Consider the ILV: International Lighting Vocabulary of the “Commission Internationale de l’Éclairage”  (CIE ILV) to identify what areas of vision science regard the adaptation phenomena [2, 3, 4, 5].
Process by which the state of the visual system is modified by previous and present exposure to stimuli that may have various luminance values, spectral distributions, and angular subtenses.
NOTE Adaptation to specific spatial frequencies, orientations, sizes, etc. is recognized as being included in this definition.
- State of Adaptation
State of the visual system after an adaptation process has been completed.
NOTE The terms light adaptation and dark adaptation are also used, the former when the luminances of the stimuli are of at least 10 cd/m2 and the latter when the luminances are of less than some hundredths of a cd/m2.
The HVS, after being adapted to a bright light, may need a time in the order of more than one half hour to become completely dark adapted, while, after being adapted to darkness, it may need only a few minutes to become completely daylight adapted. These two processes are not symmetrical and are considered separately.
Dark adaptation is at the basis of the duplicity theory that states that two transduction mechanisms exist, which are related to two different kinds of photosensitive cells: the rods and the cones. In each retina, there are approximately between 75 and 150 million rods and six to seven million cone cells . The rods are extremely sensitive to light; they contain rhodopsin as the light-absorbing pigment responsible for the transduction and provide achromatic vision. The cones are of three different classes (L cones, M cones, S cones) containing three different light-absorbing pigments responsible for the transduction. They are less sensitive than the rods and provide color vision. The distributions of cones and rods on the retina are very different and very nonuniform.
- Photopic Vision
Vision by the normal eye in which cones are the principle active photoreceptors.
NOTE 1 Photopic vision normally occurs when the eye is adapted to levels of luminance of at least 10 cd/m2.
NOTE 2 Color perception is typical of photopic vision.
- Scotopic Vision
Vision by the normal eye in which rods are the principle active photoreceptors.
NOTE 1 Scotopic vision normally occurs when the eye is adapted to levels of luminance of less than ~10−3 cd/m2.
NOTE 2 In comparison to photopic vision, scotopic vision is characterized by the lack of color perception and by a shift of the visual sensitivity toward shorter wavelengths.
- Mesopic Vision
Vision by the normal eye intermediate between photopic and scotopic vision.
NOTE In mesopic vision, both the cones and the rods are active.
(The mesopic vision is between two luminance levels from 0.01 to 10 cd/m2.)
- Perceived Color
Characteristic of visual perception that can be described by attributes of hue, brightness (or lightness), and colorfulness (or saturation or chroma).
NOTE 2 Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observer’s visual system, and on the observer’s experience of the prevailing and similar situations of observation.
NOTE 4 Perceived color may appear in several modes of color appearance. The names for various modes of appearance are intended to distinguish among qualitative and geometric differences of color perceptions. Some of the more important terms of the modes of color appearance are given in “object color,” “surface color,” and “aperture color.” Other modes of color appearance include film color, volume color, illuminant color, body color, and ganzfeld color. Each of these modes of color appearance may be further qualified by adjectives to describe combinations of color or their spatial and temporal relationships. Other terms that relate to qualitative differences among colors perceived in various modes of color appearance are given in “luminous color,” “nonluminous color,” “related color,” and “unrelated color.”
(Different adaptations correspond to different color-appearance modes.)
- Chromatic Adaptation
Visual process whereby approximate compensation is made for changes in the colors of stimuli, especially in the case of changes in illuminants.
- Adaptive Color Shift
Change in the perceived color of an object caused solely by change of chromatic adaptation.
- Illuminant Color Shift
Change in the perceived color of an object caused solely by change of illuminant in the absence of any change in the observer’s state of chromatic adaptation.
- Resultant Color Shift
Combined illuminant color shift and adaptive color shift.
- Contrast Sensitivity [Sc]
Reciprocal of the least perceptible (physical) contrast, usually expressed as L/ΔL, where L is the average luminance and ΔL is the luminance difference threshold.
- Luminance Difference Threshold [ΔL]
Smallest perceptible difference in luminance of two adjacent fields.
Unit: cd/m2 = lm/(m2 · sr)
NOTE The value depends on the methodology, luminance, and on the viewing conditions, including the state of adaptation.
- Luminance Threshold
Lowest luminance of a stimulus which enables it to be perceived.
NOTE The value depends on field size, surround, state of adaptation, methodology, and other viewing conditions.
Moreover, there exists another phenomenon involving different photopic adaptation levels, known as Bezold-Brücke phenomenon, and regarding the shifts of hues produced by changing the luminance of a color stimulus while keeping its chromaticity constant.
The mechanisms behind the different adaptation processes are still not completely understood; therefore, here the main phenomenological aspects of the adaptation are described. First, general aspects of the adaptation are considered and then the adaptation phenomena in the following order: the time-course of adaptation, subdivided into dark adaptation and light adaptation, and then brightness adaptation, chromatic adaptation, and color adaptation. Adaptation to specific spatial frequencies, orientations, sizes, etc. are not considered although included in the definition of adaptation given by CIE ILV.
