Spectral Lights Research Articles

Hypotheses for chromatic opponency functions and their performance on classical psychophysical data

General hypothesis for the definition of chromatic-opponency functions are given in a black-box approach to the problem. It is supposed that the color signals in the visual color processing can be factorized into the product of the lightness times a pair of chromatic opponency functions and the whole chromatic processing consists of three independent processes: a linear transformation, a logarithmic compression, and a chromatic opponency actuation. The main chromatic opponency functions, obtainable by very general hypothesis on the symmetry and on the homogeneity degree, are supposed equal to the logarithms of tristimulus-value ratios in a proper reference frame of the tristimulus space. The perceptual chromatic functions, individually with uniform scales, are a linear mixing of the main chromatic opponency functions. The Bezold–Brücke hue shift is not considered. A performance of these hypothesis is successfully realized on the OSA-UCS system, for extra macula vision, and on the chromatic discrimination ellipses, for macular vision. Unique hues are derived from the main chromatic opponency functions of spectral lights. © 2004 Wiley Periodicals, Inc. Col Res Appl, 30, 31–41, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20072

Suggested optimum primaries and gamut in color imaging

Suggested optimal “primaries” and concomitant optimal gamut for any form of device or product whose output serves as input to the normal human visual system: TV-like images, or images reflected from illuminated hardcopy. For those devices and products using additive coloration, three spectral primaries 450–530–610 nm, and their associated gamut, are suggested as goals. For those using subtractive coloration, three reflective components, of width perhaps 50–60 nm at half-height and peaking at the same wavelengths, are suggested. Thus, in either case, the light from each pixel of the generated color image entering the pupil of the normal human observer is composed of a mixture of that observer's prime colors. “Prime colors” are defined as usual as (a) the wavelengths marking the peaks of the three spectral sensitivities of the (trichromatic) normal human visual system, and, thus, (b) those spectral lights to which the normal human visual system responds most strongly per watt of power content input to the pupil. Spectral sensitivities of the normal human visual system are carefully distinguished from those of the CIE Standard Observers. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 148–150, 2000

Rod-cone color mixture: effect of size and exposure time.

Chromaticity coordinates of monochromatic lights were obtained 8 deg extrafoveally at a retinal illumination of 50 photopic td during the cone-plateau period and also with the subject in a dark-adapted state, while size and exposure time of the test field were varied. Unexpectedly, we found that for the dark-adapted condition the points representing the chromaticity matches of the spectral lights generally moved away from the achromatic point in the chromaticity diagram when size or exposure time of the test stimulus was increased. Furthermore, the chromaticity shift toward the achromatic point, obtained between the cone-plateau period and the dark-adapted state (i.e., with rod intrusion), tended to decrease with these two test parameters. In fact, when size and exposure time both were at the maximum level investigated (7 deg, 500 ms), there was no measurable shift in chromaticity with rod intrusion. Our results suggest that the cone system may become progressively more effective in suppressing the rod system as an effect of both size and exposure time.

Trichromatic colour vision in the hummingbird hawkmoth, Macroglossum stellatarum L.

Hymenopterans have long been shown to choose colours by means of the spectral distribution and independently of the intensity (true colour vision). The same ability has only very recently been proven for two butterfly species. We present evidence for the existence of true colour vision in the European hummingbird hawkmoth, Macroglossum stellatarum. Moths were trained in dual-choice situations to spectral lights of a rewarding and an unrewarding wavelength. After training, unrewarded tests were performed during which the intensities of the lights were changed. The results confirm that the species has three spectral receptor types and uses true colour vision when learning the colour of a food source. If colour vision is not possible since only one receptor type is receiving input from both stimuli, the moths learn to associate some achromatic cue correlated to the receptor quantum catch, with the reward. The moths learn spectral cues rapidly and choose correctly after one to several rewarded visits even when trained to different colours in sequence.

Chromatic rod-cone interaction during dark adaptation.

