From Photon to Pixel

4Radiometry and Photometry

During the recording of a photograph, the light wave emitted by the scene is transformed into an image on film or on a photodetector. The image is the result of the transformation of the energy of the photons into electrical or chemical energy. The role of radiometry and photometry is to describe the parameters involved in this flux of photons.

Radiometry and photometry are two closely related sciences which have as a common subject of study the energy effects of light radiation. Therefore, they describe two complementary aspects:

• – radiometry is only concerned with the objective and physical aspects of radiation;
• – photometry considers the subjective, perceptual aspects of radiation and therefore the energy balances involved, such as the measurements of a reference observer, i.e. the human eye, along with its metrics.

Nevertheless, according to several authors, the term “photometry” is also associated with the physical aspects of radiation in the visual spectrum (radiometry being extended to areas not perceptible to humans). As a precaution, energy or objective photometry will be distinguished from visual or subjective photometry throughout this chapter [KOW 72, LEG 68, SMI 90].

In the field of photography, one is driven to maintain a close connection between these two aspects since a physical sensor is available. This is precisely related to the first aspect, but most often it is sought to assimilate it as closely as possible to criteria ensuing ...