As with all constancies, there are conditions that create illusions that overwhelm the processes that create constancy. The Gelb effect is an exception to the principle of lightness constancy, but in an odd way, it reveals some of the ways that lightness constancy happens. The Gelb effect is a phenomenon whereby a black object is lit separately from the area that surrounds the object. This black object appears to be gray or white in a homogeneously dark space. Think of a cat caught in the headlights of a car at night. In your mind, hit the brakes quickly, so as not to hurt the cat. The headlights illuminate only a small space in front of the car: dark pavement in addition to the cat. Because there is nothing to compare the object to, the cat appears white because it is reflecting a lot of light in an otherwise dark space. However, if we suddenly place a white object next to the cat, the cat now appears black, its actual color, as our visual systems now have something with which to compare it. The Gelb effect neatly shows the importance of the ratio principle in explaining lightness constancy (Gilchrist et al., 1999).
The ratio principle states that the perceived lightness of an object is explained by the ratio of light it reflects rather than the absolute amount of light it reflects, assuming even illumination across the visual scene. As long as illumination is constant across the field of view, the ratio captures the properties of reflectance. In an illusion such as the Gelb effect, the constant illumination requirement is violated—the cat is more illuminated than its surroundings, and lightness constancy is compromised. Of course, situations such as the Gelb effect are rare in the real world, and for the most part, the ratio principle allows us to correctly interpret the lightness of objects.
Since we cannot separately illuminate one part of the screen to another, to demonstrate the Gelb effect, we do the opposite: The intensity of the background will be changed. In this activity, you can examine different objects that are all the same or one unchanging object against a changing background. The ratio changes. In the ratio principle, our visual system determines the ratio of light reflected by comparing adjacent areas: Think the center-surround receptive fields. As the background changes intensity, the question is this: What happens to the perceived lightness of the object?
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
Below is a list of the ways that you can alter the illustration. The settings include the following:
Stimulus Type: change stimulus to a series of identical squares you can
add and remove or a continuous bar that stretches across the whole stimulus.
Number of Squares (Square Stimulus Only): change the number of squares on the screen starting from the darkest part of the background on the left.
Clear Background: remove the background to the stimulus. This change helps you see that the squares are all the same and the continuous stimulus does not change.
Brightness: adjust the intensity of the squares or continuous stimulus.
Gradient Change: adjust the steepness of the change in intensity of the background.
Pressing this button restores the settings to their default values.