Dark adaptation time of a normal eye. Vision adaptation

It is known that the human eye is capable of working at very wide range brightness. However, the eye cannot perceive this entire range at the same time. In the process of vision, the eye adapts to the level of brightness prevailing in the field of vision. This phenomenon is explained by the dependence of the light sensitivity of the eye on the level of its excitation photosensitive elements. The eye has maximum light sensitivity after a long stay in the dark. In light, the sensitivity of the eye decreases. Adaptation process visual organ a person to different levels of brightness is usually called brightness adaptation.

It has been experimentally shown that the range of perceived brightness at a given level of adaptation is very limited. All surfaces that have a brightness less than the minimum for a given range appear black to us. Maximum brightness creates the feeling of white. If a surface appears in the field of view, the brightness of which exceeds the maximum for a given range, then the adaptation of vision will change, and the entire range of vision will correspondingly shift towards higher brightnesses. Moreover, those surfaces that seemed gray to us at a lower level of adaptation will be perceived as black.

Brightness adaptation occurs as a result of changes in the brightness of the visual field, and, consequently, the illumination of the retina in the image area. Special cases of brightness adaptation are dark And light adaptation. Dark adaptation occurs when the brightness of the visual field instantly decreases from a certain value to zero adaptation brightness value. Light - when brightness increases from its zero value to a certain finite value. The duration of the processes of light and dark adaptation is different. While a decrease in visual sensitivity (light adaptation) occurs in a time from a fraction of a second to several seconds, the process of dark adaptation lasts 60...80 minutes.

If you observe a sheet of white paper for 10...15 seconds, half of which is covered with something black, and then remove the black, then the previously covered part of the sheet will seem lighter than the rest of it. In this case, it is customary to talk about local brightness adaptation. The phenomenon of local brightness adaptation can be explained by the fact that when details of different brightness are simultaneously observed, i.e., when the illumination of different areas of the retina at the same time is different, the level of excitation of some areas affects the light sensitivity of others.

Color adaptation occurs as a result of a change in the color of the visual field while its brightness remains unchanged. While luminance adaptation is characterized by a discrepancy between lightness and brightness, color adaptation is characterized by a discrepancy between the chromaticity of the radiation and the sensation of this chromaticity.

The phenomenon of color adaptation is explained by a change in the sensitivity of the eye as a result of a change in the ratio of the excitation levels of its three receivers when the eye is exposed to radiation of a certain color. Color, on

which the eye adapts, fades, as it were. This occurs as a result of a decrease in the sensitivity to a given color of the part of the retina that is adapted to this color. So, if, after observing a green figure for 15...20 seconds, you turn your gaze to an achromatic background, then a consistent image (a trace from the previous irritation) of a reddish color appears on the background. If you look through yellow glasses for some time, then after the glasses are removed, all surrounding objects will appear bluish. The change in color as a result of the preliminary action of other colors on the eye is called consistent color contrast. It has been experimentally shown that changes in the sensation of color during the process of color adaptation can be quite large, and the nature of the change in color does not depend on the brightness of the observed color.

Depending on the presence of details of different colors in the field of view, changes in visual contrasts may occur due to both changes in lightness and changes in chromaticity. Details covered in dark background, they lighten, but on light they darken. Thus, two pieces of the same paper, placed in one case on black velvet, and in the other on white fabric, seem unequal in lightness. The lightness of a detail changes under the influence of the background color, regardless of whether the background and the detail viewed on it are achromatic or colored.

Having placed pieces of the same gray paper on backgrounds of different colors, we will notice that these pieces will seem to us different in color tone. On a red background, a gray field will take on a greenish tint, on a blue background it will become yellowish, and on a green background it will become reddish. Similar phenomenon It is also observed if pieces of paper of colors different from the background color are placed on colored backgrounds: yellow on red will seem slightly greenish, yellow on green will appear orange, etc. This phenomenon, in contrast to consistent contrast, is called simultaneous color contrast.

It is known that the same sheet of white paper is perceived as “white” in any lighting conditions: by candlelight, incandescent lamps and daylight. Despite the fact that differences in the spectral composition of “white” light sometimes exceed differences in the spectral reflectance curves of most objects, the eye almost always accurately determines the colors of objects. So, for example, although surfaces that are blue in daylight conditions turn out to be greenish when illuminated by incandescent lamps, a person continues to consider them blue. This is explained by the fact that in any lighting conditions, white details are most easily recognized, since they are always the lightest. All other colors are evaluated by the eye in relation to them. In other words, when observing a certain scene containing a number of colored objects, under certain lighting conditions, the relative sensitivities of the three eye receivers change in such a way that the ratio of their excitation levels in the part of the retina where the image of the lightest object in the scene appears becomes equal to the ratio of excitation levels sensational white. This phenomenon is called the phenomenon color constancy, or lighting corrections. This phenomenon explains, for example, that the viewer, when watching films (in a darkened room), does not notice

The peripheral organ of vision reacts to changes in lighting and functions regardless of the degree of brightness of the lighting. Adaptation of the eye is the ability to adapt to different levels illumination The reaction of the pupil to the changes that occur gives the perception of visual information in the millionth intensity range from lunar to bright lighting, despite the relative dynamic response volume of visual neurons.

