# Photometry Introduction | Physics Grade XI Reference Note

### Luminous Flux

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The amount of light energy emitted by a source per second is called luminous flux. It is denoted by Q. its unit is lumenLumen is the unit of flux and is defined as the amount of luminous flux emitted per unit solid angle by a point source of one candle power.

Luminous Intensity is defined as the amount of flux emitted by a source per unit solid angle, Ω in that direction. It is also known as illuminating power and is denoted b I.

If Q is the amount of luminous flux emitted by a source uniformly in all directions, then the luminous intensity is,
L = Q/ Ω
or, L = Q/ 4π,     Where Ω is the solid angle and at the center of sphere, Ω = 4π.

Its unit is lumen per steradian or simply candela, cd. Eq. (i) is the relation between luminous intensity and luminous flux.

Intensity of Illumination
The amount of light energy incident on unit area of a surface per second is called intensity of illumination. Let us consider a source of light placed at the center of a hollow sphere of radius r. if Q is the amount of light energy emitted per second by the source and 4πr2 is the area covered by the light energy, from the definition, we can write,
I = Q/ A
I = Q/ 4πr2 .. (i)

Again, from luminous intensity, we have,
L = Q/ 4π,
Then equation (i) becomes,
I = L/ r2

This is the relation between intensity illumination of a surface and luminous intensity of a source. Since L is constant, so
I α 1/r2

This shows that the intensity of illumination of a source at a point is inversely proportional to the square of the distance from the source of light. The unit of intensity of illumination is phot in CGS system and lux in SI system.

Phot or centimeter Candle
It is the amount of light energy falling normally on a square centimeter spherical surface of radius of ne centimeter, when a source of one candle is kept at the center of the sphere i.e. on phot = one lumen per square centimeter.

Lux or meter Candle
It is the amount of light energy falling normally on a square meter spherical surface of radius one meter, when a source of one candle power is kept at the center of curvature i.e. one lux = one lumen per square meter.
1phot = 104 lux

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