Automobile Lighting fundamentals

Automobile Lighting fundamentals


Joseph Swan in the UK demonstrated the first light bulb in 1878. Much incremental development has taken place since that time. The number, shape and size of bulbs used on vehicles is increasing all the time. Most bulbs for vehicle lighting are generally either conventional tungsten filament bulbs or tungsten halogen.

In the conventional bulb the tungsten filament is heated to incandescence by an electric current. In a vacuum the temperature is about 2300 ° C. Tungsten is a heavy metallic element and has the symbol W; its atomic number is 74; and its atomic weight 2.85. The pure metal is steel grey to tin white in color. Its physical properties include the highest melting point of all metals: 3410 ° C. Pure tungsten is easily forged, spun, drawn and extruded, whereas in an impure state it is brittle and can be fabricated only with difficulty. Tungsten oxidizes in air, especially at higher temperatures, but it is resistant to corrosion and is only slightly attacked by most mineral acids.


The glass envelope has a tubular shape, with the filament stretched between brass caps cemented to the tube ends. This bulb was commonly used for number plate and interior roof lighting.

Cap less bulb

These bulbs have a semi-tubular glass envelope with a flattened end, which provides the support for the terminal wires, which are bent over to form the two contacts. The power rating is up to 5 W, and these bulbs are used for panel lights, sidelights and parking. They are now very popular due to the low cost of manufacture.

Single contact, small bayonet cap (SBC)

These bulbs have a bayonet cap with a diameter of about 15 mm with a spherical glass envelope enclosing a single filament. A single central contact (SCC) uses the metal cap body to form the second contact. The size or wattage of the bulb is normally 5 W or 21 W. The small 5 W bulb, is used for side or tail lights and the larger 21 W bulb is used for indicators, hazard, reversing and rear fog-lights.

Double contact, small bayonet cap

Similar in shape and size to the large SCC 15 mm SBC bulb, as described above. It has two filaments, one end of each being connected to an end contact, and both of the other ends are joined to the cap body forming a third contact, which is usually the earth. These caps have offset bayonet pins so that the two filaments, which are of different wattage, 292 Automobile electrical and electronic cannot be connected the wrong way around. One filament is used for the stop light and the other for the tail light. They are rated at 21 and 5 W (21/5 W) respectively.

External lights

Regulations exist relating to external lights, the following is a simplified interpretation and amalgamation of current regulations, the range of permissible luminous intensity is given in brackets after each sub heading. Such as

Sidelights (up to 60 cd), Rear lights (up to 60 cd), Brake lights (40–100 cd), Reversing lights (300–600 cd), Day running lights (800 cd max), Rear fog lights (150–300 cd), Front spot and fog lights.

Headlight reflectors

Light from a source, such as the filament of a bulb, can be projected in the form of a beam of varying patterns by using a suitable reflector and a lens. Reflectors used for headlights are usually parabolic, bifocal or homifocal. Lenses, which are also used as the headlight cover glass, are used to direct the light to the side of the road and in a downward direction.

Parabolic reflector

A parabola is a curve similar in shape to the curved path of a stone thrown forward in the air. A parabolic reflector has the property of reflecting rays parallel to the principal axis when a light source is placed at its focal point, no matter where the rays fall on the reflector.

It therefore produces a bright parallel reflected beam of constant light intensity. With a parabolic reflector, most of the light rays from the light-bulb are reflected and only a small amount of direct rays disperses as stray light.

Lastly Comment

Lenses work on the principle of refraction – that is, the change in the direction of light rays when passing into or out of a transparent medium, such as glass (plastic on some very recent headlights). The headlight front cover and glass lens, is divided up into a large number of small rectangular zones, each zone being formed optically in the shape of a concave flute or a combination of flute and prisms. The shape of these sections is such that, when the roughly parallel beam passes through the glass, each individual lens element will redirect the light rays to obtain an improved overall light projection or beam pattern.

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