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Electric Lighting Pt.10: Automotive Lighting The design and construction of lights used in cars – especially headlights – has changed considerably over the years. This month we look at current headlight designs, while in the next issue we will examine automotive High Intensity Discharge lighting. JJANUARY anuary 1999  73 Sealed beams reduced the problem of glass blackening and being pre-focused assemblies, gave more consistent results than earlier designs. the whole lamp body with systems using manual levers and even pneumatics to do this. Electric switching of filaments to dip the beam was introduced in the 1930s. However, this was different to the present system – in the dipped position one headlamp was extinguished and the other mechanically dipped by means of a solenoid. Twin filament bulbs allowing the pure electrical dipping of lights were introduced in the 1940s. Sealed Beams Since vehicles have been driven at night there has been a need for effective illumination of the road ahead. Very early cars used lamps of polished brass and copper that contained a single candle. However, they could scarcely light the way of the man walking in front carrying the red flag! This type of lamp was replaced with lamps burning oil and in some cases petrol, common until about 1910 when acetylene designs became popular. Early Lamps The acetylene lamp used two containers mounted one above the other. The lower one was filled with carbide in solid form; the upper one contained water which was dripped onto the carbide, with the flow regulated by a needle valve. The ensuing chemical reaction released acetylene gas which was transferred to the lamp itself through a tube. Here it burnt with a bright green flame. Some models of this type of lamp even had a primitive dipping function! The first electric headlights were powered by non-rechargeable batteries with quite limited life. The light output of these lamps was little better than oil or candle lamps, which meant they made little headway against acetylene lamps. Only the introduction of the generator saw the popularity of acetylene lamps begin to wane. Early automotive electric lighting systems used a constant current dynamo complete with a magnetic cutout which disconnected the dynamo from the battery when it rotated too slowly to charge. A typical battery of the time was described as a “12 volt 40 Actual Ampere Hour Accumulator”. Headlights ranged in diameter from 12.7cm (5-inch) to 33cm (13-inch), with systems normally incorporating a switchboard complete with ammeter and voltmeter. Some lamps were even available with sealed, gas-filled reflectors plated in either silver or gold. The brightness of these lights meant that a dipping system was needed. This generally took the form of moving Many low beam headlights use a capped bulb. The cap shades the bottom half of the reflector, preventing light from being reflected in an upwards direction. The edge of the cap demarcates the light/dark cut-off on the road surface. 74  Silicon Chip It is the luminaire design (comprising the bulb, reflector and lens) that is critical to headlight performance. Early reflectors used a system where it was possible to vary the position of the bulb in relation to the reflector, leading owners to adjust the focus of the light beam with, in some cases, poor results. To overcome this (and other) problems, the sealed beam was introduced. This consisted of an integral lens, filament and reflector – effectively a large bulb with an inbuilt reflector and diffuser. A further advantage of sealed beams over conventional bulbs was in reduced glass blackening. This occurs as the tungsten of the filament vaporises and is deposited on the inside of the bulb. In a sealed beam there was a very large area of glass on which the tungsten could be deposited, resulting in less blackening than previously occurred when using small bulbs. The sealed beam design also protected the reflector from physical damage and corrosion. Some Citroen vehicles have used swivelling headlights that turn in conjunction with the steering. The inner light that can be seen here is so equipped. (1) low beam filament (2) cap Fig. 1: a low beam headlight using a capped bulb. Note how the lower half of the reflector is not used in this type of lamp. (Bosch) The 17.8cm (7-inch) headlight was standardised in the 1940s and remained current until the 1970s. The change in the style of cars then required a smaller size and the 12.7cm (5-inch) headlight was introduced. Aerodynamic development of cars in the 1980s reduced the popularity of discrete round headlights and together with the introduction of halogen bulbs, meant that some of the previous advantages of sealed beams were no longer valid. This resulted in the widespread adoption of headlights unique to each model of car, using commonly available interchangeable bulbs. A bit like 40 