LIGHTNESS AND RETINEX THEORY PDF

Main article: Color vision Color vision is how we perceive the objective color, which people, animals and machines are able to distinguish objects based on the different wavelengths of light reflected, transmitted, or emitted by the object. In humans, light is detected by the eye using two types of photoreceptors, cones and rods , which send signals to the visual cortex , which in turn processes those colors into a subjective perception. Color constancy is a process that allows the brain to recognize a familiar object as being a consistent color regardless of the amount or wavelengths of light reflecting from it at a given moment. This is due to an ignorance of all possible sources of illumination. Although an object may reflect multiple sources of light into the eye, color constancy causes objective identities to remain constant.

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MCCANN Polaroid Corporation, Cambridge, Massachusetts Received 8 September Sensations of color show a strong correlation with reflectance, even though the amount of visible light reaching the eye depends on the product of reflectance and illumination. The visual system must achieve this remarkable result by a scheme that does not measure flux. Such a scheme is described as the basis of retinex theory. This theory assumes that there are three independent cone systems, each starting with a set of re- ceptors peaking, respectively, in the long-, middle-, and short-wavelength regions of the visible spectrum.

Each system forms a separate image of the world in terms of lightness that shows a strong correlation with reflectance within its particular band of wavelengths.

These images are not mixed, but rather are com- pared to generate color sensations. The problem then becomes how the lightness of areas in these separate images can be independen t of flux.

Most of us assume that, subject to a variety of com- look light, objects with reflectance higher in the long- pensatory factors, we see in terms of the amount of the wave portion of the spectrum than in the short-wave light coming from objects to our eye; we think that in look reddish, objects with reflectance higher in the a particular scene there is more light coming from short-wave portion than in the long-wave look bluish, white objects than from black objects; we think there and so on.

It is with reflectance that sensation of color is more long-wave light so-called red light coming is strongly correlated when we view the world around from red objects than from blue objects. For example, the eye cannot insert a comparison the laboratory, we find that there is no predictable standard next to the object which it is regarding. Accord- is clearly the product of the reflectance and the illumi- ingly, we believe that the eye must have evolved a nation.

The illumination from the sun is modulated by system which, though using light as the communication clouds, atmosphere, water, mountains, trees, houses, medium with the world, has become as nearly inde- etc. As every photographer knows, the sun and sky pendent of energy as is biophysically possible.

In short, produce every conceivable combination of sunlight and color sensations must be dependent on some as yet skylight. Even less uniform illumination is provided by undefined characteristic of the field of view, a char- artificial light because the distance from the light acteristic that can be communicated to us by the light source drastically affects the illumination falling on with which we see, even though the amount and com- any point.

This of the reflectance and illuminance, our eye could not paper describes our search for that characteristic. We turned the photometer to another area, such as a dark brown. We then separately adjusted the Lu transformers to settings such that the three luminances z0 at the eye were 6, 35, Thus the luminances from Ca the new area were identical to the three luminances Z previously reaching the eye from the first rectangle. H- The color sensation from the new area remained es- sentially unchanged dark brown despite the fact that the wavelength-luminance composition for that area had changed from whatever it might have been to 6, 35, The illumination of each area was FIG.

Spectral transmittances of bandpass interference filters. After each of view, neither reflectance nor illuminance is known; of the new illuminations was adjusted so that the and neither is uniform. When the variable transformers were To demonstrate the extent to which color sensation changed in this way to produce the standard set of is independent of flux-wavelength distribution, we will three luminances for any square, then all the other describe a simple quantitative laboratory experiment.

Dramatically, the retention of Mondrian. The Mondrian- only information reaching the eye. The flux from each of the three pro- one point.

The luminance-vs-wavelength distribution jectors is changed by a separate variable transformer. The mystery then is how we can all agree as wide a band of wavelengths, and as much light as with such precision about blacks, whites, grays, reds, possible. Then, using one projector altering the output of the long-wave projector relative at a time, and hence only one waveband at a time, we to other projectors, is countered by a compensatory measure with a telescopic photometer the luminance at adaptation in the eye.

