It deals with measurement of illuminating power and involves

measurement of the quantity of light.

Illuminating power of two sources of light is directly

proportional to the squares of the distances at which they

can produce equal intensities of illumination.

The instruments used for comparing the illuminating powers

of two sources of light are called photometers. They are.

1.Runford?s Shadow Photometer

2. Bunsen?s Grease-Spot Photometer

3. Flicker Photometer.