Maintenance of light fittings
Dust, dirt and use will progressively reduce the light output of lamps. Attention to good general cleaning, maintenance, and a realistic lamp replacement policy will help maintain the standard of illuminance.
Factors affecting the quality of lighting
The eye has the faculty of adjusting itself to various conditions and to discriminating between detail and objects. This visual capacity takes time to adjust to changing conditions as, for example, when leaving a brightly lit workroom for a darkened passage. So, sudden changes of illuminance and excessive contrast between bright and dark areas of a workroom should be avoided. A recent problem, resulting from the introduction of word-processors and other equipment using video screen displays, is the effect on eye discomfort and general wellbeing of viewing screens for extended periods of time.
Glare
Glare causes discomfort or impairment of vision and can be considered at three separate levels, viz. disability glare, discomfort glare, and reflected glare.
Disability glare, or dazzle, impairs the ability to see clearly without necessarily causing personal discomfort. The undipped headlights of a car are an example of this.
Discomfort glare is a type of glare that causes visual discomfort without necessarily impairing the ability to see and may occur from unscreened windows in bright sunlight or when over-bright or unshaded lamps in the workroom are far too strong in terms of brightness for the workroom environment.
Reflected glare is the effect of light reflecting from a shiny or polished non-matt surface. The visual effect may be reduction of contract, or distortion, and can be both irritating and, in certain workplaces, dangerous.
Glare indices
The Illuminating Engineering Society (IES) has produced a glare index system for the numerical evaluation of direct glare. This has become the standard system in the UK and is also used in certain other countries.
A set of comprehensive tables has been produced by the IES giving glare indices for a wide range of situations, types of luminaires etc., and these should be referred to for specific advice. Examples of numerical values of maximum desirable glare indices range from about 19 for a general office to about 28 for industrial work. Figures above the recommended levels for a given location may lead to visual discomfort.
Protection from glare
The most common cause of glare results from looking directly at unscreened fluorescent tubes or incandescent lamps from normal viewing angles. Any form of diffuser fitted over the lamp, or suitably placed reflector used as a screen will help to reduce the effect of glare from a lamp. The minimum screening angle below the horizontal should be about 20°. Reflected glare can only be really eliminated by changing the offending shiny surface for a matt one.
Glare from sunlight coming through windows can be reduced by using exterior or interior blinds but this reduces the amount of natural lighting.
Effect of shadow
Shadow will effect the amount of illumination, and its impact on people in working area will depend on the task being performed, and on the disposition of desks, work benches, etc. The remedy is to use large luminaires or to increase their number.
Stroboscopic effect
Stroboscopic effect which gives the illusion of motion or non-motion as the case may be. For example, it was thought that the pulsating effect of alternating current operating at 50 Hz produced a 'flicker' which when imposed on rotting machinery would appear to indicate that the machinery was slowing down or even stationary. The danger of stroboscopic effect is self-evident but with modern fluorescent tubes the problem has been minimised by reduced flicker effect and should not cause problems. Wiring adjacent lighting tubes to different supply phases overcomes this effect.
Colour effect
The reflection of light falling on a coloured surface produces a coloured effect in which the amount of colour reflected depends upon the light source and the colour of the surface. For example, a red surface will only appear red if the incident light falling upon it contains red: under the pure yellow of sodium street lighting, for example, a red surface will appear brown. The choice of lamp is important if colour effect of 'warm' or 'cool' effect is required and can be as important a consideration as the degree of illuminance itself. Where accurate colour judgements have to be made the illuminance should not be less than 1000 lx.
Use of light measuring instruments
The human eye is unreliable as an indicator of how much light is present. For accurate results in the measure of illuminance at a surface it is necessary to use a reliable instrument. Light meters are available for this purpose.
A pocket light meter, normally adequate for most locations, is a photocell which responds to light falling upon it by generating a small electric current which deflects a pointer on a graduated scale measured in lux. Most light meters have a correcting factor built into their design to allow for using a filter when measuring different types of light (daylight, mercury vapour lamps, fluorescent tubes, etc.). The recommended procedure for taking measurements with a light meter is to:
- cover the cell with opaque material until the pointer is at zero on the scale or zero adjustment in accordance with the manufacturer's instructions.
- Allow a few minutes for the instrument to 'settle down' before taking a reading. A longer period will be required in the case of fluorescent tubes and discharge lamps which have just been switched on.
- Select the appropriate scale on the instrument, i.e. the one that gives the greatest pointer deflection within the limit of the scale. If the pointer moves to maximum a higher scale should have been chosen.
- If readings are to be taken during daylight two readings are necessary:
- with the lights on and with the window blinds drawn back so as to record the combined effect of natural and artificial light, and
- with the same natural light conditions as in (a) but with the artificial lights switched off.
The result required, i.e. the measure of the artificial light, is the difference between the two readings. If the two readings are large and approximately equal it will be necessary to re-check the artificial light reading after dark.
The level of illuminance measured should be checked against the appropriate standard service illuminance for the location and task. It is important to avoid great contrast and shadow, particularly when people are working or passing because of the visual adaptation problem of the eye. The correct use of a light meter is an important aid to establishing good levels of illuminance. However, to ensure accurate readings, the instrument should be kept in its case when not in use and away from damp and excessive heat.
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