Lightness and brightness

What is lightness? Lightness is the perceptual dimension that runs from black, through gray, to white. The physical counterpart of lightness is the permanent property of a surface that determines what percentage of light the surface reflects. Surfaces that appear white reflect about 90% of the light striking them. Black surfaces reflect about 3%. In short, lightness is perceived reflectance.What is the basic problem? The problem is that the light reflected to the eye from a gray surface, by itself, carries no hint of the reflectance of the surface from which it comes. A black surface in sunlight can easily reflect more light to the eye than a white surface in shadow, but they are not confused by the eye. The black surface still looks black; the white looks white. Indeed reflected light of any intensity, that is, any luminance, whatsoever, can be perceived as any shade of gray between black and white, depending on the context. To get the lightness of a surface right, the visual system must use the visual context. But there is no consensus on how this is done.What is the current theoretical landscape? Theories of lightness fall into three levels: low level, high level, and mid level. Low-level theories, which stem largely from Ewald Hering (ironically called brightness theories) emphasize known, relatively simple, peripheral neural mechanisms, such as lateral inhibition and spatial filtering. These theories, of which there are many, are very concrete and consistent with the received wisdom of sensory physiology. But they fail badly to predict what humans actually perceive when viewing typical scenes from our everyday world: scenes that contain multiple depth planes or projected illumination boundaries (such as the edge of a shadow).High-level theories view the perceptual process as a thought-like process. The classic high-level theorist is Helmholtz, who argued that, through an unconscious process of inference, we take into account the level of illumination in different parts of the scene. This approach, while intuitively appealing, has resisted further operationalizion. The computer age spawned more modern high-level theories which argue that reflectance can be recovered by decomposing the retinal image into those physical properties, like reflectance and illumination, that became entangled in the formation of the image. But the highly complete internal representation of the world implied by these decomposition models has not found support in the empirical data. High-level theories resonate with our visual experience of surfaces, especially in complex scenes, but are criticized as abstract and physiologically implausible.Low-level theorists argue that one must start with a very concrete model, even if it works only for very simple images, and then try to extend the reach of the model. High-level theorists argue that one must first make a theory that captures the process intuitively and then work to fill in the operational details.The serious shortcomings of low- and high-level theories have led to the emergence of mid-level theories. These theories are more sophisticated than the low-level theories, yet more concrete than the high-level theories. Generally, these models use simple properties of the image, like fuzzy boundaries or depth edges, to parse the image into separate frameworks of illumination and then apply a simple computation, using a different standard of white within each framework.What is the neural basis of lightness? As the preceding discussion suggests, we are far from an answer to this question. What is known of the sensory mechanisms for processing luminance fails to map onto lightness values as reported by human observers. The kind of mechanisms that would seem to be required have not been discovered yet.What is brightness? Brightness is the perceptual dimension that runs from dim to bright. Like lightness, brightness is a perceptual term. The physical counterpart of brightness is called luminance — that is, the absolute intensity of light reflected in the direction of the observer's eye by a surface (or at least coming from a certain part of the visual field). In short, if lightness is perceived reflectance, brightness is perceived luminance. The reflectance of an object is a relatively permanent property, whereas its luminance is transient.Lightness concerns the objective side of visual experience while brightness concerns the subjective side. What does this mean? Normally when we look at a scene, we are interested in the objects it contains, not the specific impact made on our receptor surface. We perceive a dinner plate on the table as round, but ignore the fact that it projects an elliptical image toward the eye. When we track a golf ball in flight we attend to the path of the ball, ignoring that the image of the ball remains stationary in our field of view. But by adopting a special visual attitude we can become aware of the nature of the stimulation reaching the eye, rather than the objective properties of the object itself. This special attitude, typically adopted by an artist painting a scene, is known as the proximal mode of perception. Brightness perception then, falls within this mode.Brightness is our experience of the raw energy emanating from a surface, and the impact of that raw energy on our sensory system. That energy includes the combined effects of the surface reflectance and the intensity with which it is lit. We put on sunglasses, for example, when the light reaching our eyes becomes too bright, either when facing a moderately illuminated white surface, or an intensely illuminated gray surface. What matters is only the intensity of light reaching the eye. Note that a white surface appears brighter than a gray surface in the same illumination, but a sunlit white surface also appears brighter than a shadowed white surface.A useful analogy can be found in the domain of size. We make an important distinction between the perceived size of an object and the size of its retinal image (its visual angle). Perceived size, like lightness, concerns an objective property of an object — in this case its actual physical size. Perceived visual angle, the counterpart of brightness, concerns the proportion of an observer's visual field occupied by the image of the object. A small object close to the eye and a large object far from the eye can have the same perceived visual angle, just as a black surface in sunlight and a white surface in shadow can have the same luminance.Why has the distinction between lightness and brightness proven so confusing? The distinction is confusing because it is easy to confuse lightness with light, the message with the medium. The lightness we experience depends on the pattern of light reaching the eye, carried on the vehicle of light. Light is merely the medium while modulations in the light form the information. All the properties of the world that we perceive through vision — size, motion and reflectance — come to the eye via light. But while no one would confuse size with light or motion with light, lightness is easy to confuse with light.The situation has become aggravated by frequent misuse of terms. Prior to the computer revolution the distinction between energy and information was less clear, and the term brightness was used to refer to both lightness and brightness, without distinction. Sometimes the term brightness is used for the perceived brightness of the illumination level. But perceived illumination and perceived luminance are separate, and by consensus, the term brightness is now used to refer only to the latter.Why are there so many theories of brightness? There is little adaptive value in determining the intensity of light reflected by a surface. It is far more useful to know if the surface is black, white or gray. Thus, just as the perception of object properties requires a theory of perceived object size, not a theory of perceived visual angle, doesn't this also require a lightness theory rather than a brightness theory? The answer is obviously yes. But why then are there more theories of brightness than of lightness? Because brightness theorists make the tacit assumption that lightness is based on brightness. This assumption has its roots in the old sensation/perception distinction, with brightness values serving as sensations that correspond to local stimulation. But this remains an assumption. And there is little reason to reject the claim of gestalt theory that lightness is perceived directly, without a prior stage of raw sensations.