Apart from making the home look more comfortable and welcoming, sufficient and suitable lighting makes all visual tasks easier. For example, it is much easier to read small print or do fine needlework if there is plenty of light on the task. Lighting can be so arranged so as not to produce excessive glare, and with the minimum of annoying reflections – for example, from the screen of the television or home computer or from a polished table top.
Of the many advances in lighting practice in recent years, very few have found their way into the home, and with little or no public education in the subject acceptance of a low standard of domestic lighting is common. Sadly, many people give it little thought beyond replacing failed bulbs.
It has been estimated that more than half of British homes have insufficient light even for ordinary visual purposes.
The optometrist has, therefore, both an opportunity and responsibility to inform patients of ways in which visual performance can be maximised by careful attention to basic lighting principles.
Unfortunately, many people believe that their existing lighting arrangements are satisfactory, and unlikely to need improvements or modification, simply because they are not aware that anything is wrong.
Few would disagree that the amount of light on a dull, overcast day is not very great and yet the average illumination in many homes at night is approximately one hundred times less. Because, however, of the eye's remarkable power of adaptation to different levels of brightness, it is rarely assumed that vision is affected to any great extent. In other words the pages of a book poorly illuminated by a single bulb situated in the middle of the ceiling may appear subjectively as bright as the same book seen near a window during daylight. However, even though most people with average visual capability can read with little difficulty during the day there will be many whose vision is seriously impaired when reading or performing more exacting visual tasks, with insufficient artificial light.
One of the problems of home lighting design is that the home contains a great variety of activities, the visual demands of which may be very different. For example, in the small house, the living room is used for highly exacting visual tasks such as sewing, often done by elderly people whose eyesight is not what it was. In the same room children may be studying whilst others may be watching television, all at the same time. The solution lies in ensuring that both the lighting and activities are flexible and adjustable.
Typical causes of complaint associated with domestic lighting
The most common reasons for visual difficulty in the home are :-
- inadequate light levels
- uneven brightness distribution between rooms and from one room to another
- excessive brightness between the task and immediate background
- shadows cast by poorly positioned lamps
- glare from naked or inadequately shielded lamps
- unsuitable room decor
- unsuitable, or sometimes merely dirty, spectacles
Every home needs a good general level of lighting for background and general purpose illumination together with local light on specific seeing tasks. Both are necessary. In other words, general and local lighting are distinct and complementary.
A satisfactory level of general room illumination can be achieved with overhead light fixtures, such as ceiling pendants, and some indirect sources such as floor and wall lamps. Uplighters are again becoming popular. Writing, reading or sewing needs good local lighting which can be achieved with a table lamp, floor lamp beside the chair, or a desk lamp.
Daylight is an important source of light which is frequently wasted. Curtains should draw well clear of the window so as not to obstruct the light. If daytime privacy is required only the very lightest net curtain should be used. For the same reason, window frames and glazing bars should be light in colour. It is important that windows, like spectacles, should be kept clean to transmit the greatest amount of light. It is also an advantage if the furniture can be arranged so that light from the windows provides maximum benefit, e.g. by falling on the task from over the shoulder. Table or floor lamps should then be positioned to serve this furniture layout or, if the electric light is fixed (i.e. ceiling pendant or wall brackets), an alternative reading position should be used when daylight is no longer adequate.
In the main decoration of walls and ceilings should be in light colours to reflect more light into the room and aid seeing.
What determines how much light is needed for any particular task?
Because the eyes have the ability to adapt to low illuminances, once the process of dark adaptation has taken place, it is possible to move about quite safely in an illuminance of a few lux such as, for example, in a bedroom lit only by the dim light from a 5 watt neon glow lamp used as a night light - an illuminance about the same as moonlight. Under such levels of illuminance, however, it would be impossible to read a newspaper, even though it may be possible to read the headlines. Under higher levels of illuminance, such as a well lit room, the eyes adapt to that level and it becomes possible to read the small print. In other words, the smaller the detail to be seen, the higher must be the illuminance provided on the task.
The main factors that determine the need for light are:
- the apparent size of detail to be seen i.e. the combination of the size of detail and the distance at which it must be seen. For example, it is more difficult to thread a needle at arm's length than nearer to the eyes.
- the contrast in reflection factor between the detail to be seen and its background. For example, it is more difficult to thread a needle with dark thread against a dark background than against a light one.
- the duration of the task. For example, it may just be possible to thread a needle under a low level of illuminance, but it would be very tiring to have to thread needles under this level of light hour after hour.
- the age of the individual. The amount of light needed for best vision will vary from person to person. Although most elderly and partially sighted people need relatively high light levels, remember that a few may benefit from less light (see later). Cataract, for example will differ in its effects depending on its size and position within the lens. If the cataract is located centrally, a high level of lighting will cause the pupil to contract perhaps to the extent that the cataract will prevent light entering the eye. Less light will cause the pupil to dilate and it may be possible to see 'around' the cataract. Similarly, for those with central corneal opacities there may be a need for less, not more light. In both cases, glare may cause a much greater problem.