Chromatic rod-cone interaction in mesopic vision was investigated by measuring chromaticity coordinates of spectral lights 3 deg extrafoveally during long-term dark adaptation, under conditions where the eye was chromatically adapted. It was concluded that (a) the chromaticity shift obtained between the cone-plateau period and the dark-adapted state is due, at least under some conditions, to rod signals elicited by the test stimulus, (b) the chromaticity points of cone-mediated colors may change in different directions in the chromaticity diagram as an effect of the rod intrusion, (c) cone-cone and rod-cone color-mixture processing may, at least under some conditions, be different, and (d) chroma-related processes of rods and cones tend to suppress each other, with rods dominating at low and cones at high mesopic intensities.

Rod influence on hue-scaling functions

Rod influence on hue appearance of spectral lights was characterized by comparing the scaling of red, green, yellow, and blue hue sensations for an 8°-diameter, 7°-eccentric test spot under conditions that minimized (cone plateau) and maximized (dark adapted) rod influence at two mesopic light levels (1.5 and 3.0 log scoptic trolands). At the lower light level, the hue-scaling functions showed that rod signals influenced the spectral range and magnitude of all four primary hues. The rod influence could not be characterized as a ubiquitous augmentation or diminution of any hue over the entire spectrum. This constrains models of rod influence on color vision.

The detection and discrimination of categorical yellow

Threshold detection of most spectral lights presented on a white background is subserved by colour opponent mechanisms which produce distinct percepts of colours. Five ranges of wavelengths can be discriminated: red, yellow, green, blue and violet [Mullen and Kulikowski (1990) Wavelength discrimination at detection threshold. J. Opt. Soc. Am.A7, 733–742]. However, under most viewing conditions yellow can also be detected and discriminated by activating the achromatic mechanism. Using 2-alternative forced-choice we studied detection and discrimination between spots (1 deg) which were either spectral colours or white (matching the background in colour temperature), with and without masking the achromatic mechanisms. For low colour temperatures of white (2700 K), yellow could be discriminated from white at slight suprathreshold levels of detection. However, at a colour temperature of 6800K and with masking, the yellow-white discrimination threshold for 565–574nm was also close to detection threshold (as for other wavelengths). We conclude that it is possible to demonstrate the role of the blue—yellow opponent mechanism in categorical perception of yellow (at threshold) by sensitizing its yellow branch and by suppressing the achromatic mechanism.

Chromatic content of spectral lights.

Using three novel formats, we compare four estimates of the spectral sensitivity of the opponent stage channels: a Linear Model, Jameson and Hurvich's [Journal of the Optical Society of America, 45, 546-552 (1955)] hue cancellation sensitivities, Gordon and Abramov's [Optical Society of America Technical Digest Series, 15, 12-15 (1987)] hue scaling, and hue matching. The three formats are: the spectrum locus in the opponent equiluminant plane, null lines in the CIE XYZ chart and response functions for unique hues. All sensitivities show departures from the Linear Model and from each other. Relative to the model, common features of all estimates are that violets are compressed; long-wavelength reds are amplified; the redness component of violet lights is greatly attenuated; and saturation of violet stimuli is underestimated.

The prediction of hue and saturation for non-spectral lights

The Jameson and Hurvich opponent-colors model of hue and saturation was tested for spectral and non-spectral lights. Four observers described the color of lights by scaling hue and saturation. The lights ranged from 440 to 640 nm and consisted of five purities: 1.0, 0.80, 0.60, 0.40 and 0.20. Admixtures of monochromatic and a xenon-white light yielded the different colorimetric purities. For each subject, chromatic response functions were measured by the method of hue cancellation at each purity, and an achromatic response function was measured by the method of heterochromatic flicker photometry for spectral lights. Chromatic response functions measured for a particular purity and the achromatic response function were used to predict hue and saturation for that purity. The model successfully predicted hue at each level of purity, but failed to predict precisely the Abney effect. The model made relatively poor predictions of saturation, tending to overstimate short-wave lights and underestimate long-wave lights. An additional experiment found that stimulus parameters that favor rod contribution weaken the model's predictions of saturation, while stimulus parameters that do not favor rod contribution improve the model's predictions of saturation.

