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Posts Tagged ‘lightness theory’

Seeing Black and White

In Design Theory on January 27, 2010 at 8:55 am

In his book Seeing Black and White, Alan Gilchrist explored how they human eyes perceive colors of object surfaces in context with the everyday environment. Specifically, he discussed the achromatic aspect of colors, or the colors’ gradation from light to dark. He also discussed lightness constancy, which means, “the perceived lightness of a surface remains roughly constant even though the illumination, and thus the luminance of the surface, changes” (Gilchrist, page 7).

Gilchrist began by explaining some commonly used terms in the field of visual perception: distal stimulus, proximal stimulus and percept. Distal stimulus is the physical appearance of the object, without the observer’s interpretation; proximal stimulus refers to the interaction between light reflected by the object’s surface and the retina; and percept is the human’s interpretation of the object’s appearance, which may or may not correspond to the actual appearance of the object. Gilchrist also explained the difference between lightness and brightness, terms that are commonly misunderstood. According to Gilchrist, lightness is the human’s perception of the percentage of light being reflected by the object’s surface, whereas brightness refers to the intensity of the object’s image on the retina.

Contrast is another term that Gilchrist explored. He explained three different uses of this term, one of which was “the ratio (sometimes difference) between the luminance on one side of and edge (or gradient) and the luminance on the other side” (Gilchrist, page 8). This is also sometimes referred to as luminance ratio, or tone.

In writing his book, Gilchrist researched and cited historical scientific studies and developments of lightness theories, but also pointed out the shortfall of how these studies were influenced by different factors at different times. He also argued that many of the existing theories predicted errors in human lightness perception do not occur, and many errors that do occur were not predicted by these theories. At the end of the book, Gilchrist analyzed the strengths and weaknesses of each theory and presented his opinions about the status as well as the current challenges for lightness theory.

This book is an eye opener for me. After reading merely the introduction of Seeing Black and White, I realized that I have always taken my ability of sight for granted. I go through my every day seeing things are I perceive them, thinking that’s what they are like. I never stop and think about why I perceive them a certain way. As Adelson’s checkered shadow illusion (Gilchrist, page 4) illustrated, two squares with identical illuminance appear differently to the human eyes. One of these squares is light gray and the other one is dark gray. Scientifically, their illuminance are the same because one is in the light and the other one is in the shadow. However, they human eyes can still tell them apart. This is astonishing to me. What else is being interpreted by the human vision that is different from the object’s physical appearance? How can I know whether my perception corresponds to reality? I hope this book will help answer these questions.

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