As stated before, the method to be introduced relies on photography analogies. There are two ways how light can be measured in photography:
The first is incident light metering. It was used at the beginning of the photography era by portrait photographers. The main idea is to measure the light falling on the subject that is photographed. The incident light meter is placed at the subject position and aimed to the camera (see fig. 7.1). Portrait photographers had the possibility of walking up to the subject, measuring the light, and then walking back to the camera to adjust the exposure. Actually, this is still done in professional photography, and in the movie industry, where the incident light is measured even outdoors via fill panels, and stand-ins are paid to stand around the scene and be metered for light adjustment purposes (that is the reason why they are called stand-ins).
For landscape photographers it is not possible to walk to the subject to measure the light, so they use reflected light metering only, where they measure the light coming to the camera from the subject direction (fig. 7.1). When people started to take photos of moving targets, it was also more convenient to measure the light from the camera position. Unfortunately, in contrast to incident light metering, reflected light metering is not independent of subject reflectivity or BRDFs of the scene, and the relative preponderance of light or dark areas in the scene. This is still the biggest drawback of reflected light metering. Nowadays, however, in spite of all its drawbacks, it is used more often than incident light metering, since a lot of improvements in reflected light metering have been introduced. All these improvements try to compensate for the subject variations over the imaginary average gray scene.
Figure 7.1: Reflected and incident light metering
Figure 7.2 shows a light meter that can be used for reflected and incident light metering. Note the white hemisphere that acts as a half-space integral.
Figure 7.2: Incident and reflected light meter