The effect of the stimulation on the photoreceptors is a modification of its own effectiveness. Whenever there is a change in retinal illuminance and/or spectral power distribution of the stimulus, the visual mechanism starts readapting to the changing stimulus. Thus, in the real situation, when the light flux crossing the retina is continuously changing, the visual sensitivity at any particular time and place on the retina is the resultant of the effects of the stimulations due to the light flux in the time, previous and actual, and on the considered place and contiguous places. The term local adaptation refers to the effect of a stimulus which has been confined to a specific region of the retina.
The time change in sensitivity depends on the duration and also on the degree of stimulation. If the adapting light level changes by a relatively small amount, the visual system compensates for the change almost immediately, but, if the light level changes by a large amount, it takes a long time to reach complete adaptation. If a new stimulus remains the same for long enough time, the adaptation level reaches an equilibrium and the adaptation is complete. The time required for a complete adaptation depends on the starting level and on the new light level. The sensitivity change is very sudden in the initial phase, but its duration is a small fraction of the time required for a complete adaptation.
The term adaptation level defines the kind and degree of steady stimulation that would produce the same state of sensitivity as exists at any moment and place on the retina.
The adaptation level determines the range of responsiveness. The fully dark-adapted visual system cannot discriminate any luminances below an absolute threshold nor can the fully light-adapted visual system discriminate any luminances above an upper terminal threshold. The overall range within which the optimally adapted visual system is effective is approximately from 10−5 to 105 cd/m2. The retina is bound to be adapted to some level at any time and only a small part of the full range is available at any time. The ratio of maximum luminance over minimum luminance detectable for the full range is 10 billion to 1, while the momentary range for ordinary levels of luminance is in the order 1,000 to 1.
The adaptation mechanisms produce changes in threshold visibility, color appearance, visual acuity, and sensitivity over time.
The adaptation process operates over a luminance range of nearly 14 log units: the light of the midday sun (~108 cd/m2) can be as much as ten million times more intense than moonlight (~10−6 cd/m2). The process which allows this great extension of retina sensitivity is called brightness adaptation.
Switchover from rods to cones in the passage from scotopic to photopic vision
Bleaching/regeneration of the photopigments in the photosensitive cells (rods and cones)
Feedback from the horizontal cells to control the responsiveness of the photosensitive cells
These four mechanisms have the combined effect of making the retina more sensitive at low light levels and less sensitive at high light levels, with important consequences for perception.
The pupil size has only a small part in the adaptation process: the luminous flux entering the eye is proportional to the pupil area; therefore, since the pupil diameter ranges from 1 or 2 mm to approximately 8 mm, the luminous flux is modulated by a factor of 16–64. A change of 10 log units of luminance induces the pupil to change in diameter from approximately 7–8 mm down to approximately 1–2 mm . This range of variation produces a little more than one log unit change in retinal illuminance, so pupillary action alone is not sufficient to completely account for visual adaptation .
The neuron net of the retina has a very limited response range: −80 mV to +50 mV of graded potential in the non-spiking cells of the retina (rods, cones, and horizontal cells) and 0 to approximately 200 spikes per second for the ganglion cells.
The main parts of the adaptation process are due to the mechanisms (3) and (4) producing changes in retina sensitivity.
The effect of light on photopigments is their bleaching or depletion (photochemical effect) and, after bleaching, there is a regeneration of photopigments (chemical effect). Pigment bleaching makes the receptors less sensitive to light and, at high flux of light, produces a compression in their response. However, pigment bleaching cannot completely account for adaptation because the time-courses of the early phases of dark and light adaptation are too rapid to be explained by pigment bleaching alone .
Adaptive processes sited in the neural network of the retina (horizontal cells) have a multiplicative process effectively scaling the input by a constant related to the background luminance. This process acts very rapidly and accounts for changes in sensitivity over the first few seconds of adaptation. A slower acting subtractive process reduces the base level of activity in the system caused by a constant background. This last process accounts for the slow improvement in sensitivity measured over minutes of adaptation . Sensitivity increases in dim light and decreases in bright light inducing a more or less constant range of response of the visual system.
For a given set of visual conditions, the current sensitivity level of the visual system is called the brightness adaptation level. These combined phenomena have the effect that the perceived brightness is approximately constant in a wide range of brightness adaptation levels.
The response of photosensitive cells may no longer increase if the light flux is so intense that the regeneration of the photopigments is not able to counterbalance the bleaching completely. This situation is known as saturation.
The term brightness adaptation is not defined in the CIE ILV; it is used seldom, and the phenomena here described are generally considered in the light adaptation process.
