2.5.1. Biological effect and technical characteristics
Subjective perception of visual information makes about 90% of the total. In conditions of insufficient or changing illumination visual organs have to get accustomed to the environment what’s possible owing to such functions as adaptation, accommodation and convergence.
Adaptation is the process by which the eye’s retina becomes adapted to varying luminance.
Accommodation is the automatic or voluntary adjustment of the thickness of the lens of the eye for far or near vision.
Convergence is the turning of the eyes inwards in order to fixate an object nearer than that previously being fixated.
Moreover, light has a great physiological effect. Poor lighting brings to fast fatigue and risk of accident, it also may cause occupational disease, such as myopia or short sight and accommodation spasm.
Color is the characteristics of light by which a human observer may distinguish between two structure-free patches of light of the same size and shape. Appropriate coloring can rise 20 - 40 % of illumination, make smooth shadows, improve uniformity of illumination.
Illumination variations and unequal brightness of surroundings frequently lead to eyes’ re-adaptation and fast fatigue. Illumination of occupational areas is evaluated by quantitative and qualitative characteristics. Quantitative characteristics include luminous flux, intensity, illuminance, luminance, and reflectance. Qualitative characteristics include background, contrast of an object against its immediate background, visibility, uniformity of luminous flux.
Luminous Flux: the time rate of flow of light. Lumen (lm) is the SI unit of luminous flux. The luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.
Intensity: luminous intensity or radiant intensity often misused for level of illuminance. Candela (cd) is the SI unit of luminous intensity; one candela is one lumen per steradian.
(cd).
Luminance: the physical measure of brightness: luminous intensity per unit projected area of any surface, as measured from a specific direction. (English unit: Footlambert. Metric unit: Candela per square meter).
(cd/m2),
where – reflectance, S – surface area.
Illuminance: the density of the luminous flux incident on a surface, expressed in units of footcandles or lux.
(lux).
Reflectance: the ratio of reflected flux to incident flux.
.
Vision always deals with two subjects: object and background. Background is the surface adjacent to object being observed. Background can be light, medium and dark, depending on reflectance. It’s light when >0.4, medium when =0.2..0.4, dark when <0.2.
Owing to the difference between luminances of object and background and also color people distinguish things around. That’s why there is studied another characteristic called contrast.
Contrast: the relationship between the luminance of an object and its immediate background. Mathematically, the difference between the luminances divided by the luminance of the background.
,
where Lo, Lb – object and background luminances.
Contrast is high when C>0.5, medium when C=0.2..0.5, low when <0.2.
Visibility: the quality or state of being perceivable by the eye. It evaluates visual acuity and considers illuminance, size of object, its luminance, contrast of object against background, exposure duration:
V=C/Cthr,
where C – contrast of object against background; Cthr – threshold visible contrast.