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- visual acuity (the recognition ability) is the ability of the visual analyzer to recognize the smallest elements of the object. It’s determined by the smallest angle, under which the two adjacent spots are recognized as separate. The visual acuity is conventionally considered to equal to one angular minute. The visual acuity grows proportionally to the illuminance until it reaches 130-150 lux. When the illuminance is above that point, the visual acuity growth slows down.
- contrast sensitivity is the ability of the visual analyzer to perceive the minimum difference between the brightness of the object and the background. It reaches its highest level when the illuminance is 1 000-2 500 luxes;
- visual perception speed is the time, required to recognize the details of the object. This speed grows until the illuminance reaches 150 luxes. After that point, the growth slows down unproportionally to the illuminance growth;
- visibility is the integral function of the visual analyzer, which is the combination of its main functions – visual acuity, contrast sensitivity, visual perception speed;
- clear vision stability is the time, during which the object can be clearly seen to the total time of the object examination. Physiologically this function of the visual analyzer based on the destruction of the visual purple (rhodopsin) under the influence of the light and formation of the protective black pigment on those parts of the retina, where the picture is the brightest. This function reaches its optimal value at the illuminance of 600-1 000 luxes. Its reduction is the evidence of the visual analyzer fatigue;
- color recognition function. White, black, grey – achromatic colours are only characterized by brightness and light flow intensity. Chromatic colours (monochromatic) are characterized by brightness and chromaticity. Vision is the most sensitive to the yellow and green part of the visual spectrum and the least sensitive to the violet light. During the twilight or under the artificial illumination (especially with incandescent lamps) the visual analyzer’s colour recognition reduces and may distort;
- adaptation is visual analyzer’s ability to reduce its sensitivity during the change from low to high illumination (light adaptation), (achieved very quickly, (in 2-3 minutes) and is caused by the visual purple conversion into the protective black pigment in the retina), and to increase it again when the illumination changes from high to low level (adaptation to darkness), which takes much longer – up to 40-60 minutes and is caused by the restoration of the visual purple in the retina;
- accommodation is the ability of the eye to regulate the visual acuity depending on the distance to the examined object and illumination due to the changes in the light refraction in the optic system of the eye, which is mostly caused by the chrystalline lens curvature change. The curvature will increase when the illumination is less than 100-75 luxes. So, in such circumstances the object must be closer to the eye for the proper recognition;
The insufficient illumination leads to the overstrain of accommodation system, overstrain of the visual analyzer, and for children and adolescents (their eye has not yet formed completely) it may cause the myopia (short-sightedness) especially if they have the congenital disposition;
- critical flicker frequency is determined by the time, during which the afterimage remains in the visual analyzer: the image of an object, which has disappeared from the visual field still remains visible for some time depending on the object brightness. This visual function is based on the same processes of visual purple destruction and restoration. The cinema, one of the most important human inventions, is based on it. The frequent change of the frames and the almost similar objects (25 frames per second), and the darkening of the screen provides dynamic and continuous picture.
The sources of artificial illumination may be electric and non-electric. Non-electric sources are kerosene, carbide lamps, candles and gas lamps. Their use nowadays is mostly limited to the field conditions and emergency situations. The electric sources of artificial illumination may be arc lamps (in searchlights, floodlights, spotlights etc), incandescent lamps, gas-discharge lamps and luminescent lamps.
The disadvantage of incandescent lamps is the spectrum parallax in the yellow-red direction, the distortion of the color perception, the dazzling (blinding) effect of direct rays.
The luminescent lamps have the spectrum, almost similar to the day light with modifications, depending on the luminophor, that covers the internal surface of the glass tube and transforms the ultraviolet luminescence of mercury vapour in the tube into the visible light. There are the daylight lamps (DL), white light lamps (WL), warm white light lamps (WWL) etc.
The disadvantage of luminescent lamps is the stroboscopic effect - the flickering of moving objects.
One of the disadvantages of both the direct sunlight and bright sources of artificial illumination is their ability to cause the dazzling effect. We protect ourselves from the bright sunlight using the curtains and jalousies, dark-toned windows, the sun glasses.
The lighting fixtures (also used for the aesthetic purpose), are used for protection from the dazzling effect of artificial light sources.
The lighting fixtures are divided onto 5 types according to light flow formation (see fig. 5.1):
- direct light type, directing the whole light flow into one hemisphere (the table lamp with the opaque lamp-shade, spotlights, floodlights, and other fixtures used in photo and movie shooting);
- evenly-diffusing the light (dim or light-white sphere);
- reflected light (when the lamp with the opaque lamp-shade directs the light flow towards the upper hemisphere);
- directed-diffused light type, when the main light flow is directed towards the lower hemisphere through the aperture in the lamp-shade and the other part is diffused to the upper hemisphere through the lamp-shade made of plastic, dim or light-white glass;
- reflected-diffused light type, when the main light flow is directed towards the upper hemisphere and is reflected from the ceiling but a part of it is diffused to the lower hemisphere through the lamp-shade with dim or light-white glass.
The allowable values of dazzling at the workplace are:
-20 cd/m2 for types 1 and 2 of the visual work;
-40 cd/m2 for the types 3-5 of the visual work;
- 60 cd/m2 for the types 6 and 7 of the visual work.
Fig.5.1. Types of lighting fixtures
(1 - direct light type, 2 - directed-diffused light type; 3, 4 - evenly-diffused light type;
5 - reflected-diffused light type)
Appendix 2
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