years ago, really! Current Headlight Design The majority of headlight use, especially for city driving, is on low beam. This requires lamps with sharply defined, bright beams giving extended range on the passenger’s side of the road without creating glare for oncoming drivers. Many low beam headlights use a light source mounted forward of the parabolic reflector’s focal point. A cap The JE Holden Camira uses a homofocal headlight reflector. From left to right: the high beam inner light, the homofocal combined high/low beam and the indicator. (1) Basic reflector; (2) Supplementary reflector. Fig. 2: this graph shows the luminous intensity on the passenger side, as a function of the horizontal reflector diameter. As can be seen, wide headlights can have high luminous intensities. (Bosch) Fig. 3: a homofocal headlight uses two reflectors within the one housing. (Bosch) within the bulb keeps the lower portion of the beam from being reflected from the bottom half of the reflector in an upwards direction. Fig.1 shows this approach. Other low beam headlights mount the low beam filament above and slightly to one side of the reflector focal point. This causes almost all of the effective luminous flux to be directed downwards and to the left (in righthand-drive countries!). However, this approach does not give the clearly defined light/dark cut-off of those headlights using a capped bulb. The edge of the cap in bulbs so equipped demarcates the light/dark cut-off on the road surface. While it first might appear that this should be as sharp a cut-off as possible, this is not the case. For practical driving reasons, the light/dark contrast must not exceed a prescribed value. An extremely high contrast will produce unfavourable dynamic contrast of the road surface during vehicle pitching, leading to disorientation as the road is alternately plunged into darkness and then well lit. To achieve a maximum visual range with a minimum of glare, the light distribution close to the vehicle is critical. For example, there must be sufficient illumination of both the lefthand and righthand edges of the road to allow cornering. In the past, some manufacturers have mounted headlights on swivels such that they turned in conjunction with the steering. Citroen and Maserati did this on some models. More recently “turning” lamps that operate when the indicators are on have been introduced. As one would expect, the larger the reflector and the higher it is mounted, the more effective is the illumination for a given power. Unfortunately placing two 20cm headlights a metre above the ground is practical only for large trucks, not modern sleek and aerodynamic cars! This has meant that other strategies have needed to be adopted to improve illumination. Increasing the size of the reflector is normally achieved by fitting wider headlights. This is advantageous because the horizontal diameter of the reflector is a major determining factor Variable foci reflectors can be optimised to produce whatever light distribution is required, with the entire reflector surface being employed. This type of reflector is used with a clear lens Some headlight clusters incorporate a variety of lamp designs. From left to right – indicator, parking light, projector style low beam, homofocal high beam. January 1999  75 (1) Bulb; (2) Basic reflector; (3) Supplementary reflector. (1) Lens; (2) Shield; (3) Reflector; (4) Bulb. (1) Lens; (2) Shield; (3) Reflector; (4) Bulb; (5) Auxiliary beam. Fig.5: a projector headlight uses an elliptic reflector and imaging optics ahead of the bulb. (Bosch) Fig.6: an auxiliary short-distance version of the projector light uses a stepped reflector and a shortened shield. (Bosch) Reflectors are available in a number of different types. Stepped reflectors consist of paraboloid sections of different focal lengths, allowing a shorter effective focal length without the disadvantage of a tall reflector. Stepped reflectors are available in two configurations – homofocal and bifocal. A homofocal reflector uses a supplementary reflector which has a shorter focal length than the main reflector. This increases the effective luminous flux with the supplementary reflec- tor improving near-field and lateral illumination. Fig.3 shows this type of reflector, which is normally made from plastic to accommodate the large steps between the different parts of the reflector. The Holden Commodore VL and some Camiras used this design in a combined high/low beam application. Bifocal reflectors use reflector sections with different focal points. Used only in low beam applications, the design makes use of the lower portion of the reflector which normally receives no light. This section of the reflector is shaped such that light from this area is directed downwards, improving near-field illumination. Fig.4 shows this type of design. Note that the two reflectors do not have a common plane surface behind the