AP Psychology Practice Test 2025 - Free AP Psychology Practice Questions and Study Guide

Lightness constancy refers to the ability of our visual system to perceive the color or lightness of an object as relatively constant under varying lighting conditions. This phenomenon relies heavily on relative luminance, which is the luminance of an object in relation to the luminance of its background or surrounding areas.

When we see an object, we don't just consider its inherent brightness, but rather its brightness compared to other objects in the scene. For instance, a gray box will appear the same shade of gray regardless of whether it's in a bright room or a dimly lit one, as long as the surrounding objects provide a constant reference for comparison. This ability to maintain a consistent perception of lightness amid changes in lighting is what enables us to recognize objects accurately in different environments.

Absolute luminance, while it does contribute to the overall brightness we perceive, does not account for the surrounding context that is crucial for lightness constancy. The size or color of the light source can affect the overall illumination but are not determinants of the constancy effect seen in our perception of object lightness. Thus, it is the relative luminance that plays a vital role in maintaining lightness constancy in our visual perception.