Types of adaptation

Scientists have studied the following types:

light - vision adaptation in daylight or bright light;
dark - in darkness or low light;
color - conditions for changing the color of the illumination of objects located around.

How is it happening?

Light adaptation

Occurs during the transition from darkness to strong light. It instantly blinds and initially only white is visible, since the sensitivity of the receptors is tuned to dim light. It takes one minute for the cones to hit the sharp light to capture it. With addiction, the retina's light sensitivity is lost. Complete adaptation of the eye to natural light occurs within 20 minutes. There are two ways:

a sharp decrease in retinal sensitivity;
reticular neurons undergo rapid adaptation, inhibiting rod function and favoring the cone system.

Dark adaptation

The dark process occurs during the transition from a brightly lit area to a dark one.

Dark adaptation is the reverse process of light adaptation. This happens when moving from a well-lit area to a dark area. Initially, blackness is observed as the cones cease to function in low intensity light. The adaptation mechanism can be divided into four factors:

Light Intensity and Timing: By increasing pre-adapted brightness levels, the cone mechanism's dominance time is extended while the rod mechanism's switching is delayed.
Retinal size and location: The location of the test spot influences the dark curve due to the distribution of rods and cones in the retina.
The wavelength of threshold light directly affects dark adaptation.
Rhodopsin regeneration: when exposed to light photopigments, structural changes occur in both rod and conical photoreceptor cells.

It is worth noting that night vision has much more low quality than vision in normal light, as it is limited by reduced resolution and provides the ability to distinguish only shades of white and black. It takes about half an hour for the eye to adapt to twilight and acquire sensitivity hundreds of thousands of times more than in daylight.

It takes much longer for older people to adjust their eyes to the dark than for younger people.

Color adaptation

For humans, color objects change under different lighting conditions only for a short period of time.

It consists of a change in the perception of the retinal receptors, in which the maxima of spectral sensitivity are located in different color spectrums radiation. For example, when changing natural daylight to indoor lamp light, changes will occur in the colors of objects: green color will be reflected in a yellow-green tint, pink - red. Such changes are visible only for a short period of time; over time they disappear and the color of the object appears to remain the same. The eye gets used to the radiation reflected from the object and is perceived as in daylight.

Light perception is an ability visual analyzer perceive light and distinguish between degrees of brightness. When studying light perception, the ability to distinguish between minimal light irritation - the threshold of irritation - and to capture the smallest difference in the intensity of illumination - the threshold of discrimination.

The process of adaptation of the eye to different conditions lighting is called adaptation. There are two types of adaptation: adaptation to darkness when the light level decreases and adaptation to light when the light level increases.

Everyone knows how helpless you feel when you go from a brightly lit room to a dark one. Only after 8-10 minutes does the discrimination of poorly lit objects begin, and in order to navigate sufficiently freely, it takes at least another 20 minutes until visual sensitivity in the dark reaches the degree necessary for this. With dark adaptation, sensitivity to light increases, maximum adaptation is observed after an hour.

The reverse process of adaptation to high light levels occurs much faster than adaptation to darkness. When adapting to light, the sensitivity of the eye to the light stimulus decreases; it lasts about 1 minute. Upon leaving a dark room, visual discomfort disappears after 3-5 minutes. In the first case, during the process of dark adaptation, scotopic vision appears, in the second, during light adaptation, photopic vision appears.

The visual system responds adequately to both fast and slow changes in radiant energy. Moreover, it is characterized by an almost instantaneous reaction to a rapidly changing situation. The light sensitivity of the visual analyzer is as variable as the characteristics of light stimuli in the world around us are varied. The need to adequately perceive the energy of both very weak and very strong light sources without undergoing structural damage is ensured by the ability to rearrange the operating mode of the receptors. In the bright light light sensitivity eyesight decreases, but at the same time the reaction to spatial and temporal differentiation of objects intensifies. In the dark, the whole process happens in reverse. This set of changes in both photosensitivity and resolution of the eye depending on external (background) illumination is called visual adaptation.

