2. QUANTITATIVE MEASUREMENT
OF LIGHT
BY: SUMAYYA NASEEM
INTERNEE OPTOMETRIST

3. AIMS & OBJECTIVES OF TODAY’S LECTURE
 How do we measure light, Quantitatively?
 What is Radiometry and Photometry?
 Some important terms explaining photometry and
radiometry.
 Its Clinical application.
 And Surgical application.

4. Light: A form of energy, to which human eye is sensitive.
Optical radiation:
b/w X-rays and microwaves
Seven bands
UV-C 200nm to 280nm
UV-B 280 to 315nm
UV-A 315 to 400nm
Visible rad. 400-780nm
IR. A 780-1400nm
IR. B 1400-3000nm
IR. C 3000-10000nm

5. ELECTROMAGNETIC SPECTRUM

6. Light is essentially an electromagnetic wave which travels through
space described as quantized particles known as photons. However,
only a small band of the spectrum of possible electromagnetic
waves are actually visible and perceived by the eye as light.
 Furthermore, not all frequencies within that band of visible light
are detected equally by the eye, as it is sensitive to some
wavelengths significantly more than others. In fact, the response is
almost more of a bell curve, centered on about a 550nm
wavelength.
 This curve of sensitivity is known as the Visual Response curve
 What this curve says, roughly, is that given red (750nm), blue
(450nm) and green (550nm) lasers of equal power, the eye would
perceive the dot from the green one as being far brighter than either
the red or blue.

8. Radiometry
&
Photometry

9. RADIOMETRY
 In radiometry, we measure the radiant energy in absolute terms
from any part of electromagnetic spectrum.
 It is the measurement of radiant power or energy within that
part of the electromagnetic spectrum that is optical, meaning it
is refracted by glass or can be focused by a lens. This includes
microwave, infrared, visible and ultraviolet wavelengths
approximately in the range of 1 millimeter to 100 nanometers
(10-3 to 10-7 meters, or frequencies of 3 x 1011 to 3 x 1016 Hz).
Radiometry excludes radio waves, x rays and gamma rays.

10. PHOTOMETRY
 In photometry, we measure the energy from the
visible part of spectrum, in terms of the visual
response it produces.

11. We basically make 4 types of measurements in
radiometry & photometry:
1. Radiant flux and luminous flux
2. Radiant intensity and luminous intensity
3. Irradiance and Illuminance
4. Radiance and luminance

12. Radiant Flux
&
Luminous Flux

13. Radiant flux and luminous flux
When we measure the total amount of energy,
emitted in all directions from the point source,
then in radiometry we call it radiant flux.
Unit: Watt
In terms of response of the eye, i.e. in photometry
we call it luminous flux.
Unit: Lumens.

14. Luminous flux

15. Radiant Intensity
&
Luminous Intensity

16. Radiant intensity and luminous intensity
 Instead of measuring total energy, we measure the energy
going in a particular direction, then we call it radiant
intensity in radiometry and luminous intensity in
photometry.
 We measure this energy per unit of solid angle, which is
known as Steradian.
Unit:
 Watt per Steradian (in radiometry)
 Lumens per Steradian (in photometry)
 Lumens per Steradian is known as Candela.

17. Luminous intensity

18. Irradiance
&
Illuminance

19. Irradiance and Illuminance
 When we measure energy or light incident or
falling on a surface, we call it irradiance in
radiometry and Illuminance in photometry.
Unit:
 Watt per square meter (in radiometry)
 Lumens per square meter or lumens per square
foot (in photometry)
 Lumens per square meter= Lux
 Lumens per square foot = Foot Candle.

20. Illuminance

21. What is this figure showing?
I
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22. An example from practical life

23. Radiance
&
Luminance

24. Radiance and luminance
 When energy or light is reflected or emitted from a
surface then we call it radiance in radiometry and
luminance in photometry.
 Here we measure the amount per solid angle reflected or
emitted by a unit area of the surface.
Unit:
 Watt / Steradian /square meter (in radiometry)
 Lumens / Steradian / square meter (in photometry)
 Candela / square meter or Candela / square foot
 1 / Π Candelas / square meter = Apostilb
 1 / Π Candelas / square foot = Foot Lambert

25. What is this figure showing?
Illuminance & Luminance

26. R
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T
I
V
E
L
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I
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27. SUMMARY
Radiant Watt= J/sec Luminous Lumens
(1 candle emits
flux flux 4Π lm)
Radiant Watt/Steradian Luminous Candela
=1lm/steradian
intensity intensity
Irradiance Watt / sq.meter Illuminance 1 Lux
=1 lm/sq.meter
Radiance Watt/Steradian Luminance Apostilb
1/ΠCandelas/sq. met
/sq.meter Foot Lambert
1/Π Candelas/sq. foot

28. PHOTOMETRY SUMMARY

30. The relationships among the five photometric units

31. Clinical
Applications

32. Clinical Applications (assessment of visual functions)
 Visual acuity (luminance 220 to < 1 cd/m²)
 Visual field (background: 31.5 abs., light stimulus: 0.8 to 10,000 abs.)
 Color vision (FM 100 Hue Test)
 ERG (Stimulus strength 3.43 & Background illumination 17-34 cd-m-2)
 VEP (Luminance of stimulus 80 cd. m-2)

33. VISUAL DRUM

34. PERIMETER (Visual field testing with a
Humphrey Field Analyzer device)

35. FM 100 Hue Test

36. ERG

37. VEP/ VER
E
- -
+ +

38. Surgical
Applications

39. Surgical Applications
 LASER surgeries (e.g. xenon arc)
 Ablative surgeries
• LASIK
• PRK
• Radiotherapy