Brightness is defined as:

Brightness is understood as the subjective impression of light intensity that an individual experiences when observing an object. It reflects how light or dark a color seems to a person based on their perception, which can vary greatly depending on factors such as surrounding colors and the context in which the light is perceived.

This subjective quality means that different people may perceive brightness differently, even under the same lighting conditions, due to individual differences in vision and psychological factors.

The other options address related concepts but do not accurately define brightness. The objective measure of luminance refers to a physical measurement of light intensity, while color perception relates to how colors are distinguished under different lighting, and contrast deals with the differences in color or brightness between objects in a visual field. These aspects may influence brightness perception, but they do not capture its essence as a subjective experience.