The scotopically adapted retina is maximally sensitive to the light energy of the low level, but at the same time its spatial resolution sharply decreases and color perception disappears. The photopicly adapted retina, being low-sensitive for distinguishing weak light sources, at the same time has high spatial and temporal resolution, as well as color perception. For these reasons, even on a cloudless day, the moon fades and the stars go out, and at night, without backlighting, we lose the ability to read text, even in large print.

The range of illumination within which visual adaptation occurs is enormous; in quantitative terms it is measured from a billion to several units.

Receptors in the retina have very high sensitivity- they can be irritated by one quantum of visible light. This is due to the action of the biological law of amplification, when after the activation of one rhodopsin molecule, hundreds of its molecules are activated. In addition, the retinal rods are organized into large functional units in low light. Impulse from large quantity rods converge into bipolar and then into ganglion cells, causing an amplification effect.

As the illumination of the retina increases, vision, determined mainly by the rod apparatus, is replaced by cone vision, and the maximum sensitivity shifts in the direction from the short-wavelength to the long-wavelength part of the spectrum. This phenomenon, described by Purkinje back in the 19th century, is well illustrated by everyday observations. In a bouquet of wildflowers on a sunny day, yellow and red poppies stand out, and at dusk - blue cornflowers (shift of maximum sensitivity from 555 to 519 nm).

Factors that reduce visibility (fog, snow, rain, haze, etc.) make observation extremely difficult. sea, At night, observation conditions also worsen, and they have their own characteristics.

The duties of the watch officer while the ship is underway consist of two main ones equally important functions. Firstly, it performs various computational operations, solves navigational and other tasks, monitors the position of the vessel and calculates its path on the navigation map. Secondly, he, along with the sailor on watch, provides visual and auditory observation of the environment, using appropriate technical means. In other words, the navigator has to alternate between these two types of activity: either work in the control room on manuals and a map, or go out and stay on the open part of the bridge. This mode of action of the navigator in the dark is associated with the well-known phenomenon of eye adaptation. Adaptation of vision is called a change in the sensitivity of the eye depending on its exposure to light or darkness. A decrease in the sensitivity of vision during light stimulation is called adaptation, or adaptation of the eye to light, and an increase in sensitivity as it remains in the dark is called adaptation of the eye to darkness, or dark adaptation of the eye.

Light adaptation occurs much faster than dark adaptation and it takes 1-3 at average light brightnesses min(dark adaptation at least 5-7 min).

From the above it is clear that the phenomenon of vision adaptation is of utmost importance for night observations. In order for the sensitivity of the eye in the dark to be the same during the watch high level, the observer's vision should not be exposed to light. However, due to the operating conditions, the navigator on watch cannot avoid periodic, albeit short-term, eye glare while working in the control room over a map or with instruments. The task in this case, obviously, will be to eliminate or at least minimize the influence of light exposure.

It is known that the increase in vision sensitivity in the dark occurs much faster after being in low light conditions. According to scientific research, the red light stimulus has a weak effect on the retina of the eye - several tens of times weaker than white light.

From the foregoing it is clear that the nature of the lighting in the chart room, where the watch officer has to periodically work, as well as all instruments in the wheelhouse has exclusively important. We must strive to ensure that this lighting lies within the optimum range from all points of view.

As you know, lighting is divided into two types: general

local. The general one is intended for simultaneous illumination of both the working surface and the rest of the room, "" only for a relatively small space

different workplace, such as, for example, for part of the navigation

table occupied by a map.

It is not recommended to use general lighting in the chart room at night while the ship is moving. Local lighting above the chart table is arranged in the form of a special sconce, | reflecting a beam of light down onto the table. The lamp receives power through a rheostat, which allows you to reduce or increase the light intensity. A folding red or orange light filter is attached to the reflector.

To the watch officer during short-term visits:

In the chart room for calculations and plotting the point on the map, it is recommended to constantly keep the sconce under the filter. As a last resort, in the absence of a filter, the light intensity of the sconce must be reduced by a rheostat so that, on the one hand, one can freely work on the map, and on the other, so that the decrease in visual sensitivity is reduced to a minimum. This is necessary so that the eye is always adapted to the dark.

The illumination of compass cards, engine telegraphs, dials and displays of various instruments and installations in both the steering and chart rooms should be reduced to the minimum limit that only allows readings or indications to be distinguished, in order to exclude negative impact of this lighting on the dark adaptation of the navigator's eye. When taking direction finding of any objects, the light on compasses or repeaters must also be dimmed. When surveying at night, the radar screen should not be heavily illuminated. When setting up the device, you need to skillfully use the “Brightness” knob, setting it to the optimal position each time. The illumination of the scales is turned on only for a short moment, when it is necessary to read the bearing or heading angle, and usually only by one step.