It is therefore important to be flexible and take a trial and error approach when seeking a solution for the individual.
The Code for Interior Lighting published by the Lighting Division of the Chartered Institution of Building Services Engineers gives recommendations for illuminances needed for many kinds of interiors and tasks to be performed in them. It is important to note that the Code recommends that lighting levels should be increased by 50 to 100 per cent in old people's homes, or where the family includes elderly persons. The Code also shows that about six times more illuminance is needed for performing difficult visual tasks than for casual seeing.
The Table below gives suggestions as to the minimum quantity of light required for reasonable seeing of a variety of domestic tasks for people of different ages.
Class of Visual Task
Quantity of Light Required (Lux)
|Tasks with large details
|Fairly severe tasks
|Severe & prolonged tasks
||Sewing with dark material
In a specific location, the light can be described by its quantity and by its quality. Quantity is mathematically measurable but is by no means adequate on its own as a way of assessing a lighting scheme.
The intensity of a light source is measured in candelas. However, for practical design purposes, it is often necessary to assess the amount of light falling on a particular surface. This is known as illuminance
, and varies according to many factors – for example the number of lamps used, their total wattage, their distance from the surface, and whether or not they are modified by a shade, diffuser or similar method of control.
To express illuminance, a unit of light output is used – the lumen
. For a particular kind of interior, it can be stated that the recommended illuminance is so many lumens per metre. In metric units illuminance is measured in lumens per square metre, or “lux”
The light output of a 100W incandescent lamp is 1330 lumens
Let us assume that, because this lamp is contained in a shade, 15% of its light reaches the surface of a table top measuring 1.2m x 0.6m.
Light reaching the surface is 15% of 1330 lumens, which equals 200 lumens
The surface area of the table is 1.2m x 0.6m which equals 0.72m2
Since Illuminance = light arriving divided by the area of surface, 200 ÷ 72 = 275 lux.
Fairly severe tasks such as prolonged reading of small print or sewing require a much higher level of illumination compared with that needed for easy visual tasks. This can usually be achieved by increasing the power of the lamp or moving the lamp closer, where practical. The inverse square law states that the illumination on a surface varies with the power of the light illuminating the surface and inversely with the square of the distance away from the surface illuminated. Although, therefore, increasing the power of the light bulb from 100 to 200 watts would double the illumination, if the distance from the light source to the task was halved the illumination would be increase four times. Whenever inadequate or deficient lighting is suspected, bringing the light nearer to the task will demonstrate the dramatic increase in illuminance this can produce.
For example, from the Table below it can be seen that a 60 watt bulb in a simple reflector would give approximately 80 lux on a task at a distance of 100cm. If this bulb was contained in a lamp in the centre of a ceiling approximately 200 cm from the task (not uncommon in this country), from the inverse square law the illuminance produced would be only 20 lux!!
It is worth experimenting, therefore, with an additional source of light such as a table, floor, desk or angle-poise lamp, and alter its position, change the bulb to one of a higher wattage (but not beyond the manufacturer's recommended maximum), and reduce its distance from the task.
A very approximate, but useful, guide to the level of illuminance which can be expected from a 100 watt and 60 watt pearl bulb in a simple reflector, such as an angle-poise lamp, at various distances from a task is shown in the Table below.
The inverse square law applies only to very small sources such as filament bulbs in desk or table lamps, in which the illumination is very significantly affected by the distance of the source from the task. For comparatively large sources such as fluorescent tubes or opaque glass fittings (in which the light is diffused in all direction), the distance from the source to the task is much less important.
A reading lamp such as an angle-poise lamp has the advantage of flexibility of adjustment both for position and distance and can provide a very high level of light when required, although it does have a number of disadvantages such as instability (so needing a wide or heavy base), a tendency to 'sag', and difficulty in adjustment for those sitting in an armchair rather than at a desk. When used, particularly with a higher wattage tungsten filament bulb, the heat produced may be uncomfortable when adjusting the lamp or if it is positioned close to the face. Reading lamps using miniature or compact fluorescent lamps, however, operate at much lower temperatures ands since they give about five times as much light as the same wattage of filament lamp can produce reasonably high light levels without the problems associated with heat. For example, in terms of light output a 20-watt rating produces at least as much light as a 100-watt incandescent filament lamp.
When higher light levels are required a possible solution is to use an internally silvered spot lamp or reflector lamp to direct the light more effectively and enable the lamp to be further away from the task, so avoiding uncomfortable heat.
