Vehicle Lighting Systems
Vehicle lighting systems are extremely important specifically from road safety considerations. If headlights suddenly fail at night and at high speed the result would be catastrophic. Many techniques have been incorporated, ranging from automatic changeover circuits, to thermal circuit breakers, which pulse the lights rather than putting them out as a blown fuse would. Most modern wiring systems fuse each bulb filament separately and if the main supply to the headlights were to fail, it is likely that the dim dip would still work.
The vehicle lights must perform two functions, they must allow the driver to see in the dark and allow the vehicle to be seen in the dark (or in conditions of poor visibility). Side lights, tail lights, brake lights and others are relatively straightforÂward. Headlights present the most problems because on dipped beam they must provide adequate light for the driver but without dazzling other road users or pedestrians. Even after adoption of many techniques, it is very difficult to overcome the conflict between seeing and dazzling. One of the latest developments, ultra violet lighting, shows some promise.
The environment in which vehicle lights have to survive is hostile to say the least. There are extreme variations in temperature and humidity as well as serious shocks and vibration. This chapter covers the materials to assist with the understanding of vehicle lighting systems.
29.1.
Some Terms and Definitions
This section first covers terms associated with light itself and then terms relating more specifically to vehicle lights. The definitions presented are generally related to the construction and use of headlights.
Luminous Flux. Luminous flux is defined as the amount of light passing through an area in one second. The unit of luminous flux is the lumen, which is defined as the light falling on a unit area at a unit distance from a light source that has a luminous intensity of one candela.
Luminous Intensity. This is the power to produce illumination at a distance. The unit is the candela, and it is a measure of the brightness of the light rather than the amount of light falling on an object.
Illumination Intensity. Illumination intensity is defined as the luminous flux reaching a surface per unit area. The luminous intensity of a surface, such as a road, is reduced if the light rays are at an angle. The unit is the lux, and this is equivalent to one lumen per square meter or to the illuminance of a surface one meter from a point source of light of one candela. The illumination intensity depends on the brightness, distance from, and angle to a light source.
Brightness or Luminance. Brightness is different from illumination. For example, during night driving the illumination from the vehicle lights remains constant. The brightness or luminance of the road varies depending on its surface colour. Luminance therefore depends not just on the illumination but also on the light reflected back from the surface.
Range of a Headlight. A headlight’s range is the distance at which the headlight beam still delivers a specified luminous intensity.
Geometric Range. This range is the distance to the cut-off line on the road surface when the dip beam is set at an inclination of 1% below the horizontal (1 cm per m).
Visual Range. The visual range is defined broadly as the distance within the luminous field of vision, at which an object can still be seen. Since it is affected by many factors it cannot be expressed in units.
Signal Identification Range. The single identification range is the distance at which a light signal can be seen under poor conditions.
Glare or Dazzle. Glare is also difficult to express, as it is perceived differently by different people. A figure is used however, i.e. if the luminous intensity is llx at a distance of 25 m, in front of a dipped headlight at the height of the light centre, then the light is said not to glare or dazzle. In the old British method it is expressed that the lights must be dazzle a person standing on the same horizontal plane as the vehicle at a distance over 25 feets (7.62 m), whose eye level is more than 3 feets 6 inches (1.07 m) above the plane. In principle the lights when on dipped beam must fall below a horizontal line by 1% or one cm/m.
