Color stimulus

When considering light of a certain spectral energy distribution falling on an object with a given spectral reflectance and perceived by an eye with its own spectral response, to obtain the perceived color stimulus it is necessary to multiply these factors together as ia Eigure 6. Standards are clearly required for both the observer and the illuminant. 

The color of an object is a sensory impression received by the individual and triggered by a color stimulus. The color stimulus consists of light from the object incident on the eye, light denoting electromagnetic radiation in the range of wave-... 

Colorimetry relates the perceived color quality to the color stimulus, which in turn is based on the reflectance spectrum q (A). 

Colorimetry [1.17] - [1.19]. The principles of colorimetry are based on the fact that all color stimuli can be simulated by additively mixing only three selected color stimuli (trichromatic principle). A color stimulus can, however, also be produced by mixing the spectral colors. Thus, it has a spectral distribution, which in the case of nonluminous, perceived colors is called the spectral reflectance q (2). After defining three reference stimuli, the trichromatic principle allows a three-dimensional color space to be built up in which the color coordinates (tristimulus values) can be interpreted as components of a vector (CIE system for standards, see Table 1, Colorimetry CIE = Commission Internationale de l Eclairage). For uncolored illumination the three CIE tristimulus values depend on the spectral reflectance as follows ... 

Figure 4.5 Set of weights for three monochromatic stimuli at wavelengths 700 nm, 546.1 nm, and 435.8 nm to match a color stimulus of a given wavelength (data from International Commission on Illumination 1996).

The principles of colorimetry are based on the fact that aU color stimuli can be simulated by additively mixing only three selected color stimuU (trichromatic principle). A color stimulus can, however, also be produced by mixing the spectral colors. Thus, it has a spectral distribution, which in the case of nonluminous, perceived colors is... 

A definition of a color stimulus includes the chromaticity and a relative intensity measure, the luminance factor. This is determined by the value of Y. This primary has a spectral form which is the same as the eye s overall sensitivity in daylight. Thus, a color can be specified by the three dimensions x, y, Y). 

Fig. 6. The stimulus perceived as color is made up of the spectral power (or, as here, energy) curve of a source times the spectral reflectance (or transmittance) curve of an object times the appropriate spectral response curves (one shown here) of the eye (3).

Objective Evaluation of Color. In recent years a method has been devised and internationally adopted (International Commission on Illumination, I.C.I.) that makes possible objective specification of color in terms of equivalent stimuli. It provides a common language for description of the color of an object illuminated by a standard illuminant and viewed by a standard observer (H). Reflectance spectro-photometric curves, such as those described above, provide the necessary data. The results are expressed in one of two systems the tristimulus system in which the equivalent stimulus is a mixture of three standard primaries, or the heterogeneous-homogeneous system in which the equivalent stimulus is a mixture of light from a standard heterogeneous illuminant and a pure spectrum color (dominant wave-length-purity system). These systems provide a means of expressing the objective time-constant spectrophotometric results in numerical form, more suitable for tabulation and correlation studies. In the application to food work, the necessary experimental data have been obtained with spectrophotometers or certain photoelectric colorimeters. 

The effect of LSD on perception is sometimes referred to as illusiogen-ic, because, rather than creating a perception of a nonexisting stimulus, LSD produces a distortion of sensory input from the environment (Kulberg 1986). Visual symptoms are most frequently experienced for example, vision may be blurred, and the perception of distance and depth may be changed. Objects in the surroundings may be perceived as unusually intense in color, shape, and/or size. With the eyes closed, geometric and kaleidoscopic patterns are perceived. Synesthesia, by which a sensory stimulus of one modality is transformed into a perception from another sense, is a type of a perceptual distortion typically experienced under the influence of LSD for example, smells... 

FIGURE 5.2 (See color insert following page 336.) Illustration of the method of HFP. On viewing the stimulus directly (upper), MP attenuates the blue component of the stimulus whereas with peripheral viewing (lower), no such attenuation occurs. In each case, the subject adjusts the luminance of the blue component until it matches the luminance of the green component, which is unaffected by MP. 

Fig. 1 Active choice of intravenous nicotine in experimentally naive squirrel monkeys, a Monkeys sat in chambers equipped with two levers and distinctly colored light stimuli above the levers. Completion of the response requirement (the ratio) on the active lever produced a brief 2-s presentation of a light stimulus and an intravenous injection of nicotine followed by a timeout (TO) period of 5-60 s. Completion of the ratio requirement on the inactive lever resulted in presentation of a brief 2-s light stimulus of a different color but no injection. The fixed-ratio (FR) response requirement was gradually increased over successive sessions from one to ten (FR 1-FR 10). b Mean percentage choice for responding on the active lever by monkeys when they were experimentally naive (first week under a FR 1 schedule) and when they had learned to self-administer nicotine under the FR 10, TO 60s schedule (first week under the FR 10 schedule). P < 0.01, compared to the first week of training. From Le FoU et al. (2007b)...

The quality of experience changed, once we began moving, from being primarily explorations of inner events, with almost no environmental stimulus, to the experiencing of the external world colored by "inner" emotions and experiences. Internal and external realities coexisted, alternating somehow in a pleasing and harmonious way. As for my "objectivity"... 

To return to the data from the meditation experiment, it may be that the simpler perceptions of color, light, energy force, and movement represent a shift of the normal perceptual processes to aspects of the stimulus array previously screened out—or it may be that these percepts are registered through the operation of new perceptual processes. In the... 

Land and McCann (1971) then developed a computational theory for color constancy, the retinex theory. In their experiments, they used a stimulus similar to the famous paintings... 

Figure 1.1 A Mondrian image similar to the one used by Land to develop his retinex theory. Land used colored sheets of paper and arranged them randomly. The resulting image reminded him of the abstract paintings that were drawn by the Dutch artist Piet Mondrian. This is why this stimulus is called a Mondrian image.

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