Terminology
The amount of light emitted by a light source is called the luminous flx and is measured in lumens . The efficacy of a particular light source is the quantity of luminous flx which is created by a given input of electrical energy, and this is expressed in lumens per watt . This light now spreads out from the source, and the quantity of light hitting the working surface or task is described as the illuminance, which is defined as the amount of light per unit area. It is measured in lumens per square metre, which are also called lux (lx) . Consider a light source emitting a particular amount of light—luminous flx, measured in lumens—and illuminating the working area from a distance d . If the light source is moved further away from the surface, then the area it illuminates (the area over which the amount of light is spread) will increase. As the distance doubles, the area illuminated increases fourfold, and thus the illuminance decreases by a factor of 4. This represents the inverse square law: illuminance of an object is inversely proportional to the square of the distance of the light source from that object. Illuminance of the surface decreases if it is tilted, because this also increases the area to be illuminated ( Fig. 11.1 ). If the surface is tilted by an angle α (or the light source is placed at an angle α with respect to a perpendicular to the surface) the illuminance will be proportional to the cosine of angle α : this is the cosine law.
Combining these two relationships, it is clear that
illuminance=intensity×cosαd2
Thus, the maximum illuminance is obtained by having the most intense light source, placed as close as possible to the task and perpendicular to the surface rather than obliquely: distance is the most significant factor in determining the illuminance in a given situation. The formula given only applies to the direct illumination from a point source: indirect illumination by reflection can make a significant contribution to the illuminance created by extended sources if the distance from the working plane is greater than 5× the size of the light source.
Luminance describes the intensity of light (‘brightness’) emitted or reflected in a particular direction by an area which is either self-luminous or is reflecting incident light. It is measured in candelas per square metre (cd/m 2 ). Reflections of a light source from a shiny, smooth surface will all be in the same direction (specular reflection) but from a rough matt surface the reflected rays will be in all directions (diffuse reflection). Diffuse reflection is necessary as that is what allows us to see the object, but it is usually undesirable to have specular reflection from a task because this will obscure the detail near the reflection: it can be impossible, for example, to read text from glossy paper with the light in certain directions.
A ‘black body’ is a theoretical object which absorbs all radiation which hits its surface. It only emits light when it is heated: when heated to a specific temperature, it emits light of a particular colour, ranging from reddish white (corresponding to a low colour temperature) to blueish white (a high colour temperature). The colour temperatures typically seen in white lights range from around 2800 (red/orange—a ‘warm’ colour) to 6500 K (blue—a ‘cool’ colour).
Types of Domestic Lighting
Until recently, incandescent filament lamps with their characteristic pear-shaped envelopes of soda–silica–lime glass were the most common form of household lighting. Due to their high energy use, these bulbs are no longer sold in Europe. In these bulbs, a tungsten filament is heated and an inert gas fills the envelope to help slow the evaporation of tungsten from the filament. This increases bulb life and prevents blackening of the inside of the glass (which would reduce light output). Clear glass envelopes can give harsh shadows and act as a glare source, so it is more usual to have a frosted ‘pearl’ finish to the glass to diffuse the light without significant loss of brightness. The efficacy of incandescent lamps is approximately 10 lm/W, being higher for higher wattage lamps. This is a very poor rating, with a lot of energy being wasted as heat, but the lamps are very cheap, small and compact, relatively long-lasting and require only simple electronic circuitry. Light output is biased towards longer wavelengths, and this gives a ‘warm’ light which is favoured for household use.
If a halogen gas is introduced into the bulb to create a tungsten halogen lamp, the filament can be operated at higher temperature to give greater efficacy (up to 25 lm/W). This causes more evaporation of tungsten from the filament, but this combines with the halogen and is redeposited on the filament, rather than on the inside of the glass, leading to increased bulb life. If of high wattage, the envelope is made of fused silica or quartz and can deteriorate if touched with the fingers when oil or moisture can be transferred. Low wattage versions use aluminophosphate glass, but problems of uneven heating and subsequent failure can still occur if the glass envelope is touched.
