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of working surfaces. In many instances, the recommended standards of lighting in lumens
would be misleading if glare (dealt with in the following paragraphs) were not also considered.
(4) Instances ofexcessive glare that have been noticedand the standards which have
been used in assessing it.
The majority of fluorescent lamps in offices and shops are unshaded and many
instances of minor degrees of glare have been observed. As a first step in assessing glare
from light fittings, the light fittings themselves are viewed within an angle of 20 degrees
from the horizontal at eye-level intheworking position itself. The brightness of the surroundings
of the working position in relation to the field of view is next assessed. Agood impression
of the degree of glare and the relationship of lamps to interior decoration can often be
obtained by viewing the room or shop from outside in the dusk or dark. The rooms in which
the interior decoration and the light from the fittings are not in balance are immediately
obvious.
A simple device has been produced in order to give inspectors experience in the
degrees of brightness and reflectivity of surfaces. A piece of card, matt black on one side,
and white (reflection factor 75%) on the other, is placed on the surface whose brightness
is to be assessed. The light-meter used in this Department has a light-cell which can be
moved through a quadrant, and it can therefore be read at 90degrees to the source of light.
It is held at a distance of 6" from the surface being assessed, and a reading is taken with
the black side of the card in position, on the surface. The card is then turned over, and
another reading taken with the white side of the card in the same position. The card is then
removed and reading taken from the surface itself. The degree of reflectivity of the surface
can be calculated from the difference between the readings from the black and white sides
of the card, having in mind that the white surface of the card represents 75%.
Other useful aids have been found in the British Standards for Ready-mixed
paints, and Decorative and Building paints, the Standard Colours and luminance factors.
The worst instance of glare was in an office where the lighting source was a
completely diffused ceiling giving an even illumination of 60 lumens per sq.ft. The window
wall was white, and the other three walls were grey, dark green, and teak panelled. The
carpet was dark green. The desk-top was off-white, and the reflected glare from the desktop
caused headaches in the staff using the desk.
In shops there were a few instances of what may be termed "cash-desk glare",
in which the brightness of thecash-desk or enquiry counter is much less than the illumination
from the light fittings in the selling area visible from the occupant's seat at the desk, and
this type of glare causes discomfort. In one shop, the discomfort caused by this type of
glare was pointed out to the area manager, and the following week-end the light fittings
of the whole shop were re-located, despite the fact that the level of illumination over the
counters had previously been 50 lumens per sq. ft.
The Glare Indices listed in the Codeof the Illuminating Engineering Society are
therefore always quoted alongside the standards in lumens per sq.ft. in advice and correspondence
to occupiers.
Paragraph 3
Detailed information about lighting in premises inspected during November,
1965:-
(1) The number of office premises found where lighting, either natural or artificial,
in lumens per square foot, at the working place (or working plane) where work
is done was:-
(a) less than 5 3
(b) more than 5 but less than 10 18
(c) more than 10 but less than 15 22
(d) more than 15 but less than 25 44
(e) more than 25 104
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