There are three categories of light fitting listed in The Chartered Institution of Building Services Engineer (CIBSE’s) namely Category 1, Category 2 and Category 3. These types of lighting are differed according to their luminance limit angle. Category 1 limit angle is 55, Category 2 is 65 and Category 3 is 75 as shown in figure below;
Categorizing these light fitting is not indicating that category 1 is not ‘better’ than category 2 ‘better’ than category 3. The correct category must be selected for each individual installation otherwise undue installation and energy costs are likely to be incurred.
Light output ratio is another aspect of the installation when choosing the ‘right’ category. This ratio must be determined so that the luminance is evenly distributed as can be seen in the figure below.
From this information it can be ascertained that in general Category 1 (reflector luminance less than 200cd per sq.m from an angle of 55o)luminaires are recommended where there were a high density of VDU (Video Display Terminal) in an area, and where usage is sustained over long periods of time, or is of an intensive nature or where errors are critical.
Category 2 luminaires (reflector luminance less than 200 cd per sq.m from an angle of 65°) are recommended where there would be fairly wide spread use of VDT in an area where one VDT per desk for general use.
Category 3 luminaires (reflector luminance less than 200 cd per sq.m from an angle of 75°) are recommended where the density of VDT is low and where computer use is casual.
Credit to PCO Lighting
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