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Pt.9: The Basics Of Luminaires Electric Lighting 86  Silicon Chip Very few lamps are suspended naked in space. For aes­thetic and functional reasons, the lamp is usually mounted within a fixture – a “luminaire” in lighting parlance. The design of the luminaire has a major bearing on its luminous intensity, durabil­ity and appearance. By JULIAN EDGAR ABOVE & FACING PAGE: The same scene, by day and by night. The luminaires used to illuminate this road are the B2224 Series, manufact­ured by Sylva­nia. They use a diecast aluminium chassis, polyethylene injec­tion moulded canopy and acrylic refractor. The refractor is secured with aluminium screws and the optics are silicone gasket sealed. During lamp changes, a wire lanyard holds the refractor in its open position. In this application the luminaires have been fitted with 80W mercury vapour lamps. In addition to having a pleasing appearance, a luminaire must: •  provide electrical connection to the lamp(s);   physically protect the lamp(s); • •  control and distribute the light from the lamp(s);   be • robust; and •  be efficient in use. The wiring used inside a luminaire is normally of the solid core type. Because of its stiffness, fewer ties are needed to hold solid core wire in position and it is easily stripped of insulation. However, where the luminaire is subject to vibration or if the wire will be frequently bent (eg, in an adjustable spotlight), stranded wire is used. The ability of the wiring’s insulation to withstand high temperatures is very important. Not only is the temperature of the air within the luminaire likely to be elevated above ambient but components such as ballasts and lamp holders can become very hot. Generally, PVC insulation with a heat rating of 90°C, 105°C or 115°C is used. In high-intensity discharge floodlights, even higher temperatures may be present. In these lights, silicone rubber (170-200°C) and PTFE (250°C) insulation is used, sometimes with glass-fibre sleeves. Protection of the lamp is also important in many situa­tions. If the lamp is to be used outdoors, for example, the luminaire must prevent the ingress of dust and moisture. As well, it may also be designed to protect the lamp against physical damage; eg, from children playing ball or from vandalism. Dust and moisture protection re- quires that the lamp be fully enclosed, with a light-transmitting front panel fitted. So that the lamp can be changed when it fails, the cover needs to be detachable, necessitating the use of a seal around its aperture. Seals can be made of felt, silicone rubber, norprene, or neo­prene. Fig.1 shows two different sealing methods. Protection against accidental damage and vandalism can be obtained by covering the front face of the This indoors luminaire is designed to add to the appearance of the lamp and to provide a broad spread of light. The lamp should be cleaned at regular intervals, to maintain light output. November 1998  87 luminaire with stain­less steel mesh or by making the luminaire of polycarbonate. This very tough material is available in clear and coloured forms, making it suitable for all parts of the fitting. Light control (a) (b) Fig.1: luminaires located outdoors use sealing mechan­isms that allow them to remain weatherproof while still allowing the lamps to be easily changed. Fig.1(a) shows the waterproof edge seal used in a fluorescent luminaire, while Fig.1(b) shows the notched rubber seal used in a floodlight. (Philips Lighting Manual). Fig.2: a circular reflector gives a broad spread of illuminance when the light source is at the focus, as depicted here. (Philips Lighting Manual). (a) Fig.3: a parabolic reflector with the light source placed at the focus produces a parallel beam of reflected rays. (Philips Light­ing Manual). (b) Fig.4: combined spherical and parabolic reflectors are generally used in the two configurations shown here. In both cases, the spherical reflector diffuses the light from the source prior to reflection off the parabolic portion of the reflector. (Philips Lighting Manual). Fig.5: an elliptical reflector with the light source placed in front of the focus gives a “pinhole” effect and is commonly used in downlights. (Philips Lighting Manual). Optical light control systems range from those that produce an even, well-distributed light to those that direct a defined beam in one direction. Optical devices that are commonly used include: •  reflectors; •  refractors and diffusers; and •  screening devices. There are three different types of reflectors: specular, spread and diffuse. Specular reflectors use a mirror-like sur­face. Materials used in such luminaires include anodised alumini­um, aluminised glass and aluminised plastics. Alternatively, commercial grade aluminium can be clad with a thin layer of very pure aluminium or silver, giving a finish with reflectances of up to 80% and 90% respectively. These reflectors are used where a precise form of light distribution is required, such as in floodlights, spotlights and road lighting luminaires. A number of different shaped specular reflectors are used, including: circular reflectors (Fig.2), parabolic reflectors (Fig.3), combined spherical and parabolic reflectors (Fig.4), and elliptical reflectors (Fig.5). Unlike specular reflectors, spread reflectors do not give a mirror image of the source but the angle of maximum reflected intensity still equals the angle of incidence. A spread reflector gives a very even distribution of light, with the reflecting surface de-emphasising any hot spots caused by manufacturing inaccuracies in the shape of the reflector. Spread reflectors are commonly made from polished alumini­ u m, hammered or moulded into a pattern consisting of small bumps or dimples. Alternatively, the aluminium can be brushed. The spread reflector is used where an even light distribution is required. Diffuse reflectors Diffuse reflectors scatter the light widely. The shape of