For fairly easy visual tasks where the contrast is good, such as casual reading, or tasks with large detail, the illumination necessary can often be provided by a table lamp with a 100-150 watt bulb or 20 watt compact fluorescent lamp 3 to 4 feet away depending, of course, upon the type and darkness of the lamp shade. A table lamp permits upward light to be reflected off the walls and ceiling and will brighten the room in general especially when the room surfaces are light in colour. Its disadvantage is that the pool of light beneath the lamp cannot be easily adjusted for the particular task and individual. A white lined shade will ensure that the maximum amount of light is reflected.
Seated in an easy chair, a floor lamp just behind one shoulder with a 150 watt bulb or 20 watt compact fluorescent lamp 4 to 5 feet away will also produce the desired illumination for casual reading.
Dark rooms and heavy shades require more wattage.
Lampshades with white linings reflect the maximum amount of light. Coloured shades can give unwanted colour casts. A large shade has the advantage that not only will it obstruct less light but can also cope better than a small one with the heat from the lamp. In the case of pendant fittings, shades which are open, or at least translucent at the top will allow light to reflect off the ceiling.
It should be remembered that inadequate light levels may be caused by dirty lamps, lampshades and fittings. Cleaning, therefore, is very important if full advantage is to be taken of the light which is being produced. This is especially so in kitchens, where grease and grime tend to accumulate more quickly.
A simple and inexpensive method of assessing the adequacy of lighting levels for near tasks at home, without the need for special instruments or techniques, is by using the Visibility Indicator Card devised by the author. (A full description of the Card has been given in Optometry Today, Nov. 20th 1989). Visibility Indicator Cards with notes on their use can be obtained from the College of Optometrists.
As well as getting the correct amount of light on the task, it is also important to ensure that light levels from room to room, from one area to the next in the same room and between the task and its immediate surroundings are not too uneven.
Dimly lit halls, landings, steps and cellars are threats to safety. Accidents in the home are more common at night because there is not enough light. Such areas are often dark in colour and do not reflect much light. Extra wattage can greatly enhance seeing and provide added safety. The need for a good even level of illumination throughout the home is particularly important for older people, whose eyes are slower to adapt to differences in brightness when walking, for example, from a brightly lit room to a semi-darkened staircase, corridor, bathroom or toilet where the greatest number of falling accidents occur.
Reading in a dark room with nothing but a desk lamp or bedside lamp can soon give rise to visual fatigue when the eyes try to cope with too great a variation in brightness between the white pages of a book and the dark background.
The same applies to watching television in a dark room where the contrast between the brightness of the screen and background is too great for visual comfort. The first TV sets were only capable of producing a low picture brightness and limited contrast which necessitated subdued room lighting in order to see the picture clearly. Unfortunately, this has led to the mistaken impression that television should be viewed in darkness or a very dimly lit room.
Modern television sets are capable of producing a picture bright enough to be seen in ordinary domestic lighting, although light fittings which cause reflections on the screen should be moved, switched off or the viewing position or TV position changed. There should be no light fittings close to the screen or, as is current fashion, on top of the television, which may cause glare or distraction.
During daytime it may be necessary to draw window curtains to screen off light which causes reflections on the screen or 'washes out' the picture or causes direct glare from the window in the line of sight, especially when it is not possible to position the television away from a window. In theory the average brightness of the picture should be just slightly brighter than the background wall against which it is viewed. These recommendations are of particular importance to those TV's incorporating a Teletext system or when viewing the screen of a home computer, where a much greater control over viewing conditions is demanded for visual comfort.
Glare is the discomfort experienced when part of the visual field is excessively bright compared with the brightness to which the eyes have become adapted. Sometimes people experiencing glare complain that there is 'too much light', but glare is, in fact, often more noticeable in a room that has a poor lighting level. In the home 'disability glare' which prevents seeing, is rarely encountered (e.g. glare from the bright headlights on an oncoming car at night). Much more commonly experienced is 'discomfort glare' which can be tiring and irritating, but which does not prevent seeing. Glare may be 'direct' or 'indirect'. An example of direct glare is that caused by looking at a bare lamp or a bright light fitting seen against a dark background.
Indirect glare occurs when an image, for example, of a bare lamp or bright lighting fitting is seen by reflection from a glossy surface, such as a polished table top.
No matter how good the light, it is wasted if it produces glare. No visible light source should be close to the line of sight and the reflections produced by such sources should not be directly visible to the eye.
Glare sources of any kind put the eyes in a dilemma. They are attracted to them and away from the task. Tension and eyestrain result from the conflict in attempting to rapidly adapt to levels of markedly different brightness.
The risk of discomfort glare increases as the brightness, number and size of the visible light sources approach the direction of the field of view. It reduces as the brightness of the background to the source increases.
Glare increases rapidly with the brightness of the source but only slowly with its size so by putting a simple diffusing shade round a naked light bulb the brightness will be reduced and the size increased in almost the same ratio. As a result glare will be reduced.
