Metering method and which kind of light to be measured are two vital parts of metering technique.

Light normally refers to sunlight; for photography, this includes artificial light too. If there is no light, there will be no image, so image is directly related to light. All objects reflect light in a wide direction and our vision pick up these reflected light to form an image of them.

The intensity of reflected light related to light source and the reflection rate. The strength of light source is called lumination, and the intensity of light falling on an object is called illumination which is affected by the distance from the light source. In other words, with the same distance, illumination is high when lumination is high, while with the same light source, illumination is high when distance is shorter and low when distance is longer.

The image of an object must have different reflection including intensity and color to be real, without such difference, there would be no image at all. The reflection is based on the reflecting percentage and illumination, e.g. a 100 units of light falls on a part with reflection of 40%, the reflect light would be 40 units. The color is based on the bandwidth absorption of that part, e.g. if it absorbs red, only green and blue light will be reflected, then we see it as cyan (green plus blue).

Since image is related to the illumination and its reflection rate, therefore metering should be for the illumination, which covers the lumination and the distance between the object and light source. However, an object with high reflection rate and low illumination may be same as an object with lower reflection rate and high illumination, therefore measuring must take a fixed reflection rate to make the result, the middle gray with reflection rate of 18% is used.

Measuring light depends on a kind of light-sensitive material (selenium, silicon, cadmium sulphide etc.) to convert light into electric signal, either electric current, voltage or resistance. A light meter is a kind of voltmeter or ammeter. Traditional design uses needle and electronic design uses LED or liquid crystal display.