QUANTITATIVE MEASUREMENT OF LIGHT

BY: SUMAYYA NASEEMINTERNEE 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 microwaves

Seven 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

ELECTROMAGNETIC SPECTRUM

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&

Photometry

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 in radiometry & photometry:

1. Radiant flux and luminous flux2. Radiant intensity and luminous intensity3. Irradiance and Illuminance4. Radiance and luminance

Radiant Flux &Luminous Flux

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.

Luminous flux

Radiant Intensity

& Luminous Intensity

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.

Luminous intensity

Irradiance &Illuminance

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.

Illuminance

What is this figure showing?

ILLUMINANCE

An example from practical life

Radiance &Luminance

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

RELATIVE

LUMINANCE CHART

SUMMARYRadiant

fluxWatt= J/sec Luminous

fluxLumens

(1 candle emits 4Π lm)

Radiant intensity

Watt/Steradian

Luminous intensity

Candela =1lm/steradi

an

Irradiance Watt / sq.meter

Illuminance

1 Lux=1 lm/sq.meter

Radiance Watt/Steradian/sq.meter

Luminance

Apostilb1/ΠCandelas/sq. met

Foot Lambert1/Π Candelas/sq. foot

PHOTOMETRY SUMMARY

The relationships among the five photometric units

Clinical Applications

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)

VISUAL DRUM

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

FM 100 Hue Test

ERG

VEP/ VER

--

+

E

+

Surgical

Applications

Surgical Applications

LASER surgeries (e.g. xenon arc) Ablative surgeries

• LASIK• PRK• Radiotherapy