Adelson's Perception of Lightness

In his article, Adelson described the theories which scientists have developed to explain the perception of lightness by the human eye. Our visual system processes light in the retinal image received by the brain, adjusting the illumination and other viewing conditions and estimating the reflectance to obtain lightness constancy. As a result, we are able to perceive properties such as color, size, shape of objects in our surroundings.

The multi-level visual processing system

The visual system processes information at multiple levels of sophistication, classified as low, mid and high-level vision.

Low-level vision includes light adaptation and the center-surround receptive fields of ganglion cells. Adaptation and local interactions at a physiological level are the crucial mechanisms of low-level vision.

Mid-level vision is the region between low and high-level vision. The representations and the processing in the middle stages involve surfaces, contours, grouping, etc.  Mid-level vision emphasized the importance of perceptual organization, including grouping, belongingness, and good continuation, proximity.

High-level vision includes cognitive processes that incorporate knowledge about objects, materials, and scenes. This explains the phenomena that perception is the product of unconscious inference. A lightness judgment involves the workings of the whole visual system, and that system is designed to interpret natural scenes. Lightness constancy is achieved by inferring, and discounting, the illuminant. Simultaneous contrast and other illusions are the byproduct of such processing.

Measuring light in scientific terms

Luminance, illuminance, and reflectance, are physical quantities that can be measured objectively by physical devices. Luminance is the amount of visible light that comes to the eye from a surface. Illuminance is the amount of light incident on a surface. Reflectance is the proportion of incident light that is reflected from a surface. In scientific terms, an illuminance image, E(x,y), and a

reflectance image, R(x,y), are multiplied to produce a luminance image, L(x,y):

                                    L(x,y) = E(x,y)R(x,y)

Real life examples

The scientific founding highlighted in this article is very crucial in design. The visual perception is fundamental to design and designers must understand how the physiological and cognitive visual processing will affect the viewer’s perception. In fact, some artists and designer have been able to manipulate the viewer’s visual perception to achieve some interesting results. Here are some examples:

Artist Julian Beever's sidewalk illusions, manipulating viewers' perception of light to achieve the illusion of depth and shape.

Application in interiors (demonstrates grouping)

Eureka car park illusion (demonstrates figure)