Tubular fluorescent lamps could also be described as low pressure mercury discharge lamps. An electrical discharge passed through the mercury gas causes its atoms to lose electrons (become ionised) which collide with other atoms. These collisions cause further ionisation, or the absorption of energy with the result that some electrons are raised to a higher energy state. As these fall back, the energy is emitted in the form of visible and ultraviolet (UV) radiation. The latter is absorbed by the phosphor coating on the inside of the envelope and re-emitted in the form of visible radiation. The radiation emitted from the mercury is at certain discrete wavelengths, but the spectral composition can be broadened by careful choice of these phosphors. Because the output of short-wavelength light is increased over that produced by incandescent lamps, some people consider fluorescent lighting to be too ‘cold’ for household use. These lamps have an efficacy of at least 40 to 60 lm/W, thus using about one-quarter the power to achieve the same luminous flx compared to incandescent lamps. They also require much less frequent replacement. Some control circuitry is required to limit the electrical current through the lamp, and this can add to the physical size and weight of the installation. Compact fluorescent lamps are available where the long discharge tube is folded or bent into a circular or spiral configuration. The circular tube can be arranged around the large diameter lens in a variable-focus stand magnifier. Limiting the size using the spiral configuration allows it to be used as an energy-saving replacement for an incandescent filament lamp, but this is often not successful because the shade has been designed for an incandescent envelope which gives its maximum intensity straight down, whereas the fluorescent tube emits maximum intensity sideways. It takes up to 3 minutes for the older compact fluorescent lamps to reach maximum brightness from a starting brightness at switch-on of 50% of the maximum: if used for ambient lighting, especially on staircases or corridors where the occupant is passing through, this could create a hazard. The compact fluorescent is extremely successful, however, in purpose-made localised task lighting. The high efficacy means that there is little energy lost as heat, so that the lamp housing does not get as hot as would that surrounding an incandescent bulb. This means that the patient can place their head very close to the lamp without discomfort and can grasp the housing to adjust it without risking burning their hand. However, compact lamps without the covering envelope should not be used closer than 30 cm for more than 1 hour per day, due to a UV hazard.
Light-emitting diode (LED) lamps are becoming consistently more available and for all types of light fittings, rather than those specifically designed for them. They are extremely low power (2 W) so represent an exceptionally efficient lighting system (more so even than fluorescent lamps). The lifetime of these lamps can be up to several years, which is important for a user with visual impairment, due to the practical difficulty for them of changing a failed unit (and the potential safety issue of reaching a wall or ceiling luminaire). LED sources are often in a sealed light fitting, so when the lamp fails, the whole fitting needs to be replaced rather than just changing the LED. These lamps do not get very hot so there is less danger of the patient burning their hands, or there being a fire hazard. The LED lamps can be made to emit various ‘white lights’ and is now common to also see these used in illuminated hand-held and stand magnifiers. It is possible to have white LED light created by using combinations of red, green and blue LEDs. LEDs offer white light with a variety of colour temperatures by mixing the amount of red, green and blue LEDs that are being used to create each of these different light sources. Schweizer magnifiers, for example, are available with three alternative colour temperatures: 2700 K which is the incandescent equivalent, 4500 K is the fluorescent equivalent and 6000 K which is a bluish light (i.e. overcast sky in the northern hemisphere).
Some LEDs have the facility to tune the colour temperature to individual preference across the range from 2700 to 6500 K. On a table lamp, this may be a manual control, but ‘smartbulbs’ which can have colour temperature and brightness controlled by a smartphone app, or voice controlled via a digital assistant, are also available. The low power of LEDs means batteries last a long time in these types of magnifiers. Unlike incandescent lamps, where the light gets dimmer as the batteries lose their power, LEDs maintain their brightness over time until the batteries have not got enough power to work them and then the light stops working altogether.
