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
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.
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
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.
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.
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.
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