Measurement of the Stimulus and Dioptrics of the Human Eye

Abstract

Visible light consists of electromagnetic vibrations limited to a certain band of wavelengths—the visible spectrum. The limits are given classically as 400 nm and 760 nm, wavelengths shorter than 400 nm being described as ultraviolet, and longer than 760 nm as infrared.¹ As we shall see, however, the retina is sensitive to wavelengths shorter and longer than these limits. Light is a form of energy and, as such, is amenable to quantitative measurement in absolute units, namely ergs. For physiological purposes, however, that subjective quality of light we call ‘brightness’ or ‘luminosity’ is the one that is of greatest interest, and since this is not uniquely determined by the energy content of the light, but varies widely with the wavelength (p. 395), it has been considered desirable to base measurements of light on a somewhat arbitrary ‘luminosity basis’. Thus, on a luminosity basis, two sources of light are said to be of equal intensity when they produce equal sensations of brightness when viewed by the same observer under identical conditions.