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In this journal entry I will describe how the visual system achieves luminance constancy. I will explain how the perceptual system distinguishes surface lightness from the illumination of a surface. By doing so I will present how the visual system transforms registration to perception, solving underdetermination. Additionally, I will explain why the distinguishing operation of luminance constancy contributes to a perceptual constancy.
Luminance constancy is the perceiving of an object as having constant surface lightness under varying illumination. In other words, one sees an object as the same degree of lightness regardless of lighting. For example, one could observe a blue t-shirt under dim light or bright sunlight. Despite the differing brightness, one would perceive the shirt as roughly the same shade of blue. This perceptual constancy begins with the production of a luminance contour.
A luminance contour is a pattern of discontinuities of lightness intensity. When one sees an object, light intensities from the surface of that object strike the retina. A luminance contour is produced immediately when various intensities of light stimulate the retina. These different light intensities are composed of reflectance and illumination . Reflectance is a constant property of the surface of an object. It is roughly the lightness of its surface. Illumination of an object varies depending on the amount of light or shadow on the surface. Also illumination can distort surface lightness. The visual system’s ability to distinguish reflectance from illumination in a luminance contour is critical. Without this distinction luminance constancy would fail. What one thinks is the lightness of a surface could actually be a distortion by illumination. The visual system’s receptors are not able to make this distinction. They are limited to merely registering a pattern of light intensities as a luminance contour. However, the perceptual system can make the distinction in a luminance contour, making luminance constancy possible.
Luminance constancy is the perceiving of an object as having constant surface lightness under varying illumination. In other words, one sees an object as the same degree of lightness regardless of lighting. For example, one could observe a blue t-shirt under dim light or bright sunlight. Despite the differing brightness, one would perceive the shirt as roughly the same shade of blue. This perceptual constancy begins with the production of a luminance contour.
A luminance contour is a pattern of discontinuities of lightness intensity. When one sees an object, light intensities from the surface of that object strike the retina. A luminance contour is produced immediately when various intensities of light stimulate the retina. These different light intensities are composed of reflectance and illumination . Reflectance is a constant property of the surface of an object. It is roughly the lightness of its surface. Illumination of an object varies depending on the amount of light or shadow on the surface. Also illumination can distort surface lightness. The visual system’s ability to distinguish reflectance from illumination in a luminance contour is critical. Without this distinction luminance constancy would fail. What one thinks is the lightness of a surface could actually be a distortion by illumination. The visual system’s receptors are not able to make this distinction. They are limited to merely registering a pattern of light intensities as a luminance contour. However, the perceptual system can make the distinction in a luminance contour, making luminance constancy possible.