Colour perception of single and mixed monochromatic lights in the blowfly Lucilia cuprina

Colour perception of spectral lights and mixtures of two monochromatic lights of blue and yellow wavelengths was studied in the blowfly Lucilia cuprina by using a generalization test in which the fly had to compare these lights in memory with coloured papers (blue, green, yellow and red) represented in the test array. Flies trained to a monochromatic light in the wavelength range of 429–491 nm responded to blue; those trained to 502–511 nm to green; and those trained to 522–582 nm to yellow. The maximal generalization for blue was found at 429 nm and that for yellow at 543 nm. Flies trained to the mixtures responded neither to blue, green nor yellow, when the blue component was mixed with the yellow component in a ratio of approximately 1 ∶ 3. It seems that the fly perceives the mixtures as a neutral or an achromatic light. Colour loci of coloured papers, spectral lights and mixtures of two monochromatic lights used formed blue, yellow and neutral clusters in a colour triangle with respect to generalization responses to test colours.

Spectral-luminosity functions, scalar linearity, and chromatic adaptation

We report data for three experiments that assess the effect on the luminosity function of chromatic adaptation arising from the measurement stimuli. First, we report spectral-sensitivity functions (wavelength range, 510-640 nm) measured by heterochromatic flicker photometry for a luminance range of 25-5000 Td. The data were fitted to a linear combination of cone fundamentals. The data narrowed and the fits deteriorated with an increase in luminance level, which indicates that at high luminances chromatic adaptation that is dependent on the spectral composition of the standard and test lights is a factor in spectral-luminosity determination. Second, we report heterochromatic modulation photometry as measured with two spectral lights at constant time-averaged chromaticity and luminance for luminances from 1.6 to 1300 Td. For a time-averaged chromaticity of 570 nm, the red-green ratio of the photometric match was invariant with luminance. For a time-averaged chromaticity of 605 nm, the red-green ratio increased by almost 0.3 log unit for a 2-log-unit increase in luminance, which is indicative of chromatic adaptation to the 605-nm chromaticity. Third, we measured flicker increment detection (wavelength range, 510-640 nm) on 570- and 605-nm backgrounds of 25-5000 Td. The data were fitted to a linear combination of cone fundamentals and showed good fits at all luminances. Fits to the 570-nm-background data set showed little variation in the proportions of the cone fundamentals with luminance. Fits to the 605-nm-background data set required an increased weighting of the middle-wavelength-sensitive cone with luminance. These three experiments indicate that luminance-dependent variation in the spectral-luminosity function as assessed by flicker techniques is caused primarily by chromatic adaptation to the measurement stimuli.

Action spectra for the initiation of bioluminescent flashing activity in males of twilight-active firefly Photinus scintillans (Coleoptera: Lampyridae)

The action spectra [ S(λ)-functions] for the initiation of bioluminescent flashing were determined in males of twilight-active firefly Photinus scintillans, using horizontal and vertical beams of spectral lights to simulate closed and open space in nature, respectively. The peak of the S(λ)-function in the horizontal beams was in the long (yellow, Fig. 3) and that for the vertical in the short (near-u.v. and blue, Fig. 5) wavelength region. It is concluded that both short and long wavelength visual receptors mediate the initiation of flashing behavior in males of those fireflies which restrict their flashing to twilight-hours. Furthermore under dim illumination conditions as in a closed space, the long wavelength receptor dominate the behavioral response, while in an open space with bright illumination conditions, the short wavelength receptors dominate.

Color mixing in the pigeon ( Columba livia) II: A psychophysical determination in the middle, short and near-UV wavelength range

Pigeons were trained to discriminate between spectral lights and additive mixtures in the 350–560 nm spectral range using a successive “autoshaping” discrimination procedure [introduced in Palacios, Martinoya, Bloch & Varela, Vision Research, 30, 587–596 (1990)]. Dichromatic mixtures were found in the short and near UV region, but not in the middle-wave region. Our results suggest that color vision in the pigeon involves the active participation of five different primary mechanisms, which are differentially active in the yellow- and red-sensitive retinal fields.