If, in the previous experiments, the eye from any particular area, say a white rectangle. The subsequent pro- color appearance of objects. To produce an extremely cedures constitute a null experiment. The radiance-vs- large difference between the state of adaptation to luminance function, the particular units of measure, long waves and the state of adaptation to middle the wavelength sensitivity, and the linearity of the and short waves, we asked observers to wear deep-red, meter are not significant in the experiment.

When the observers removed the goggles, they H reported at the first instant, as well as later, that the D colors of the paper squares in the Mondrian were es- 40 sentially unchanged.

The experiment was repeated with the deep-red, dark-adaptation filter over only one eye. Luminance vs position for two-squares- of the illuminants is unimportant. Similarly, reasonable and-a-happening experiment. The experimental procedure was exactly the same as Each retinal system starts with a set of receptors peak- in the first 6, 35, 60 Mondrian experiments, with the ing, respectively, within the long-, middle-, or short- exception that the observers looked at the Mondrian wave portion of the visible spectrum.

Each system through a photographic shutter. The projectors were forms a separate image of the world; the images are set so that the long-, middle-, and short-wave lumi- not mixed but are compared.

Each system must dis- nances from a white area were 6, 35, 60 and the ob- cover independently, in spite of the variation and un- servers reported that the area appeared white.

The knowability of the illumination, the reflectances for projectors were then set so that other areas had lumi- the band of wavelengths to which that system responds. A retinex employs as much of the structure yellow, blue, gray, green, and red. These mechanisms are not controlled by processes that are time dependent, light, and the blacks being called dark. Nevertheless, following him, illumination. In our theory, it is an image in terms in the Mondrian, where it is surrounded by new sets of lightness, which is produced by each retinex for the of colored rectangles, the color sensation does not portion of the spectrum to which its pigment responds.

The color sensation depends only The color sensation for any area is determined by on the long-, middle-, and short-wave reflectances of the three lightnesses that are arrived at independently the rectangle and not on the properties of the neighbor- by the three retinexes. Because the lightnesses of an ing rectangles. This independence of the neighboring area are here defined as the biologic correlates of three rectangles holds for all flux settings of the illuminating reflectances, it follows within this conceptual frame- projectors.

Our original given wavelength-luminance combination, within limits problem is converted into a new one: How does each as wide as the reflectance variations of these papers, retinex generate for each area the appropriate light- E. We can see this by placing a pencil on the boundary between the areas in Fig. The experiment was important in the development of our ideas of how the visual system could generate lightnesses.

The fact that obscuring the edge informa- tion could change the appearance of these areas meant that the edges are a very important source of informa- tion. It suggested that the change of luminance at the junction between areas both constituted an edge and also led to the visual difference between the whole two areas. The word edge suggests a sharp, in-focus boundary.

Experiments, however, show that the sharp- ness or focus of the boundary is not at all critical. For example, Fig.

Picture of two-squares-and-a-happening experiment. Place lenses to change the boundaries from being sharp and a pencil over the boundary between the two gray areas. Areas with boundaries quite out of focus look essentially the ness, the biologic correlate of reflectance that is inde- same as when they are in sharp focus.

What mechanism can we imagine that would discover The scheme that we are about to describe for answer- edges and characterize adjacent areas in a way consis- ing this question is one of a number of approaches that tent with our experiences with the happening, a mecha- we have been investigating. All these schemes are nism that will also discover the reflectances in the designed to solve the same problem, namely: For one Mondrian even when it is in nonuniform illumination? If the illumination is nonuniform, then the reflectances, how can the biologic system generate a luminances at these two positions will, of course, be set of values that we experience as lightness?

The different. When the two detectors are placed closer and particular scheme we will describe is the first that we closer together, the luminances approach the same have found to satisfy our criteria.

This will be true of almost any two adjacent points. EDGES However, if the two detectors bridge the boundary between two areas of differing reflectance, then the The experiment that we call two squares and a ratio of the outputs of these detectors will approach happening provides striking evidence of edges as the the ratio of the reflectances. Thus, the simple pro- source of lightness information.

