PROCESSES IN BIOLOGICAL VISION: including, ELECTROCHEMISTRY OF THE NEURON This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. James T. Fulton Vision Concepts [email protected] April 30, 2017 Copyright 2001 James T. Fulton Performance Descriptors 17- 1 17 Performance descriptors of Vision1 Because of the amount of color artwork in this chapter, it has been necessary to divide it into three parts for distribution over the INTERNET. PART 1A: INTRO, LUMINANCE & NEW CHROMATICITY DIAGRAM PART 1B: EXTENSIONS TO THE NEW CHROMATICITY DIAGRAM PART 2: TEMPORAL AND SPATIAL DESCRIPTORS OF VISION PART 1B: NEW CHROMATICITY DIAGRAM DEFINITIONS AND COMPARISONS TO OTHER COLOR SPACES The press of work on other parts of the manuscript may delay the final cleanup of this PART but it is too valuable to delay its release for comment. Any comments are welcome at [email protected]. 17.3.4 New definitions based on the New Chromaticity Diagram 17.3.4.1 The general concepts of narrow and broadband colors [new intro From a spectral perspective, the color of an object involves the evaluation of the centroid of two samples of its radiance function. How this is accomplished by the visual system will be developed below. Much confusion has surrounded the fact that the radiance of an object frequently involves the product of the reflectance function of an object multiplied by the irradiance function of the light applied to the object. The definition of the color of an object depends on the situation. Using a strictly psychophysical definition related to the perceived color reported by an animal is of little use to the image reproduction specialist. The specialists in photography, television and other image reproduction fields require definitions independent of the complex system represented by the animal visual system. However, these specialists would always like to know the interrelationship between the color observed by their system and the animal system. To provide a precise yet comprehensive definition of the phenomena of color requires careful specification of the conditions involved. The following two sections should be considered as a set. While the discussion in the first section appears comprehensive, it actually contains a subtle and very significant oversight. This oversight, which is found within the description of definition #5 is most easily addressed as a separate amendment to the first discussion. It is addressed in the second section. The subject of asymmetric illumination of the two eyes of an observer also introduces another condition requiring a separate definition of color. The color reported by a subject under this condition clearly involves cognition in the cortex. Cognitive color will be used to describe this situation. 17.3.4.1.1 First Order definitions Amazing as it may seem, the visual science community has suffered from the lack of a widely accepted and precise definition of the phenomena of color. This does not mean that there are not definitions of the phenomena of color. It focuses on the fact that there are a proliferation of definitions that are inconsistent, both conceptually and mathematically imprecise, and not widely accepted. The problem is partly due to the inadequacy of language (at 1Released April 30, 2017 2 Processes in Biological Vision least the English language) to provide the needed differentiation between various aspects of color as found in the signaling chain of both animal vision and man-made image recording. This problem is highlighted at the current time by the explosion in interest concerning the poorly defined phenomena of color constancy as noted in human vision. To simplify the following discussion, only objects that are not self-radiating will be discussed . Although Roget’s Thesaurus provides about twenty multiple word synonyms for various aspects of color, it gives virtually no single word synonym for the word color. The Encarta Encyclopedia hedges its bet with the description of light as the “physical phenomenon of light or visual perception associated with the various wavelengths in the visible portion of the electromagnetic spectrum.” [italics added] In the context of this work, color can be defined with conceptual and mathematical precision by using modifying adjectives (sometimes multiple adjectives). This is most easily done by defining the phenomena of color specifically at each applicable stage in the visual process. Wyszecki & Stiles used this approach to define about ten different types of color. Unfortunately, their definitions are all conceptual and based on semantics rather than physics/mathematics. This work will define color in terms of several primary situations; 1. the intrinsic color of an object independent of how it is observed, 2. the sampled color of an object as observed by an instrument that samples the light emanating from an object, 3. the sampled trichromatic color of an object as observed by an instrument that samples the light emanating from an object using spectrally selective radiometers analogous to those of the long wavelength trichromatic animal eye, 4. the applied color of an object in terms of its spectral content at the Petzval surface of an optical system, 5. the adapted color of an object as found at the pedicels of the photoreceptor cells of the animal eye, 6. the encoded color of an object as represented by the signals within the chrominance channels of the visual system and 7. the perceived color of an object reported by an animal. Note that none of these definitions involve the employment of comparative or null techniques in object space. Discussions of the color of an object frequently become entangled with the spectral aspects of an illuminating source and the intrinsic reflectance of the object. This can be avoided if the illuminating source is defined so as to make its impact on the color of the object negligible. If the illuminating source is chosen such that it applies spectrally, spatially and temporally uniform irradiation to the object from a spherical source centered on the object, any variation in the characteristics of the radiation emanating from the object in a given direction, and possibly within a given cone angle centered on that direction, will be due to the intrinsic properties of the object. A caveat is that the spectral uniformity is measured with respect to radiant flux intensity of the source and not radiant energy intensity. This condition can be applied over any wavelength (frequency) interval without changing the experiment. However, in the case of vision, the wavelength interval need only extend over the wavelength interval to which the sensory system of the animal is responsive. For tetrachromats (including many small terrestrial mammals), this interval is not less than 300 to 700 nm. A somewhat narrower range can be used for trichromats; 300 to 600 nm for short wavelength trichromats and 400 to 700 nm for long wavelength trichromats (such as human and other large terrestrial mammals). This condition also effectively eliminates any observable form of specular reflection from the object. If the specular characteristics of the radiation from the object are required, a different illumination source will be required. As noted repeatedly in this work, the use of energy per unit wavelength as a parameter in visual research leads to endless difficulties. The visual system is a photoelectric class of device that is sensitive to the number of photons absorbed per interval. It is not properly represented as being sensitive to the amount of energy absorbed per interval, as would be the case in a thermoelectric class of device. Use of a spectral power density (SPD) to describe a radiation source is not appropriate for vision research. The use of a spectral flux density (SFD) distribution would be more useful. 1. Under the above conditions, the intrinsic color of a uniformly colored object is given by the spectral distribution of the light emanating from the object in any direction within the specified spectral limits. This intrinsic color is invariant with the intensity of the specified illumination source. 2. Since it is difficult to convey the intrinsic color of an object semantically when it is only represented by a continuous spectral distribution, an additional step is common. It is common to sample the continuous spectral Performance Descriptors 17- 3 distribution, using multiple narrow spectral band radiometric flux (not energy) detectors and report the relative intensity readings of these detectors. The individual radiometric flux detectors are calibrated to insure equal sensitivity to a uniform radiometric flux source. When implemented in a man-made instrument, this procedure is analogous to the initial photodetection stage of the animal eye. The result is defined here as the sampled color of the object. In the above procedure, it is necessary to describe both the relative readings of the individual detectors and the spectral characteristics of each detector. 3. A single valued description of the sampled color is not obtained using the above method. However, if the spectral characteristics of the individual detector channels are standardized, a simpler description of the sampled color of the object is possible. Traditionally, such a standardization has been based on the so-called tristimulus spectrums recommended by the CIE Unfortunately, these spectrums are artificial and do not conform to the spectrums of the photodetectors in the animal (including human) eye. As a result, such instruments have given confusing results. By using the actual spectrums of the chromophores of vision presented here in the instrument, results can be obtained which are more closely correlatable to the perceived color reported by an observer under conditions of constant illumination. If three radiometric flux detectors are used to emulate the spectral characteristics of a long wavelength trichromat, the instrument will provide three values that concisely describe the sampled trichromatic color of the object.