1. In this presentation you will:
explore the measurement of the quantity of light
explore the relationship between illumination and distance
ClassAct SRS enabled.

2. In this presentation you will learn about light, how it can be detected, how its luminous intensity can be measured, and how distance affects the illuminance on a surface.
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3. The Visible Spectrum
White light actually consists of numerous colors, which can be separated by passing white light through a glass prism. This causes the light to refract and disperse.
Each color has its own specific wavelength, ranging from violet with the shortest wavelength (400 nanometers) to red with the longest wavelength (700 nanometers). All these colors are part of the visible spectrum.
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4. The Electromagnetic Spectrum
The visible spectrum is one very small part of the electromagnetic (EM) spectrum, which covers an enormous range of frequencies.
106
107
108
109
1010
1011
1012
1013
1014
1015
1016
1017
1018
100m
10m 1m
10cm
1cm
1000µm 1mm
100µm
10µm
100nm
10nm
1nm
0.1nm
1000m
1000nm1µm
X-rays
Ultraviolet
Visible
Infra-red
Micro-waves
Frequency in Hz
Wavelength
700nm
600nm
500nm
400nm
Radio, tv
Long- waves
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5. The Human Eye
The human eye can only detect frequencies in the visible spectrum.
The quantity of light entering the eye is controlled by the size of the pupil, but it is cells in the retina that detect the amount of light and the color.
Pupil
Retina
The retina consists of two types of light-sensitive (photoreceptor) cells called rods and cones.
These cells react to the different light frequencies, producing different photochemical reactions that send electrical signals to the brain.
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6. The Human Eye
Rods can distinguish between different intensities of light, but not between light of different frequencies…
Nucleus
Outer segment of cone cell containing photosensitive chemicals
Cone
Rod
Nucleus
Outer segment of rod cell containing photosensitive chemicals
… whereas cones allow us to see color.
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7. Question 1
What is the name given for the part of the electromagnetic spectrum in which light can be seen by the human eye?
A) Magnetic spectrum
B) Visible spectrum
C) Frequency spectrum
D) Invisible spectrum

8. Question 2
Rods and Cones are light-sensitive cells in the eye that allow us to see light intensity and color. Which of these two types of cells detects color?
A) Rods
B) Cones

9. Crookes’ Radiometer
One instrument that can demonstrate the energy emitted by a light source is Crookes’ radiometer.
Vanes
A radiometer consists of a set of vanes, each shiny on one side and blackened on the other.
The vanes are mounted on a spindle in a vacuum.
When exposed to light, the vanes revolve. The more light, the faster the vanes revolve.
It is difficult to measure light energy with this instrument, as the vanes can move very fast and do not respond quickly to changes.
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10. Photoresistor
Modern electronics has given us several light sensing devices.
A photoresistor, also called a Light Dependent Resistor (LDR), is a sensor that reacts to the quantity of light falling on it by changing its resistance.
Illuminance in lux
Resistance in W
The resistance of a photoresistor usually decreases as the ambient light falling on it increases.
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11. Photodiodes & Phototransistors
Photodiodes and phototransistors are light-sensitive semiconductor devices.
Each of these devices reacts to light by controlling the flow of electric current passing through it.
They can be used in a variety of sensor circuits that detect the presence and the amount of light.
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12. Photometry
Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye.
The human eye is not equally sensitive to all wavelengths of light.
Photometry attempts to account for this by weighting the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength.
In this part of the presentation you will learn about 4 terms that are commonly used in photometry:
Luminous Intensity
Luminous Flux
Luminance
Illuminance
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13. Luminous Intensity
Luminous intensity (I) is a measure of the power emitted by a light source in a particular direction.
The unit of luminous intensity is the candela (cd) from the Latin word for “candle”.
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14. Luminance
Luminance (L) is the luminous intensity emitted by a unit area of a light source.
Luminance is often used to measure light emission per unit area from flat, diffuse surfaces, such as video displays.
The luminance is a measure of how bright the surface will appear to the eye from a particular angle of view.
The unit of luminance is the candela per square meter (cd/m2).
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15. Luminous Flux
Luminous flux (Ф) or luminous power is a measure of the power emitted by a light source in all directions.
The unit of luminous flux is the lumen (lm), from the Latin word for “light”.
Luminous flux is often used to measure the useful power emitted by a light bulb.
When comparing different types of light bulbs, energy conscious consumers compare the luminous flux of each type of bulb.
For example, a typical 100 watt incandescent light bulb emits about 1700 lumens.
Roughly the same amount of light can be produced by a 25 watt compact fluorescent light bulb at a quarter the electricity cost.
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16. Illuminance
Illuminance (E) or illumination is the total luminous flux falling on a unit area of a surface. It is a measure of how brightly the surface is lit.
The unit of illuminance is the lux (lx), which is the same as lumens per square meter (lm/m2).
1 m2
1 lumen
Illuminance = 1 lux
If 1 lumen (lm) of luminous flux uniformly lights a surface of area 1 m2, the illuminance of the surface will be 1 lux.
Some typical illuminance values in your everyday surroundings are: Bright sunlight 50,000 – 100, 000 lux Sunset 1 – 100 lux Full moonlight 0.01 – lux Classroom lights 100 – 300 lux
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17. Lux vs Lumen
Area=
1m2
= 100 lux
100 lm
The difference between the lux and the lumen, is that the lux takes into account the area over which the luminous flux is spread.
For example: 100 lumens, concentrated into an area of one square meter, will light up that square meter with an illuminance of 100 lux.
Area=
100m2
= 1 lux
100 lm
The same 100 lumens, spread out over a hundred square meters, will produce a dimmer illuminance of only 1 lux.
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18. Question 3
Luminous flux of 100 lumens falls on a flat surface of area 20 m2. What is the illuminance of the surface?
A) 100 lux
B) 20 lux
C) 5 lux
D) 1 lux

19. Question 4 What is illuminance measured in? A) Candela B) Lumen
C) Candela per square meter
D) Lux

20. Effect of Distance on Illuminance
Investigation of illuminance shows that it obeys an Inverse-Square Law, which states that… r 2r 3r 4r 5r
“… as a light source is moved away from a surface, the illuminance decreases by a factor that is inversely proportional to the square of the distance”.
I µ 1 r2
This can be written as:
Thus the illuminance of a surface by a source of light 2 m away is 1/4 of the illuminance at 1 m from the source.
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21. Effect of Distance on Illuminance
Conversely, for two light sources, one at 1 m from a surface and the other at 2 m, to give the same illuminance to the surface,… 1m
…it would be necessary for the source at 2 m to be 4 times as bright as the source at 1 m. 2m
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22. Question 5
If you double the distance between a light source and a sensor, how is the illuminance affected?
A) The illuminance quarters.
B) The illuminance halves.
C) The illuminance remains the same.
D) The illuminance doubles.

23. Question 6
If the illuminance at a distance r from a light source is 400 lux, what would the illuminance be at a distance of 2r?
A) 800 lux
B) 200 lux
C) 100 lux
D) 1 lux

24. Summary After completing this presentation you should be able to:
show knowledge of some of the sensing elements used for the measurement of light.
show knowledge of some of the units used in the measurement of light.
show knowledge and understanding of the Inverse-Square Law for illuminance and distance.
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