Even table-top LED lamps can be battery operated or connected to a USB socket which means the patient can move them to wherever they are required (or even take them on holiday). LEDs are also available on adhesive strips, which are a cheaper alternative to having lighting installed under kitchen wall cupboards to illuminate the worktop, or inside wardrobes to help in selecting clothes. A miniature LED lamp is also available which can be attached to the side of a spectacle frame to illuminate the reading task ( Fig. 7.28 ).
The design of the luminaire—the housing for the lamp—can be just as important as the light source itself: it controls the amount and direction of the light output as well as offering a simple physical support, the electricity supply and a means of heat dissipation for the lamp. The bare lamp envelope does not necessarily emit light in the required direction, and may also create a glare source if viewed directly, so the lamp housing can be used to control the light. This can be done by obstruction, diffusion, refraction, reflection, or any combination of these. Obstruction is used when the lamp is surrounded by an opaque material which prevents light being emitted in that direction. Light is then only emitted through a limited aperture in the shade—usually at the bottom, and sometimes at the top of a ceiling-mounted lamp in order to create diffuse reflection from the ceiling. Diffusion occurs when a translucent cover is placed over the light, increasing the spread of the light but also usually absorbing a considerable proportion of it. The lamp covering can be made in the form of multiple prismatic elements to refract the light and redirect it into the required position. Reflection of light from the inside of the luminaire is also an extremely efficient way of deflecting all the light into the required direction. At its most extreme, the reflecting surface is specially shaped and highly polished to maximise the effect (such as in car headlamps), but it is frequently used less dramatically by the inside surface of a lampshade having a matt white finish. Dirt and deterioration of the luminaire surfaces can cause light loss over time.
The illuminance on surfaces within a room also depends on the décor. If walls and ceiling are pale, they have high reflectance, then a specific light source creates a greater task illuminance than if the surroundings were dark. If light from a luminaire is directed towards the ceiling, then the ceiling must be light in order to reflect that light into the room.
Visual Performance and Lighting
Based on the investigations by , Fig. 11.2 shows schematically how an observer’s ability to perform a visual task increases with improvements in the task illuminance: this effect is more dramatic for old compared to young subjects. This general pattern of response can be found with a wide variety of tasks (ranging from laboratory-based studies of searching for a Landolt C of particular orientation among an array of letters of other orientations, to a ‘real-life’ task of scanning components on a conveyor belt looking for those which are incorrectly manufactured), and with a variety of measures of performance (such as the numbers of errors made, or the time taken to perform a search). If the visual task is very easy—using large objects of high contrast—there will be very little difference between the performance of the different age groups, and the response will appear as in point (c) in Fig. 11.2 even at relatively low luminance. If the detail within the task is small, and contrast is low, the characteristic response is that at point (a) of Fig. 11.2 , and the illuminance must be increased to produce an improvement.
Increasing the task illuminance cannot compensate completely for the small size and low contrast of difficult visual tasks, however, and Fig. 11.3 shows that changing the size of the task detail is more effective ( ). Thus, the larger size letters always support a better performance, even when illumination is optimised, and the performance with low-contrast targets cannot be improved to match that produced by high contrast letters (although for medium-contrast levels, it can be brought close to it). It is also clear that whilst large increases in performance can be created by improving the contrast, these are not so great as the effects achieved with increases in the letter size (compare the improvement in changing the 1.5 min arc target from a contrast of 0.56 to 0.97, and note that it is less than the improvement of increasing the size to 3 min arc, whilst maintaining 0.56 contrast). Extrapolating these findings to low vision, it can be seen that increasing the illumination is not a replacement for magnification of the image, but only a supplement to it: no matter how much the illumination is increased, it does not bring the performance of a visually demanding task (small detail, low contrast) up to the level of a visually easy task. An increase in illuminance will produce a greater improvement in performance on a near-threshold task than on a visually easy task, and the low-vision patient is much more likely to be working near to their visual threshold. Magnifiers can offer a much greater range of improvement in performance compared to lighting alone, but performance will still be limited for large letters if the illumination is suboptimal: no magnifier will produce optimum performance without sufficient light.
