VISIBILITY CURVE

A graph representing the sensitivity of the eye to different wavelengths of light. Two types of curves are usually constructed, one for daylight vision and the other for night vision. They are sometimes said to measure “spectral sensitivity,” since they cover the range of wavelengths which make up the visible spectrum. See visible spectrum.Our eyes are not equally sensitive to all types of light. During daylight we register all the colors in a variegated garden, with the yellows and reds appearing particularly bright. As twilight comes on, these colors darken and the greens and blues begin to stand out. The leaves of a rose bush look bright long after the roses have turned to black, and by nightfall no colors at all are visible. This shift in the relative brightness of colors in low illumination is called the Purkinje effect, named after the Bohemian physiologist who first studied it.The Purkinje effect occurs because the retina contains different cells for color (cone cells) and for brightness (rod cells), and in decreasing illumination the eye shifts from cone to rod functioning. The daylight visibility curve therefore represents cone vision and the night visibility curve represents rod vision. One method of obtaining these curves is to expose the eyes to test patches of various wavelengths, and in each case to increase the intensity of the patch from zero until the subject indicates that he sees it. This point is called the absolute threshold, and the curve shows these thresholds for all visible wavelengths.The curve for cone vision is obtained by exposing the light patch so that it falls on the fovea of the eye, a small area of the retina that contains only cones and no rods. The resulting curve, called a photopic (daylight) visibility curve, shows that the retina is most sensitive to greenish-yellow light, which has a wavelength of approximately 555 millimicrons. The curve for rod vision is obtained by making the test patch of light fall on the periphery of the retina, which contains all rods and no cones. The resulting curve, known as the scotopic or darkness vision curve, shows maximal sensitivity at 510 millimicrons, a portion of the spectrum that corresponds to bluish-green, even though the colors themselves are visible only as shades of gray. This explains why blues and greens appear especially bright in twilight.Studies of retinal sensitivity also help to explain dark adaptation. Tfie eye is actually more rather than less sensitive in darkness than in light, since less light is required to activate the rods than the cones. There is usually enough light energy at night to enable us to see the shapes of the flowers in the garden even though we cannot see the colors. These shapes come to us in various shades of gray, according to the visibility curve of the rods for different wavelengths.Visibility curves can be used in diagnosing certain visual defects. People with hemeralopia (night blindness) show no rod curve at all; only the cones are operative. In day blindness, or total color blindness, the cones do not function and therefore there is only a curve for rod vision. In this rare condition the individual sees only shades of gray