Toward a more accurate and extensible colorimetry. Part Ii. Discussion

AbstractTechnical advances in the forty years since the Stiles‐Burch color‐matching work‐among them the bright split‐field visual colorimeter coupled by quartz light pipe to a spectroradiometer‐allow measurement of an accurate absolute spectral power distribution (SPD) of every viewed light. Furthermore, the ability to measure accurately the power‐content of each spectral constituent of a viewed light has clarified both colorimetric procedures and signficance, for example, of color‐matching functions (CMFs) and tristimulus values. Three disparate sets of three spectral primaries were used throughout the work. This has not been done before, and its use elucidated much of the difficulty (as described by Judd, Stiles, MacAdam, Wyszecki, and others) inherent in traditional colorimetry. Errors in computed chromaticities of sets of 28 visually‐matching lights depend strongly on their spectral content, and are particularly large in the presence of high spectral content in the violet, deep red, and near 500 nm or 580 nm. Peaks of visual efficiency, and minima in chromaticity errors, were found with power in three intermediate spectral regions near 450 nm, 530 nm, and 610 nm. Computed luminance (1964 CIE Standard Observer) and power‐content also depend strongly on spectral content of the visually‐matching white lights. That this is true of luminance is generally unknown. Perceived brightness of single spectral lights in isolation, and of mixtures, was examined at length: As a rule, removing one spectral component from a mixture increases perceived brightness, thus invalidating both luminance and R + G + B as correlates of brightness. Visual tests of the Grassmann proportionality and additivity assumptions, with highly metameric mixtures, tended to confirm them. Nevertheless, numerous tests‐substituting pairs of actual visually‐matching lights from one primary‐set for pairs from another, which is how “transformation of primaries” is (mathematically) carried out‐led to the inescapable conclusion that the normal human visual system does not honor such transformation. At the same time, numerous indications supported a particular set of primaries as an invariant of the visual system, making transformation needless as well as invalid. Chromaticity errors are not the fault of the particular maximum‐saturation CMFs utilized; the observer's own CMFs do no better, whatever the primary‐set. Soundness of traditional construction of the chromaticity diagram is shown to depend on the primary‐set, breakdown occurring for some otherwise legitimate primary‐sets.

Toward a more accurate and extensible colorimetry. Part I. Introduction. the visual colorimeter‐spectroradiometer. Experimental results

AbstractA bright ten‐degree split‐field colorimeter is used to study pairs of visually‐matching lights. This colorimeter is coupled by quartz light pipe to a spectroradiometer, and yields accurate absolute spectral power distributions of every viewed light. Six normal human observers use three disparate sets of spectral primaries. In Part I of the article thorough descriptions of the instrument, its calibration, and experimental results are presented. (1) By the Maxwell method, sets of 28 three‐band lights, each light matching a broadband reference‐white light, are obtained. From these sets are found strong and unexpected dependence of perceived brightness per watt on spectral content, and large systematic errors in computed chromaticity (1964 CIE Observer). (2) From sets of white two‐band lights, each matching the broadband reference light, large differences in wavelength of the visually complementary spectral lights, from those indicated by the 1964 CIE chromaticity diagram are found. (3) Sets of Maxwell‐method and maximum‐saturation‐method color matching functions are obtained for each primary‐set and for each observer. (4) Study, at the colorimeter, of perceived brightness of spectral lights in isolation yields three‐component perceived‐brightness‐per‐watt curves like those of Stiles and Crawford of 1933. (5) Perceived brightness of 3‐band white mixtures always increased upon removal of any one of the three spectral components, in agreement with MacAdam's findings of 1950. (6) Visual tests were made of Grassmann's additivity assumption, and of Maxwell spot phenomena. Part II includes discussion of chromaticity and brightness errors and of other aspects of colorimetry problems‐transformation of primaries, structure and shape of the chromaticity diagram, and normalization of color matching functions. Part III gives avenues of possible improvement and general comment.

Color appearance across the retina: effects of a white surround.

