Contrast represents the difference between items on a task surface. Without this difference in contrast the items are not visible. For example, light yellow type on a yellow screen doesn't provide enough contrast for us to read it easily. If we change the type to black , increasing the contrast, then we can read it very easily.

Usually, contrast improves with higher levels of light. Contrast also depends on the composition of the light and the angle that it strikes the task surface. These both contribute to lighting quality. It is usually better to try to improve these quality issues instead of increasing the amount of light because additional light usually leads to higher energy costs. Contrast is especially important for older individuals because they lose their ability to focus on objects.

Size - In general, the larger an object or task appears, the easier it is to see. Obviously, a sign that is close appears large and is easy to see while a sign that is very far away appears small and is difficult to see. Increasing the task size can be an alternative to improving contrast to increase task visibility.

Brightness - Luminance is a photometric measure of the light reflecting from the task to our eyes in a certain direction. Brightness is perceived luminance. Brightness is a function of both the measured luminance and the surroundings or background of a task. Both the color and the texture of the task affect luminance. For example, glossy finishes reflect more light than matte finishes and light colors reflect more than dark colors. Higher luminance increases visibility for the same contrast. For example, dark letters on a light background are more visible than light letters on a dark background because more light reflects from the light background into our eyes.

As illustrated by these squares, brightness depends on the background or surroundings, too. The square on the left appears lighter, because of the background, even though both center squares are exactly the same.

Time - And finally the time a person has to view an object affects vision because it takes time for the eye to assimilate the details. If the task has poor contrast and luminance, then it will take more time for the brain to process the information. For example, it will take longer to read a book under a dim light than it would under a desk lamp, but it can be done.