The effect of background brightness is also very important. A bright light seen against a dark surface always causes more glare than when seen against a light one. This is especially troublesome in daylight conditions when a bright sky is seen through a dark window wall. The effect can be reduced considerably by painting the window wall white or a light colour.
Although general room illumination brings up the brightness of the surroundings it may take more than light to obtain a good balance for optimum seeing conditions. Dark walls, floors, desks and tables absorb light. Any room can be improved for visibility if it is made lighter. Generally speaking it is impossible to have too much light in a room - only too much shadow or unpleasant glare. In a room with light decorations only about half the power is likely to be required to achieve a given lighting level than in a room with dark decorations. However, for large surfaces glossy paint should be avoided since it reflects light sources both of windows and light fittings and gives most irritating and distracting glare spots. A careful choice of decoration, therefore, should be made. Dark and large, highly reflective surfaces should be covered and paper or paint should be used to brighten up critical seeing areas and reduce hazards in dimly lit spaces. Light coloured blotting paper or table cloth on a dark table or desk can enhance visibility considerably.
It is also important to remember that most of the light that enters our eyes has not come directly from its source but has been reflected from the ceiling and walls and from objects in the room. Decor and colour schemes consequently have an enormous contribution to make to the amount of light available in the home.
Lighting for the Partially Sighted
Lighting for persons with partial sight requires a knowledge of the conditions that caused the disability. In most cases, the principles of lighting for persons with special visual needs are not very different from those used for the normally sighted, provided the rules of good lighting practice are followed. The main differences are in the amount of light required for critical viewing tasks (often many times more than normal), the need for very careful attention to sources of disability and discomfort glare, and the problem of lighting the task when optical aids such as stand magnifiers or spectacle magnifiers are used. Educating the patient in the correct use of any supplementary lighting is very important since incorrect usage can often make the situation worse.
For the purpose of lighting design, partially sighted patients can be grouped according to the cause of their low vision:
- Those whose problem is caused by pre-retinal anomalies, which reduce the contrast of the retinal image through increased intra-ocular scatter, such as corneal scarring, cataract or vitreous opacities.
- Those whose problem is caused by retinal injury, pathology or degeneration, which reduce the resolving power or contrast sensitivity of the eye, especially at high spatial frequencies, such as age-related maculopathy, diabetic and hypertensive retinopathy, or other similar conditions.
Persons in the first group are often helped most by increasing the physical contrast (brightness difference of the details of the task and its surroundings), while limiting the amount of light entering the eye. Because the problem is primarily one of increased disability glare and the amount of light available to be scattered within the eye, simply increasing the illuminance is often counterproductive. Because people in this group often have normal retinas and contrast sensitivity, the problem is not one of insufficient light but rather one of too much light going to the wrong places. The key to successful lighting design for such persons is careful light control and, if possible, appropriate task selection. Some of the techniques that can be used to improve the visual performance of this group are:
- Removal of obvious and unnecessary sources of glare such as unshielded windows, lamps and luminaires. The use of curtains, sharp cut-off luminaires, indirect lighting, and baffles are helpful.
- Use of high contrast print and chromatic contrast to permit lower illuminances and, therefore, reduce the amount of light entering the eye.
- Elimination of contrast-reducing veiling reflections by positioning light sources to avoid specular reflections in the direction of the eyes and by using diffusely reflecting materials where possible.
- Reduction of the amount of light reflected from the task by using reverse contrast (light on dark) or apertures in low reflectance backgrounds (such as the Typoscope) or by illuminating only the portion of the task being viewed by controlling the source with optics and/or baffles.
Persons in the second group are usually suffering from retinal disease or injury that has reduced the ability of the visual system to process information. The problem is to maximise the information-processing abilities of the undamaged visual elements. This may require considerably more light than is necessary for the normally functioning retina, and usually requires supplementary task lighting, very careful glare control, and often the use of innovative methods to deliver the light on the task. Because people with macular problems often use magnifying aids, getting the light between the device and the task is a special problem.
Solutions to the need for high illuminances nearly always include a lamp incorporated in an optical system, either dioptric or reflective. Without some sort of optical control, the lamp power required and the glare created is simply too great. Lamps of small physical size, such as tungsten-halogen incandescent lamps are best suited for these applications. Small, low voltage (12V) halogen lamps with integral dichroic reflectors have proven to be very effective in such cases. They are compact, readily available through commercial lamps distributors and some high street stores, and come in a variety of beam widths and light outputs to suit nearly any working distance. In some cases, rechargeable batteries can be used where portability is important. From a safety point of view, however, heat from the lamp is often an undesirable feature, and since such sources produce large amounts of ultra-violet radiation they must be used with a cover glass.
In many cases, especially among the elderly, a combination of the above two types of visual impairment is found and the management of their visual problems remain a challenge.