the reflector has only a general bearing on the resulting light distribution, so sharp beam control is not possible. Diffuse reflectors 88  Silicon Chip are cheaply produced using glossy white-painted steel or white-coloured plastic. This type of reflector is commonly fitted to fluorescent luminaires. Refractors are used to control the direction of the light emitted by the lamp(s), primarily to stop glare. Glare occurs in the viewing angle between 45° and 90° to the vertical axis beneath the luminaire – see Fig.6. Refractors reduce the illuminance in this glare zone, directing the light down rather than out­wards. Most fluorescent luminaires use a refractor consisting of an acrylic or polystyrene panel that is smooth on top and has many small conical prisms on the underside. The refractor fitted to a 2-lamp fluorescent luminaire can have as many as 5000 prisms moulded into it. of the beam is sometimes blocked by a baffle. Luminous intensity distribution Fig.6: direct glare from a lumin­aire is most likely to be a problem at an angle of 45-90° from the vert­-ical. (Murdoch, J., Illuminat­ion Engineering). Screening devices An alternative approach to controlling glare is to use screening devices such as louvres or baffles. These are often used in fluorescent luminaires and Fig.7 shows the screening effect of the reflector used in such a luminaire. Another ap­proach is to recess the luminaire into the ceiling so that the lamp(s) cannot be seen from directions where glare could be a problem. In floodlights, spill light to one side Fig.7: the amount of glare can be considerably reduced by using a screen to obscure the light source. (de Boer, J & Fischer, D., Interior Lighting). Manufacturers generally produce a great deal of photometric data for their luminaires, with luminous intensity distribution being one of the most important. The luminous intensity distribu­ tion curve reflects (pun intended!) the design of the luminaire, being affected by the combination of direct, reflected and dif­ fused light emanating from the luminaire. Fig.8 shows a Sylvania Indy Series luminaire. This large luminaire is designed for high mounting in warehouses, loading bays and industrial plants. It is 545mm high and its spun alumin­ ium elliptical reflector has an external diameter of 420mm. The luminaire can be fitted with lamps of up to 400 watts. The luminous intensity distribution curve is shown in Fig.9 and this shows that most of the light is directed downwards, with very little illuminance at more than 40° from the vertical. As you would expect with a round reflector, the luminous intensity distribution of this luminaire is symmetrical around its vertical axis. A luminaire which is superficially similar in appearance is shown in Fig.10. This is a Sylvania Sylvaglow, designed for mounting at relatively low heights, again in warehouses, facto­ries and so on. Unlike the previous unit however, it uses a combination of spherical and parabolic specular reflectors and is fitted with a diffuser. The luminous intensity distribution curve (Fig.11) shows that the illuminance spread from this luminaire is wider than for the previous case, with effective illumination at up to 50° from the vertical axis. However, the values of luminous intensity are well down over the other Sylvania luminaire, with luminous in­ tensity being traded off against the luminaire’s spread. Light loss If uncleaned for three years, an indirect up-light in a dirty environment will typically have its light output reduced by 55%! It is the average maintained luminance that is the critical factor in assessing the effectiveness of a lighting installation. Light loss occurs through four different factors: •  lamp burn-outs; •  lamp lumen depreciation; •  luminaire dirt depreciation; and November 1998  89 Fig.8: the Sylvania Indy Series luminaire is designed for high mounting in warehouses, loading bays and industrial plants. It uses a spun aluminium spherical reflector and can be fitted with lamps of up to 400 watts. (Sylvania). Fig.9: the luminous intensity distribution curve of the Sylvania Indy shows that most of the light is directed downwards, with very little illuminance at more than 40 from the vertical. (Sylvania). Fig.10: the Sylvania Sylvaglow is designed to be mounted at relatively low heights. It uses a combination of spherical and parabolic specular reflectors and is fitted with a diffuser. (Sylvania). Fig.11: the luminous intensity distribution curve of the Sylva­glow shows that the spread of illuminance is wider than for the Indy. However, the luminous intensity values are much less. (Sylvania). •  room surface dirt depreciation. If a burnt-out lamp isn’t immediately replaced, there will obviously be a noticeable decrease in luminance. In some situa­tions, where replacement may not be immediate, the lighting design needs to take this into account. Even prior to failure, the luminous flux of the lamps will have decreased compared to their new output. As it approaches the end of its life, an incandescent lamp will typically have a luminous flux of only 78-90% of its 90  Silicon Chip “new” figure, while a fluo­rescent lamp may be down to 72%. From this, it follows that if the illuminance is only just acceptable when the lamps are new, it will be quite unacceptable after a few thousand hours of operation. Dirt build-up However, it is dirt build-up on the luminaire that is the single greatest cause of light loss. The rate at which the light output decreases is dependent on the room cleanliness, luminaire design and, of course, on the frequency and thoroughness of lumi­naire cleaning. As an indication, a bare lamp batten in a dirty environment will typically show a decease in light output of 17% after a year without cleaning! An even worse-case scenario is an indirect uplight in a dirty environment. If it’s uncleaned for three years; its light output will typically decrease by about 55%. Make sure that you regularly clean SC your luminaires!