Dark vision adaptation plays important role in ensuring safe navigation, and this issue should be given the most serious attention. Adaptation of the eye to darkness is a slow process, lasting tens of minutes, which makes it clear what a danger bright light poses during night observations on a ship. If you spend a short time in a lighted room or look at a source of bright light, such as a spotlight, adaptation to darkness will be immediately lost, and it will take a long time to restore the sensitivity of the eye.

In the Charter of service on ships navy it is said that “but upon calling the watch officer, the captain must immediately go to the bridge and, in the event of unfavorable sailing conditions, remain there as long as necessary, regardless of the time of day.” Usually such calls are received in difficult situations, when diverging from oncoming or overtaken vessels. If in daytime the captain, having climbed onto the bridge, is able to immediately assess the situation and take appropriate

decisions and issue commands, then at night he finds himself in a difficult situation, since the first 5-7 min his vision is almost completely devoid of light sensitivity. The navigator of the watch must take this important circumstance into account. During the dark period of the day, when ships or other dangers are detected, he is obliged to immediately report this to the captain, so that the latter can go to the bridge in advance and allow the eye to adapt to the darkness to some extent.

During his stay in the interior, the captain is advised to avoid strong exposure to his vision in every possible way. At night, he should not turn on the lights in the cabin, especially bright ones; The corridors through which the captain passes to the bridge must be darkened or equipped with lamps with red shades.

Visual acuity, i.e. the ability to see distant objects and distinguish between their thin and small details, but with angular dimensions, different people their ability to adapt vision is not the same. It is known, for example, that dark adaptation changes significantly with hypertension. This change manifests itself in the form of a slowdown in the process of increasing light sensitivity and a decrease in its final values. The speed and degree of dark adaptation also decreases with age.

Taking all these factors into account, it is recommended that the captain have his own separate multiple binoculars, pre-adjusted for his eyes. Such binoculars should be stored in a special and convenient location on the bridge so that the captain, upon arriving on call, can immediately, without prior configuration, use it for observation.

The darkening of the vessel is of no small importance when viewing at night. No light should be allowed to enter the deck, even from weak sources or reflected light. The duties of the watch service include ensuring complete darkness both on the navigation bridge itself and in front of it. Lookouts on the forecastle and other lookouts wherever posted must refrain from smoking or lighting matches. The use of hand-held torches for any purpose is permitted only in extreme cases with the permission of the watch officer.

The most sensitive places of the retina do not lie in the center of the visual field, but somewhat to the side, on the periphery of the eye. This circumstance determines the so-called “lateral vision”. Its essence lies in the fact that at night a weak fire cannot be detected by direct looking at the point of its source, but as soon as the observer turns his gaze slightly to the side, this light will be clearly perceived by the side part of the retina. Well-trained observers successfully use this property of vision, detecting danger in time. They are in the same

In some cases, they direct their gaze not to the point on the horizon where fire is expected, but somewhat to the side of it.

A night observer has to look either at bright light or into darkness, like, for example, a navigator when working with a locator, so he should alternately use one eye and then the other. So, you can look at the screen only with your left eye, closing the right one, which will preserve dark adaptation and allow you to see well in the dark, although the left eye will be somewhat blinded by the light. This method gives good results, but without preliminary training it quickly tires the observer’s vision.

Adaptation is the adaptation of the eye to changing lighting conditions. Provided by: changes in the diameter of the pupil opening, movement of black pigment in the layers of the retina, different reactions of rods and cones. The pupil can vary in diameter from 2 to 8 mm, while its area and, accordingly, the luminous flux change by 16 times. The pupil contracts in 5 seconds, and full expansion- in 5 minutes.

Color adaptation

Color perception may vary depending on external conditions illumination, but human vision adapts to the light source. This allows the lights to be identified as the same. Different people have different eye sensitivity to each of the three colors.

Dark adaptation

Occurs during the transition from high to low brightnesses. If bright light initially entered the eye, the rods were blinded, the rhodopsin faded, and the black pigment penetrated the retina, blocking the cones from the light. If suddenly the brightness of the light decreases significantly, the pupil will first dilate. Then the black pigment will begin to leave the retina, rhodopsin will be restored, and when there is enough of it, the rods will begin to function. Since cones are not sensitive to low brightness, at first the eye will not distinguish anything until a new vision mechanism takes effect. The sensitivity of the eye reaches its maximum value after 50-60 minutes of being in the dark.

Light adaptation

The process of adaptation of the eye during the transition from low to high brightness. In this case, the rods are extremely irritated due to the rapid decomposition of rhodopsin, they are “blind”; and even the cones, not yet protected by grains of black pigment, are too irritated. Only after sufficient time has passed does the adaptation of the eye to new conditions end and stop unpleasant feeling blindness and the eyes acquire the full development of all visual functions. Light adaptation lasts 8-10 minutes.