The evaluation of interior lighting not only includes basic photochromic electrical parameters like luminous flux, color temperature, color rendering index, service life, nominal voltage and other strict referential standards, but also some vital evaluation parameters, such as luminance, glare level and luminance limit curve. Following are crucial ones among those parameters.
Luminance refers to the physical quantity of a luminous body's (refractive body's) surface luminous intensity. The human eye observes the light source from one direction, and the ratio of the light intensity in this direction to the area of the light source "seen" by the human eye is defined as the unit luminous intensity of the projected area. The unit of luminance is Candela/m2 (cd/m2), and the luminance intensity can be tested with a spectroradiometer.
The luminous quality is related to the luminous limit of the light. If the luminous intensity is too high in the field of view, it can produce uncomfortable glare, and even disabling glare. Serious glare directly affects labor productivity and even induces eye disease.
Glare: if there is object of high luminous intensity, sharp luminous contrast or extreme contrast in time or space in field of view, it can cause discomfort to our visual sense or lower visual performance, or both. Glare is the most important factor that affects quality of lighting. The degree of glare is related with luminous area, lighting surface luminance, background luminance, as well as direction and position of view. In this regard, the design of light should limit the lighting luminance. In normal circumstances, eyes mostly stay horizontally indoors. Thus, lights are prone to be affected by glare between 60°~90°degree. Direct glares are generated between 45°~90°.
According to the different requirements for lighting quality in lighting application, CIE divides the direct glare of indoor common lighting into 5 quality grades from A to E. The luminance limit curve recommended by CIE is the standards and methods to evaluate the direct glare of general indoor luminaires. Controlling direct glare is controlling the intensity of light in this direction. That means limiting the brightness of the luminaire between 45 <γ< 85 degrees. To evaluate whether the brightness of the light meets the requirements of limiting glare, first is to draw the brightness curve of the light. The curve is then placed in the luminaire brightness limit curve table for comparison to see if the recommended brightness is out of recommended luminous range. However, the lights of the lighting is transmitted in three-dimensional space, in order to formulate a plan easy to practice, we only limit the luminance of typical plane C. If optical axes are symmetric, we can take half-plane of C0 and C90.
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