Divergent and Convergent

Divergent and Convergent

Beams If the filament is between the focal point and the reflector, the reflected

Light source behind

Light source in front

beam will diverge - that is, of the focal point spread outwards along the

the focal point

principle axis. If the filament is positioned in front of the focal point, the reflected beam will converge towards the principle axis.

Asymmetric Headlights ฀฀ The intensity of reflected light is strongest near the beam axis, except for the light cut off by the bulb itself. The intensity, therefore, dropping off towards the outer edges of the beam. A common type of reflector and bulb arrangement is shown here, where the dip filament is shielded. This gives a nice sharp cut-off line when on dip beam. It is used with asymmetric headlights.

Headlight Lenses A good headlight should have a powerful far- reaching central beam, around which the light is distributed both horizontally and vertically in order to illuminate as great an area of the road surface as possible. The beam formation can be considerably improved by passing the reflected light rays through a transparent block of lenses. It is the function of the lenses to partially redistribute the reflected light beam and any stray light rays. This gives better overall road illumination.

Lenses Lenses work on the principle of refraction. The headlight front cover is the lens. It 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 Headlight lens details combination of flute and prisms. Each

individual lens element will redirect the light rays to obtain an improved overall light projection or beam pattern

Complex Shape Reflectors Many headlights are now made with clear lenses, which means that all the direction of the light is achieved by the

Modern

reflector. The clear lens does not restrict the headlights light in any way. This makes the headlights more

efficient as well as attractive.

Other Lights Sidelights, taillights, brake lights and others are relatively straightforward. Headlights present the most problems. This is

Rear lights

because on dipped beam, they must provide adequate light for the driver, but not by dazzling other road users.

Headlight Alignment The conflict between seeing and dazzling is very difficult to overcome. One of the latest developments, UV lighting,

Beam setter in use

which is discussed later, shows some promise. The main requirement is that headlight alignment must be set correctly.

Headlamp Levelling ฀฀ The function of a levelling actuator is to adjust the dipped or low beam in accordance with the load carried by the car. This will avoid dazzling oncoming traffic. Manual electric levelling actuators are connected up to a control on the dashboard. This allows the driver to adjust beam height.

Automatic Headlight Levelling ฀฀ Automatic static actuators adjust beam height to the optimum position in line with vehicle load conditions. The system includes two sensors (front and rear), which measure the attitude of the vehicle. An electronic module converts data from the sensors and drives two electric gear motors (or actuators) located at the rear of the

headlamps, which are mechanically attached to the reflectors.

 Look back over the previous section and write out a list of the key bullet points here:

STOPLIGHTS AND REVERSE LIGHTS

Introduction Stoplights, or brake lights, are used to warn drivers behind that you are slowing down

or stopping. Reverse lights warn other drivers that Stop and you are reversing, or intend to reverse. The reverse lights

form part of

circuits are quite simple. One switch in each case the rear light operates two or three bulbs. A relay may be used. cluster

Stoplights and Reverse Lights ฀฀ The circuits for these two systems are similar. Shown here is a typical stoplight or reverse light circuit. Most incorporate a relay to switch on the lights, which is in turn operated by a spring-loaded switch on the brake pedal or gearbox. Links from the stoplight circuit to the cruise control system may be found. This is to cause the cruise control to switch off as the brakes are operated. A link may also be made to the antilock brake system.

Switches The circuits are operated by the appropriate switch. The reverse switch is part of the gearbox or gear

Stoplight switch

Reverse light switch

change linkage. The stop switch is usually fitted so it acts on the brake pedal.

A Real Lighting Circuit ฀฀ The diagram shown is the complete lighting circuit of a vehicle. The colour codes used are discussed in the basic electrical learning sections. However, you can follow the circuit by looking for the labels on the wires. ‘N’ for example, means ‘Brown’ but this has no effect on how it works! Operation of part of this circuit is as described over the following screens.

Reverse Lights ฀฀ The ignition must be on for these lights to operate. The reverse light switch gets its feed from fuse 16 on the GY wire. When the switch is operated, the supply is sent to the rear lamps on a GN wire. The switch is usually mounted on the gear change linkage or screwed into the gearbox.

Stoplights ฀฀ The ignition must be on for these lights to operate. The brake or stoplight switch gets its feed from fuse

16 on the GY wire. When the switch is operated, the supply is sent to the rear lamps on a GP wire. A connection is also made to the centre high mounted stoplight. The switch is usually mounted on the pedal box above the brake pedal.

Light Emitting Diodes Light emitting diodes (LEDs) are more expensive than bulbs. However, the potential savings in design costs due to long

Brake light

life, sealed units being used and greater freedom using LEDs of design, could outweigh the extra expense. LEDs

are ideal for stoplights.

Enhanced Safety ฀฀ A further advantage is that they illuminate quicker than ordinary bulbs. This time is approximately the difference between 130mS for the LEDs, and 200mS for bulbs. If this is related to a vehicle brake light at motorway speeds, then the increased reaction time equates to about a car length. This is potentially a major contribution to road safety.

Centre High Mounted Stop Lamps An LED centre high mounted stop lamp (CHMSL) illuminates faster than conventional incandescent lamps,

CHMSL

improving driver response time and providing extra braking distance. Due to their low height and reduced depth, LED CHMSLs can be easily harmonised with all vehicle designs. They can be mounted inside or integrated into the exterior body or spoiler.

Summary Reverse lights are operated by a simple on/off gearbox switch. Stoplights are operated by a simple on/off switch on the pedal box. Both circuits operate in much the same way. High mounted stoplights are now quite common, many of these using LEDs.

 State TWO advantages of an LED centre high mounted stop lamp (CHMSL).