intensity the light source; the lower the lumen output,
the less bright or lower intensity the light source.
● When you purchase light bulbs based on their intensity or
brightness, it is lumens you‘re after, not watts – watts simply
determine a bulb’s energy consumption. By understanding lumens,
we can explore other lighting metrics such as illuminance
(footcandles/lux) and how this plays a key role in assessing the
intensity of a light source.

13. What is Lux ?
● Lux is simply the unit of measure used to describe the number of
lumens falling on a square foot (footcandles) or square meter
(lux) of a surface. So let’s say you have a light source with 1,000
lumens. If all of those 1,000 lumens are spread over a surface
area of 1 square meter, you’d have an illuminance of 1,000 lux – i.e.
the brightness of an overcast day. But what if we spread this
over 10x the area i.e. 10 square meters? Well, the illuminance or
lux would decrease to a less intense and dimmer 100 lux. We use
the same approach for footcandles, only our units are lumens per
square foot.
● The reason we measure light intensity is to ensure a certain
“standard” of illumination is met. it makes a valuable difference
for a photographer (whose work is specifically centered around
light) as it would in surgical theater or other indoor environments
such as offices.

14. What is a Footcandle ?
● A footcandle is a measure of light intensity – it’s
the number of lumen per square foot. Now, you
might be thinking we already covered lux, so why
add in this metric? Different people use
different metrics and for different reasons.
Simply put, where 1 lux equals 1 lumen in a square
meter, 1 footcandle equals one lumen per square
foot.

15. What is Luminous Flux ?
● Luminous flux is how to measure the perceived
power or total amount of light output from a
light source. When the number of lumens – the
unit-amount of visible light a human eye can see,
is used to measure the intensity of a light source.
A one-meter sq. surface area is required (lux) to
determine the luminous flux value.

16. How to Measure Light Intensity Using a Light Meter
1. Measure Ambient Light In the Room
To begin, turn off any lighting in the room you’re about to measure. Turn
on the light meter to establish what’s known as the baseline measurement
– the ambient light. This means you’re able to see how much the existing
lighting adds to the room once the lights are then turned on.
2. Turn On the Lights, Take Your Measurement
From a central area of the space, ensure your light meter is set up to
record your new reading. Remember not to rush – allow the lighting a few
moments to reach full brightness (especially if you’re measuring light
from CFLs).
3. Note Your Differential Reading
Simply subtract the ambient light level from the illuminated level –
known as the differential (or delta) measurement, this is the amount of
light the existing luminaires produce. With this light measurement unit,
you can asses how it compares to the optimal lighting levels required.
4. Check Other Areas of a Room
For open office lighting or a corridor, the reading you get from your
light meter should, in theory, be consistent. However, it might be worth
checking any potential “blind” spots just to be sure you have consistency.

21. To measure quantity of light from a source
● Historically, a lumen was the amount of
light from one candle which fell on one
square foot of area, one foot away from
the candle.
● Our historic “standard” candle produces
12.57 lumens A four foot fluorescent
lamp produces about 3000 lumens Your
standard 60 Watt light bulb
(incandescent) produces about 900
lumens
● One foot One lumen of light falls on a
surface one foot square, which is held one
foot away from the candle.

22. To measure quantity of light striking of surface
● Lumens per square foot, other wise
known as footcandles (fc) •
● or, Lumens per square meter
otherwise known as Lux (lx) These
are the common measurements made
by a light meter One foot-candle =
10.76 lux

23. Remember , every lightning application have 3 element :
● The Source is all the hardware, This is what most people think of The as
the lighting system, but just as important are…
● The Surface which is the room environment Source and whatever you are
looking at
● The Purpose
The reason you need the light, the activities or tasks that people are doing
that require certain visual conditions