Hue and saturation scaling was used to measure the appearance of spectral lights as a function of stimulus size at loci spaced across the horizontal meridian. At a given locus, each hue (red, yellow, green, and blue) grew as a function of stimulus size to some asymptotic value. Growth functions fitted to the hue data were used to derive the sizes of perceptive fields of the hue mechanisms. We had previously investigated the size scales of these fields by using stimuli presented on a dark background: Field sizes for all mechanisms increased with eccentricity, this increase was greater on the temporal than on the nasal retina, and the retinal size scales of the different hue mechanisms were not the same. We now report the results of repeating the study with stimuli embedded in a white surround rather than in darkness. The main effects of the white surround were as follows: Sizes of perceptive fields of all mechanisms were smaller everywhere than with the dark surround. For small fields, stimuli appeared everywhere more saturated with the white surround, but larger stimuli appeared less saturated.

Color appearance in the peripheral retina: effects of stimulus size

Hue and saturation scaling were used to measure the appearance of spectral lights as a function of stimulus size for nine loci across the horizontal retinal meridian. At a given locus, each hue (R, Y, G, and B) grew as a function of stimulus size up to some asymptotic value. The parameter values of Michaelis-Menten growth functions fitted to the hue data were used to derive the sizes of the so-called perceptive fields of the hue mechanisms. The fields for all mechanisms increased with eccentricity, and this increase was greater on the temporal than on the nasal retina. By increasing stimulus size it was possible to achieve fovealike color vision to eccentricities of 20 deg. However, even the largest stimuli failed to produce fully saturated hues at 40 deg. The retinal size scales of the four hue mechanisms were not the same; those for R and B were similar, and these mechanisms had the smallest perceptive fields everywhere. The perceptive fields of the hue mechanisms at all loci were larger than anatomical estimates of the sizes of retinal receptive fields.

Color mixing in the pigeon. a psychophysical determination in the longwave spectral range

Pigeons were trained to discriminate between spectral lights and additive mixtures in the 580–640 nm range. Two behavioral procedures were used: (I) a simultaneous instrumental discrimination and (II) successive “autoshaping” discrimination. Pigeons were able to make color mixture matches within this spectral range with satisfactory precision. Matchings determined by the animal correspond well to those predicted on the basis of the spectral sensitivities of two (or even three) pigment-droplet combinations present in the pigeon retina.

The ellipsoidal representation of spectral sensitivity

When plotted in three-dimensional color-space, thresholds of colored lights fall on or near the surface of an ellipsoid. Using data reported in the literature, we estimate the deviation between sets of spectral threshold measurements and the ellipsoid that passes closest to the data. Seventy-three percent of the reported spectral thresholds fall within 0.1 log units of the best-fitting ellipsoid. Our ability to distinguish one ellipsoidal fit as significantly better than another is limited by the choice of sampling directions in color-space. Spectral lights do not provide a good set of sampling directions for reducing the uncertainty about the estimated best-fitting ellipsoid. Complete characterization of visual sensitivity requires measuring thresholds to mixtures of spectral lights.

Blue light hazard in rat

Rats have been extensively used in light damage studies. Retinal damage threshold for white light were found at 1–10 J/cm 2, and the action spectrum resembled the absorption spectrum of visual pigment. We wished to answer the question whether a different class of light damage, the “blue light hazard”, with white light damage thresholds at about 300 J/cm 2, and an action spectrum peaking in the ultra-violet, could also be demonstrated in rat. To that purpose 5 deg patches of retina were exposed to white xenon light with exposure times between 10 sec and 1 hr. We found that for funduscopic threshold damage the product of irradiance and exposure time was constant at a level of 315 J/cm 2. Thereafter, the action spectrum was measured by exposing rat eyes to narrow band spectral lights. Threshold irradiant dose ranged from 4 J/cm 2 at 379 nm to 2000 J/cm 2 at 559 nm. Thus, susceptibility for damage sharply increased towards the ultra-violet, just like in earlier monkey studies. We conclude that in similar experimental conditions susceptibility to photic injury in rat is comparable to that in primates. Rat is the first species for which two different action spectra of photochemical damage have been established.