Off roadroll Bar lights

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.

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.

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 ).

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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.

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.

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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.

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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).

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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.

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.

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.

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. Fig. 11.1 The illumination by a light source onto a working surface. As the distance of the working surface from the light doubles from d to 2 d , the area illuminated increases by a factor of four. The area illuminated also increases (and so illuminance decreases) when it is tilted by an angle α . Combining these two relationships, it is clear that