(From Page 84)
paired in split-phase (lead-lag')_cir- cuits to smooth the 100-cycle flutun- tions. For maximum ethciency, 'white OC 'warm white' lamps are generally wed, and at present have the slight ad- vantage that they reduce the amplitude of 100-cycle fluctuations because they employ luminescent powders with a re- lative.y long 'after-glow.' However, the 'day-light' type may be preferred if the installation is intended "as
a per- manent supplement to daylight.
Levels of illumination
As already mentioned, the level of illumination is not by itseif an ade- quate guide to the suitability of a lighting system. However, the survey gave an indication of the levels of illumination found satisfactory. Where local lamps are installed at the boards. a general illumination of not less then 25-30 lumens sq. it. is adequate for studying plans and other reference ma- terial. If local lights are to be omitted and the whole of the illumination is to
be provided by a general lighting sys- tem, the level of ilumination must be at least 50 lumens sq. ft. At such a level particular care must be taken to prevent direct or reflected glare. In any case, with the intense light output required to produce these high levels of illumination, correct balance of the brightnesses throughout the entire en- vironment becomes more critical. For instance, too much light from above as- sociated with dark surroundings (i.e., dark walls and dark furniture) can cause visual discomfort when reflected from large areas of white paper.
Installation design
To illuminate the drawing board evenly, and at the same time to light the surroundings to give maximum visual comfort, calls for fittings design- ed to give the correct distribution of light and having satisfactory surface brightness characteristics. As already stressed, the higher the illumination,
the
more critical is the balance of brightness. With a plain white ceiling and with fittings suspended well above the line of sight, the brightness of the
fittings, seen at an angle less than 45° above the horizontal, should be accep- able, if not greater than 750 foot-lam- berts, assuming that at least 20% of their light is thrown upwards. Fittings designed to meet the Ministry of Edu- cation requirements for classroom lighting come within this category and include designs Idor both incandescent and fluorescent lamps.
Although the ceiling itself must be reasonably bright so that the lighting fittings seen against it do not stand out 23 sources of glare, the ceiling must not compete in brightness with the drawing paper. Where a luminous ceil- ing is used, the balance of brightness from the ceiling, drawing paper and the general surroundings is even more critical. Pending further research it is suggested that a brightness relationship for the three elements in the ratio of 7:10: should be aimed at. If this is to be achieved it is very likely that a system of louvres or baffles will be re- quired to reduce the brightness of the ceiling. On the other hand, in insta!- lations where large areas of trough re- flectors themselves constitute the ceil- ing, they may appear dark when viewed from the draughtsman's position unless they are designed to diffuse enough light sideways to make the ceiling bright enough to overcome possible glare from the drawing paper.
The relation of source brightness to the illumination of the drawing board also needs special consideration in the case of general lighting schemes, because specular reflection may make pencil lines invisible. The recommendation in the Australian Lighting Standard* may be taken as a useful guide; this states that the illumination of the drawing board (in lumens/sq. ft.) should not be less than twelve times the luminance (measured in candelas/sq. in.) of the lamps or bright areas reflected by pen- cil lines, or not less than twenty-four times this luminance when high gloss materials such as tracing cloth are to be used. Thus, if unshielded or louvred
*Australian Standard No. CA. 30 1957, Artificial Lighting of Buildings: Standards Association of Australia.
fluorescent fittings are used with a lamp having a luminance of 6 candelas/* in.. the illumination on the boar should be 72 lumens sq. ft., unlem of course the fittings can be arranged to avoid specular reflection entirely, for instance, by placing them so that the light reaches the board from behind the draughtsman. For tracing it would be better to rely on a local lamp which can add 100 lumens sq. ft. OF INNOT to the general lighting at the working point, instead of trying to produce à very high illumination for the whol office area by means of the genera
lighting installation.
In conclusion, it cannot be emphasiz ed too strongly that the colour finishe of walis and furniture have a vital pan to play in helping to achieve a correct balance between the brightness of the various surfaces and favourable visual conditions. It is not possible in the space of this Digest to do more than re-affirm that no scheme of natura lighting or artificial lighting can b wholly successful unless the design of the colour scheme has been taken into account. The subject is fully discussed in the following:
'Rational Aspects of Colouring in Building Interiors. I and II": "H.L. Gloag and M.J. Keyte: Architects J., 1957. 125 (3237) March 14, and (3238) March 21,
399-402, 443-8.
Other general references are: "Colouring in Factories": H. I. Gloag: Factory Building Studies No. 8: London, 1961, H.M.S.O. "A Survey of Drawing Office Light-
ing Requirements": J. B. Collins ¦ and F. J. Langdon : Trans. Illum. Eng. Soc., 1960, 25(3), 87-114. The I.E.S. Code "Recommendations for Good Interior Lighting, 1961′′. London, Illuminating Engineering Society.
and the Digests, Nos. 70, 80, 81 and 135 (first series), already men tioned.
Prepared
at the Building Research Station, Garston, Herts. February. 1902.
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THE HONG KONG & FAR EAST BUILDER—VOLUME 17. NUMBER 1