Processing the entire image A fluorescent tube is mounted in front and to the left in terms of the ratios of luminances at closely adjacent of the papers. The light, tectors is decreased, each number approaches a limit being a line source, produces an approximately linear equal to the ratio of the reflectances, the reflectances gradient across the papers, and the reflected luminances themselves having not yet been ascertained.

Figure 3 Given a procedure for determining the ratio of re- is a photograph of the experiment. We solve between the left and right areas, the two areas are then the problem in the following way: Find the ratio of perceived as having the same lightness.

Long narrow luminances at the edge between a first and a second strips of colored papers in parallel, or three-dimensional area, and multiply this by the ratio of luminances at objects such as a pencil or a piece of yarn, make the the edge between the second and a third area.

Similarly, we can obtain the ratio of reflectances of any two areas in an image, however remote they are from each other, by multiplying the ratios at all the boundaries between the starting area and the remote area. We can also establish the ratio of the reflectance of any area on the path to the reflectance of the first area by tapping off the sequential product at that area. Consider a Mondrian similar to the colored one in complexity and randomness, but consisting of black, gray, and white papers see bound transparency, Fig.

That is, in this Mondrian each piece of paper has the same reflectance for all wavelengths. The reflectances of each area along one path between the top and the bottom are shown in Fig.

If we apply the sequential- multiplication technique to these reflectances, we can determine the ratio of the top reflectance to the bottom reflectance, as shown in Fig. How can the eye ascertain the reflectance FIG. Reflectance along one path between the top and bottom of an area without, in effect, placing a comparison of a black-and-white Mondrian.

The numbers at the bottom standard next to the area? The sequential product can indicate the ratios of reflectances at adjacent edges along the be used as a substitute for the placement of two areas path. This number is equal to the ratio of reflectances bottom of the display than on the top. We adjusted of the top and bottom areas. Thus, without determin- the position of the light so that exactly the same ing the reflectances and without determining the illumi- luminance was coming to the eye from a high-reflectance nation, however it varies, we have determined a number area at the top of the display and a low-reflectance area exactly equal to the ratio of reflectances of these two near the bottom.

If the luminance determined the areas. Yet the two areas have the same luminance as lightness of an area, the low-reflectance area and the each other and are remote from each other by the high-reflectance area should look essentially alike; in whole width of the display.

Furthermore, this pro- fact, they do not. Although the luminances of the two cedure of sequential multiplication of edge ratios can areas are equal, the high-reflectance area at the top generate values equivalent to relative reflectance for looks dramatically lighter than the low-reflectance area all areas along the path. Figure 7 shows the luminances along a path from the Let a number of different paths start from a given top of the Mondrian to the bottom.

Note that the area and wander back and forth over the display, all luminance at the center of the top area is the same as to arrive finally at a distant area, which we wish to the luminance at the center of the bottom area. Con- evaluate with respect to the starting area. If we com- sidering the top area alone, note that the luminance pute the sequential products along each of these paths, in arbitrary units increases from at its center to we obtain the ratio of reflectances of the remote area at its lower edge.

The ratio between the bottom to the starting area for each path. In this case the edge of the first area and the top edge of the second starting and remote areas for all the paths are the area is to The luminance of the second area same, therefore the terminal sequential products are increases from 80 to from upper edge to lower edge. The ratio of the second area to the third is to

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The Retinex Theory of Color Vision SCIENTIFIC AMERICAN

Abstract Sensations of color show a strong correlation with reflectance, even though the amount of visible light reaching the eye depends on the product of reflectance and illumination. The visual system must achieve this remarkable result by a scheme that does not measure flux. Such a scheme is described as the basis of retinex theory. This theory assumes that there are three independent cone systems, each starting with a set of receptors peaking, respectively, in the long-, middle-, and short-wavelength regions of the visible spectrum. Each system forms a separate image of the world in terms of lightness that shows a strong correlation with reflectance within its particular band of wavelengths.

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