The Time-course of Adaptation
In this experiment, the observer is first adapted to a high background luminance with the rod system depleted, and then the light is switched off abruptly. In the darkness, the detection threshold is measured continuously over more than 30 min. The detection threshold is the smallest perceptible luminance of a light spot on a black background. Violet light of short wavelength is used. The graph of Fig. 1 shows the detection threshold in the darkness as a function of time. In the first 5 min, after the adapting field is switched off, the threshold drops rapidly, but then it levels off at a relatively high level because the cone system has reached its greatest sensitivity, and the rod system is still not significantly regenerated. After about 7 min, rod system sensitivity comes over that of the cone system and the threshold begins to drop again. A change in slope separates the two curve branches which represent the rod and cone systems, respectively. This separation point is known as the Purkyně break (Purkyně shift) ( Purkyně, Jan Evangelista)  and indicates the transition from detection by the cone system to detection by the rod system. Changes in the threshold can be measured out to approximately 35 min, at which point the visual system has reached its absolute levels of sensitivity, and the threshold has dropped nearly 4 log units. The relatively slow time-course of dark adaptation means that vision can be impaired for several minutes when the observer moves quickly from high illumination levels to low ones.
The course of dark adaptation is influenced by the intensity and duration of light preadaptation, and different but analogous curves are measured in correspondence to different preadaptations.
Visually, light adaptation is experienced as the temporary blindness that occurs when the observer goes rapidly from a scotopic to a photopic level of illumination. The bright light momentarily dazzles the observer, to whom everything appears as a white light because the sensitivity of the receptors is set to dim light. Rods and cones are both stimulated, and large amounts of the photopigment are broken down instantaneously, producing a sensation of glare. During the light adaptation process, the HVS has to adapt quickly to the background illumination to be able to distinguish objects on this background. The sensitivity of the retina decreases dramatically. Retinal neurons undergo rapid adaptation inhibiting rod function and favoring the cone system. Within approximately one minute, the cones are sufficiently excited by the bright light to take over. The process for light adaptation, in which visual accuracy and color vision continue to improve, occurs over 5–10 min. During light adaptation to photopic vision, rod sensitivity is lost.
The HVS operates over a wide range of luminance levels. The sensitivity defines the state of brightness adaptation. Under low levels, the HVS has a very high sensitivity and can discriminate a luminous spot as small as 100 photons (light quanta) against a black background. In the photopic range, the HVS requires a luminous spot of thousands or millions of photons to be seen against a background of higher illumination. HVS sensitivity is represented by the absolute intensity threshold, which is measured by a psychophysical experiment known as the threshold versus illuminance (TVI) experiment. The measured quantity is the retinal illuminance I. This experiment measures the minimum illuminance increment ΔI of a test spot, a sharp-edged circular target in the center of the visual field, required to produce a visual sensation on a uniformly lit background with retinal illuminance IB, to which the observer is adapted. This can be achieved by placing an observer in front of a background wall of a given luminance (adapting luminance), and, once the adaptation is obtained, by increasing the retinal illuminance of the test spot ΔI from zero until it is just noticeable to the observer. The test phase has to be very quick avoiding any conditioning on the adaptation.
The TVI below −4 log units is almost constant and the low background luminance does not significantly affect the threshold.
The TVI between −4 and −2 log units increases in proportion to the square root of the background retinal illuminance .
The TVI between −2 and +2 log units is in proportion to the background adapting retinal illuminance and the slope ΔI/IB is constant. This section of the TVI is known as Weber’s Law.
These results depend on the test size and retinal eccentricity because the distribution of the rods and cones on the retina are not uniform.
Chromatic adaptation refers especially to those transient changes in sensitivity which are ascribable to photopic chromatic stimulation and are reflected in changes in chromatic sensation and perception. Visually, chromatic adaptation is experienced when a sudden illuminant change (e.g., in a room, the passage from daylight to a tungsten light) causes a global change in perceived color which is recognized by the observer as a change in illuminant. In spite of this evident change in the perceptual appearance of the scene caused by significant changes in the wavelength composition of the light reflected from different objects under the new illuminant, the perceived color of the objects remains largely unchanged. This adaptation phenomenon is termed instantaneous color constancy. In literature, chromatic adaptation and color constancy are often considered as synonymous.
- Adapted White
Color stimulus that an observer who is adapted to the viewing environment would judge to be perfectly achromatic and to have a luminance factor of unity.
- Achromatic Stimulus
Stimulus that, under the prevailing conditions of adaptation, gives rise to an achromatic perceived color.
- Chromatic Stimulus
Stimulus that, under the prevailing conditions of adaptation, gives rise to a chromatic perceived color.
NOTE In the colorimetry of object colors, stimuli having values of purity greater than 0 are usually considered to be chromatic stimuli.
The term color adaptation is not defined in the CIE ILV; it is used seldom and generally the term successive contrast is used.
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