The following table for rule 2 specifies recommended shielding angles for cut-off type luminaires. Some manufacturers in UK provide data for the CIE method in a graphical form. The degree of glare is assessed in terms of luminances in the axial and transverse planes between 45 and 85. The following tables are used to check if the luminaire chosen will be acceptable in a room of specific size for a specific class of interior. Quality class Valid for service illuminance, lux A 2000 1000 500 300 B 2000 1000 500 300 C 2000 1000 500 300 D 2000 1000 500 300 E 2000 1000 500 300 Curve letter a b c d e f g h K.F. Chan (Mr.) Page 1 of 10 Sept., 2005
Class Quality Example A Very high Drawing office B High General office C Medium Supermarket D Low Toilets E Very low Iron Foundry Luminaires described in this system use the following terms:- luminous sides luminous side panels with a height greater than 30mm elongated when the ratio of the length to width of the plan luminous area is not less than 2:1 Diagram 1 Valid for * all luminaires without luminous sides * all elongated luminaires viewed endwise K.F. Chan (Mr.) Page 2 of 10 Sept., 2005
Diagram 2 Valid for * all non-elongated luminaires with luminous sides * elongated luminaires with luminous sides when viewed crosswise ------------------------------------------------------------------------------------------------------ The angles are defined in the following diagram K.F. Chan (Mr.) Page 3 of 10 Sept., 2005
Discomfort glare will not result if the value of the luminance given by the limiting curve exceeds the actual luminance of the luminaire over the whole range of emission angles. If the result is otherwise the design will have to be changed; for example, by choosing a different type of luminaire. K.F. Chan (Mr.) Page 4 of 10 Sept., 2005
EXAMPLE (taken from the book Lighting by DC Pritchard) Ceiling mounted luminaires with the following photometric property are installed perpendicular to line of sight in a drawing office, 6m x 6m x 3m high, 750 lux maintained illuminance. Initial flux output from each luminaire is 5100 lumens. From the above table, drawing office is class A, with 750 lux illuminance, the curve would be either b or c. The relevant range of angles for luminance values comparison is 45 o upwards as can be seen from the above 2 curves. The upper angle for luminance comparison is tan -1 (a/h s ) where a = 6m h s = 3 1.2 = 1.8m Thus the upper limit angle is 73 o So only 45 o to 73 o need to be considered. K.F. Chan (Mr.) Page 5 of 10 Sept., 2005
From the above photometric data for a transverse plane Luminance at 45 o = 553 x 5100/1000 = 2820 cd/m 2 Luminance at 55 o = 459 x 5100/1000 = 2340 cd/m 2 Luminance at 65 o = 419 x 5100/1000 = 2136 cd/m 2 Luminance at 75 o = 497 x 5100/1000 = 2534 cd/m 2 It can thus be seen that luminance of this luminaire exceeds that prescribed by both curves b and c especially at the higher angles (Diagram 2). Therefore, this indicates excessive glare from this luminaire installation if this room is used as a drawing office. As can be seen later when we discuss British Glare Index system, the same example calculated using BGI gives us the same conclusion not suitable for use as drawing office. K.F. Chan (Mr.) Page 6 of 10 Sept., 2005
Reflected glare Reflected glare is much more serious and difficult to control than direct glare. Reflected glare is also called veiling reflection. The following 2 figures demonstrate direct and reflected glare:- Direct glare the girl sitting at the desk in head-up position can see all the ceiling fixtures in front of her and all the desk lamps each one being a possible direct glare source K.F. Chan (Mr.) Page 7 of 10 Sept., 2005
Reflected glare -- the ceiling fixtures in the outlined area reflect onto the desk, causing reflected glare from the drawing, triangle, parallel straight edge and instruments. Veiling reflections are high luminance reflections, which overlay the detail of the task. Such reflections may be sharp-edged or vague in outline, but regardless of form they may affect task performance and cause discomfort. Task performance will be affected because veiling reflections usually reduce the contrast of a task, making task details difficult to see and may give rise to discomfort. Two conditions have to be met before veiling reflections occur: part of the task, task detail or background, or both, has to be glossy to some degree part of the interior, called the 'offending zone', which specularly reflects towards the observer, has to have a high luminance. K.F. Chan (Mr.) Page 8 of 10 Sept., 2005
The most common sources of veiling reflections are windows and luminaires. Generally applicable methods of avoiding veiling reflections are to use matt materials in task areas, to arrange the geometry of the viewing situation so that the luminance of the offending zone is low or reduce the luminance by, for example, using curtains or blinds on windows. Visual display screen (VDS) areas While there is a need for sufficient illuminance level for reading data to be input into the computers, there is a concern in reduction in contrast in a display screen due to reflections of the surrounds, including the luminaires, the latter being a form of veiling disability glare. In this case, glare is not just discomfort but disability. Effect of veiling reflections from electric lighting on a VDT screen VDT screen without veiling reflections K.F. Chan (Mr.) Page 9 of 10 Sept., 2005
CIBSE Lighting Guide 3. UNIVERSITY OF HONG KONG The visual environment for display screen use used to define 3 categories which refer to the situation and the control on luminance design. The control relates to luminance of less than 200 cd/m 2 above specified angles. is a relatively low level, and may be difficult to achieve. This Cat 1 luminaires Cat 2 luminaires Cat 3 Luminaires Angle to the downward 55 0 65 0 75 0 vertical at which luminance is limited to 200 cd/m 2 or below Application For screens containing safety-critical information, high density of Fairly wide spread use of display screens, one terminal per desk for general Casual use of display screen, low density, not used for sustained periods screens, intensive usage, sustained long periods usage, or few terminals used continuously Efficiency Least More Most Remarks :- Cat 2 or Cat 3 luminaries are also acceptable where space planning is either small cellular offices, or open plan with screen dividers where, by simple geometrical checking, it can be shown that the luminaires will not be seen at angles below their limiting angle from the display screens. Because there was a high tendency in the industry to always specify Cat fittings without any thought to the actual working environment into which they are to be installed, on 23 rd Oct., 2001, CIBSE issued an addendum to LG3 withdrawing the above Category rating system. This Cat rating system is discussed here because it still appears in many catalogues issued by a number of manufacturers. K.F. Chan (Mr.) Page 10 of 10 Sept., 2005