bulb –they are indeed stepped. With computer aided design it is possible to have reflectors with variable foci. The shape of the reflector can be optimised to produce whatever light distribution is required, with the entire reflector surface being em- ployed, even for low beam applications. This approach has been recently adopted with multi-faceted reflectors used with a clear lens. Projector headlights use imaging optics located in front of the light source. Fig.5 shows this type of design. A light opening area of only 28cm2 (the equivalent of a 6cm dia-meter round headlight) allows light distributions of the type only previously achievable with much larger headlights. A CAD-calculated elliptic reflector is used in conjunction with a convex lens. The light/dark contrast can be defined with either a high degree of sharpness or with an intentional lack of sharpness, depending on the pattern required. Alternative designs of this type of lamp are also available. Fig.6 shows an auxiliary short-distance lamp which uses a stepped reflector. This taller unit (130mm versus the previous design’s 80mm) has improved near-field illumination. Both types of projector lamp are used only in low beam applications. Placing a translucent plastic panel in front of the assembly shows the beam pattern of each lamp. The projector lens low beam has its highest intensity in the middle, with a sharply defined spread left and right. The homofocal high beam is much deeper, to light the near-field as well as distant objects. On this car, this is required because the low beam light does not remain illuminated when high beam is selected. An H1 halogen bulb. This type is used in fog lamps, supplementary high beam and the low beam in 4-headlight systems. Fig.4: a bifocal reflector uses two reflector sections with different focal points. (Bosch) in the achievable luminous intensity. Data from Bosch indicates that if the width of the reflector is doubled from 130mm to 260mm, the luminous intensity is approximately doubled at the lefthand edge of the road surface at a distance of 50 metres from the vehicle. Fig.2 shows this. Reflectors of the same size but with different focal lengths perform differently. A reflector with a shorter focal length develops a broader beam with better close and lateral illumination. Reflector Types 76  Silicon Chip The problems of light/dark cut off and glare are not experienced with high beam designs. Instead, the light source is always situated at the reflector’s focal point, resulting in a beam parallel to the reflector’s axis. Fig.7 shows this type of design. Reflectors can be made from sheet steel or plastic. Steel reflectors are galvanised or powder coated to protect against corrosion. A coating is then applied to smooth the surface, after which a reflective aluminium layer is applied by evaporation. A protec- Bulbs As in other forms of high intensity lighting, the type of incandescent bulb used in automotive applications has moved from tungsten to tungsten halogen. Halogen bulbs have a far higher luminous efficacy than tungsten designs, with associated advantages in alternator loading and cable thickness. To preclude inappropriate fitting, automotive bulbs have differently shaped bases. Common categories are R2, H1, H2, H3, H4 and H7. The table below shows a variety of bulbs used SC in headlight applications. Application Category Nominal Power (Watts) High/low beam R2 45/40 Specified Luminous Flux (Lumens) 400/550 Fog lamps, supplementary high beam, low beam in H1 4-headlight systems 55 1550 High beam 55 1800 H2 Fog lamps, H3 supplementary high beam High/low beam H4 55 1450 60/55 1650/1000 Shape SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. ELECTRONIC COMPONENTS & ACCESSORIES • RESELLER FOR MAJOR KIT RETAILERS • • PROTOTYPING EQUIPMENT • FULL ON-SITE SERVICE AND REPAIR FACILITIES • LARGE RANGE OF ELECTRONIC DISPOSALS (COME IN AND BROWSE) CB RADIO SALES AND ACCESSORIES Croydon Ph (03) 9723 3860 Fax (03) 9725 9443 Mildura Ph (03) 5023 8138 Fax (03) 5023 8511 M W OR A EL D IL C ER O M E Fig.7: in a headlight used only for high beam the light source is always situated at the reflector’s focal point, resulting in a beam parallel to the reflector’s axis. (Bosch) tive layer is then evaporated onto the aluminium, hermetically sealing the sheet steel. The reflective surface typically has a residual roughness of only 1/10,000mm. Plastic reflectors are produced by injection or compression moulding. Lenses are made from glass or polycarbonate. During construction, particular care is paid to surface quality to ensure light is not deflected upwards, causing glare problems. The shape, number and location of the prisms in the lens depends on the type of reflector design used. Truscott’s Low beam in 4-headlight systems, fog lamp H7 55 1500 ELECTRONIC WORLD Pty Ltd ACN 069 935 397 30 Lacey St Croydon Vic 3136 24 Langtree Ave Mildura Vic 3500 January 1999  77