Vision

Luminance is the intensity of light reflecting off an object, and influences visual acuity. Dim light activates only rods, and visual acuity is poor. As luminance increases, Figure 2. If you were to draw a line down the center of this scene, what you observe left of that line is your "left visual field," and to the right is your "right visual field." Images from the left visual field project to the right half of each retina, and images from the right visual field project to the left half of each retina. Illustration by Hans & Cassidy. Courtesy of Gale Group.
more cones become active and acuity levels rise sharply. Pupil size also affects acuity. When the pupil expands, it allows more light into the eye. However, because light is then projected onto a wider area of the retina, optical irregularities can occur. A very narrow pupil can reduce acuity because it greatly reduces retinal luminance. Optimal acuity seems to occur with an intermediate pupil size, but the optimum size varies depending on the degree of external luminance. The difference in luminance reflected by each object in an image produces varying degrees of light, dark, or color. Contrast between a white page and black letters enables us to read. The greater the contrast, the more acute the visual image.