# Introduction to Photometry

It is the branch of optics that deals with the study of measurement of visible light.

Luminous body: A body is said to be luminous if it posses light of its own. Example: Sun, star, candle etc

Solid Angle: The angle subtended by the surface at that point is called solid angle at that point. It is represented by l. It is measured as the ratio of surface area to the square of distance between the points.

$${\text{i.e., Solid angle (l)}} = \frac{\text{A}}{\text{r}^2}$$

Unit of solid angle is steradian.

1 Steradian is the angle subtended by the surface at the point when the area is 1m2 and radius is 1 metre.

Luminous energy: The radiant energy which is capable of producing the sensation of vision to our eyes is called luminous energy.

Luminous flux: The luminous energy emitted by the source per second is called luminous flux. It is denoted by Q. Its unit is lumen.

Luminous intensity: The luminous energy emitted by the source per unit solid angle is called luminous intensity.

$$L = \frac{Q}{4 \pi}$$

The unit of luminous flux is candela.

If we consider total solid angle, then n = 4 π

or Φ = 4 πI

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