Radio and Electronics Cookbook

44

Measuring light intensity – the photometer

Introduction

Before the days of automatic ‘point-and-shoot’ cameras, a photographer would use a light meter or photometer to measure the light level, then manually convert this reading into shutter speed and lens aperture settings to ensure a correctly exposed negative. Modern cameras have quite sophisticated photometers, which control the shutter speed and iris settings automatically.

A short explanation

A simple photometer circuit is shown in Figure 1, and is based on a device called a light-dependent resistor, or LDR. As its name tells us, its resistance depends upon the amount of light falling on it. In bright light, the resistance is relatively low (about 1 kΩ), whereas in the dark, its ...