# What is a Photometer? – Principle, working and Types

## Photometry and Photometer

The measurement of light's brightness or luminous intensity is known as photometry. Photometry focuses on the perceived brightness to the human eyes. The photometry takes into account the eye's sensitivity to varying degrees of light and focuses primarily on the visible light spectrum.

Thus, the photometry involves the measurement of candle power. The candle power of a given source of light in any given direction is measured by comparison with a standard source. In order to eliminate the errors in the measurement of the candle power, the measurement is performed in a dark room with dead black walls.

"The device which is used to measure the candle power or luminous intensity of a given source of light is known as photometer."

## Principle and Working of a Simple Photometer

The photometer used for the measurement of the candle power of a source is based upon the inverse square law. A simple arrangement for the measurement of candle power of a light source is shown in the figure.

From the figure, it can be seen that the photometer bench consists of two sources, one is the standard source 'S' whose candle power is known, and the other is the test source 'T' whose candle power is to be determined.

The photometer head acts as the screen for the comparison of the illumination of standard lamp and the test lamp. The photometer head is moved smoothly in between the two fixed sources until the illumination on both the sides of the screen is same.

Now, if the distance of the standard source 'S' from the photometer head is 𝐿1 and the distance of the test source 'T' from the photometer head is 𝐿2. Then, according to the inverse square law, if the illumination on both the sides of the screen are equal, then the candle power of the source is proportional to the square of the distance between the source and the photometer heat.

Therefore,

$$\mathrm{Candle\: power\: of\: standard\: source\: \propto\: \mathit{L}_{1}^{2} }$$

$$\mathrm{Candle\: power\: of\: test\: source\: \propto\: \mathit{L}_{2}^{2} }$$

On comparing the above two equations, we get,

$$\mathrm{\frac{Candle\: power\: of\: test\: source}{Candle\: power\: of\: standard\: source}\: =\: \frac{\left ( \mathit{L}_{2} \right )^{2}}{\left ( \mathit{L}_{1} \right )^{2}} }$$

$$\mathrm{\therefore Candle\: power\: of\: test\: source\: =\:CP\: of\: standard\: source \times \frac{\left ( \mathit{L}_{2} \right )^{2}}{\left ( \mathit{L}_{1} \right )^{2}} }$$

## Types of Photometers

Based on the type of head used in the photometer, the photometers are classified into three types, viz. −

• Bunsen Grease Spot Photometer

• Lummer Brodhun Photometer

• Flicker Photometer