QUANTITATIVE MEASUREMENT OF LIGHT BY: SUMAYYA NASEEM INTERNEE OPTOMETRIST
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.
Light: A form of energy, to which human eye is sensitive.Optical radiation:b/w X-rays and microwavesSeven bandsUV-C 200nm to 280nmUV-B 280 to 315nmUV-A 315 to 400nmVisible rad. 400-780nm IR. A 780-1400nmIR. B 1400-3000nmIR. C 3000-10000nm
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.
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.
PHOTOMETRY In photometry, we measure the energy from the visible part of spectrum, in terms of the visual response it produces.
We basically make 4 types of measurements inradiometry & photometry:1. Radiant flux and luminous flux2. Radiant intensity and luminous intensity3. Irradiance and Illuminance4. Radiance and luminance
Radiant flux and luminous fluxWhen we measure the total amount of energy, emitted in all directions from the point source, then in radiometry we call it radiant flux.Unit: WattIn terms of response of the eye, i.e. in photometry we call it luminous flux.Unit: Lumens.
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.
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.
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
What is this figure showing? Illuminance & Luminance