Older people are likely to gain more benefit from improved task illuminance than the younger age group. The performance of these two groups can be equated if the illuminance is high enough, and it is suggested that the decrease in the amount of light reaching the retina is the cause of the poorer performance in the elderly subjects. There is increased absorption and scattering of light by the ocular media with advancing age, in addition to senile miosis ( ). reported a threefold decrease in the amount of light reaching the retina of a 60-year-old compared to that of a 20-year-old: describe even more dramatically the 22-fold decrease in transmission of light of wavelength 400 nm by the ocular media between the ages of 1 month and 70 years.
Thus, people performing difficult visual tasks (and a given task will always be more difficult for the low vision patient as it will be nearer to the limit of their ability) require the highest level of illumination. A further consideration of Fig. 11.2 suggests, however, that there are limits to how high this illuminance can be raised. In point (d), the performance has reached an optimum plateau for both age groups, but it may well decrease due to glare if excessive illumination is used. There is also an increase in the amount of light scatter by the ‘normal’ crystalline lens after the age of 40 years which will contribute to a loss of contrast of the retinal image, even if the object itself is of high contrast. Thus, the decreasing performance with excessive illuminance is represented by point (e), showing that the effect is likely to be more marked in the older subjects. For some low-vision patients, the plateau (c) may not be reached: performance may be affected by glare even at modest levels of illumination.
The effect of slowed adaptation in older eyes is also dramatic: an object has to be 10× brighter to be seen by an 85-year-old compared to a 20-year-old after an equivalent period of dark adaptation ( ). Light adaptation (going from the dimmer indoor environment to bright outdoor space) is also slowed. So uniformity of illumination, and gradual transitions in illuminance level, are very important to individuals with visual impairment.
Nonvisual Effects of Light
As well as the rod and cone photoreceptors, the retina contains intrinsically photosensitive retinal ganglion cells (ipRGC) which contain a photopigment called melanopsin. This has a peak absorption around 470 nm, and the ipRGC are part of the signalling pathway which sets the body clock to its 24-hour cycle (circadian rhythm). The exposure to ‘daylight’—intense blue light especially in the morning—increases the production of melatonin towards the evening time, and this causes sleepiness. Some visually impaired individuals with retinal disease may have an abnormality in the circadian system because their ipRGC are also affected, and they experience sleep disturbance, which can have further consequences for physical and mental health. However, some individuals with total vision loss can have a normal sleep-wake cycle because their circadian receptors are preserved ( ). To try to reduce sleep disturbance, individuals should have exposure to high light levels (and preferably natural daylight) during the early part of the day. If they need a light to be on continuously at night (in case they need to get out of bed), this should be a red/amber light to avoid stimulating the ipRGC. Melatonin tablets are sometimes prescribed to be taken at night, to regulate the sleep-wake cycle.
Lighting Levels for Older and Visually Impaired People
Experimental data obtained by several researchers in a variety of ‘performance versus illuminance’ studies allowed a determination of the level of lighting required to optimally detect a target of a particular size and contrast. When these studies are applied to subjects with ‘normal’ vision, the absolute level of illuminance which will allow a task of ‘normal’ size and contrast to be performed efficiently and safely can be determined, and these results have influenced the lighting codes developed in various countries. These standards usually relate to the working environment, with regulations for domestic lighting being based on energy efficiency. The Illuminating Engineering Society Aged and Partially Sighted Committee have, however, published detailed recommendations on lighting design for these populations, and selected illuminance values are shown in Table 11.1 . Separate values are given for the overall ambient space (A) and the specific task (T). It should be emphasized that these are minimum levels, and may need to be increased in specific cases, especially if visual impairment is more severe.
