Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 107

Photometry is the quantitative measurement of visible light. Photometry measures a number of interrelated properties of light, using a basic unit called a candela. Two important characteristics of light are its luminous intensity, which describes the intensity of a light source (as measured in candela), and its luminance, which describes its brightness reflected from a surface (as measured in foot‐lamberts or cd/m²). These two properties are related, but they are not necessarily proportional. A handheld transilluminator is a bright source of light, but it possesses low intensity and therefore cannot be used to illuminate a football stadium. On the other hand, a streetlight provides high‐intensity light, which illuminates a large area, but it is not bright and does not provide enough illumination to conduct cataract surgery (Table 2.12).

Table 2.12 Luminances of natural and artificial light sources.^a

Source	Luminance (cd/m2)
Sun	109
Car light	107
Incandescent tungsten lamp	106–107
Fluorescent lamp	104–105
Clear sky at noon	104
Full moon	103
Street lamp	0.1–1.0
Moonless night sky	10−3 to 10−6

^a In general, only the photopic system is active at a luminance >3 cd/m²; at a luminance <0.03 cd/m², the scotopic system functions alone. Both systems are active at intermediate luminance values, which are defined as mesopic vision.

Luminance is measured using photometers, which are divided into two major classes. Visual photometers provide a subjective reading, because the observer compares the illumination of the measured light with that of a standard light. Photoelectric photometers convert the measured light into an electric current, which is displayed by the instrument. Photometry measurements are extremely important in electroretinographic (ERG) recordings because they are used to describe such variables as threshold, ambient light, and stimulus parameters.