The behaviour of light sources and photometric terms.
The Inverse SquareLaw- Warning lamps generate light which is then transmitted as energy to the eyes of the viewer. Almost all lamps are considered to be a point source, due to their small cross section. Light energy is propagated in all directions. As the distance from the source to the viewer increases, the amount of available light decreases.
The Inverse Square Law states that the intensity of light falling on a flat surface at right angles to a point source is inversely proportional to the square of the distance.
Candelasare a photometric unit of luminous intensity (old candle power).
Luminanceis the property of emitting light either as a source, or by reflection.
The intensity of lightis directly proportional to the brightness of the background. The sky is a common background from most viewing angles and typical luminance values on a bright sunny day are 10,000 cd/m2, reducing dramatically to levels of 100 cd/m2 or less at dusk (Fisher & Paine, p4).
The viewing range of a light source (eg. a traffic signal) can change by more than ten times depending on the lighting conditions and background it is viewed against. Light sources that are relatively low intensity may be seen over long distances on dull overcast days but will not be seen at much shorter distances on bright days.
The human eye is more sensitive to a light source the closer that source is to the line of sight. This means the further a light source is from the line of sight the brighter it will need to be to elicit a response.(Paine & Fisher, p4)
Colour temperatureis a measurement of spectral emission of light sources recorded in degrees Kelvin or oK.
Light emitted from lamps becomes “whiter” at higher temperatures and the total output of light increases rapidly as colour temperature increases. Daylight can be measured;
sunlight at sunrise and sunset = 2,000 to 4,000 oK
at = 5,500 oK
clear blue skylight = 12,000 to 27,000 oK.
It becomes clear that when artificial light sources compete with sunlight there can be vast differences in the individual light levels. Generally, the rule is, the higher the colour temperature, the shorter the lamp life. The colour temperature of some lamps are described in the next section.
Light energy travels in straight lines, but it can be bent (refracted) and reflected by flat or curved reflectors, optical lenses and the many angular surfaces the light strikes on the path between the source and the viewer. When light strikes a flat reflector (mirror) or surface, it splits into spurious or specular reflections, causing glare.
A properly constructed reflectorcan increase the intensity of the illumination from a given lamp by as much as three times over a field of about 60 degrees. A polished reflector will produce a visible hot spot and a satin finished reflector will moderate the hot spot, reflecting diffuse light. New automotive reflectors (circa 1999-2000) were constructed with angular surfaces to produce multiple specular reflections, visually increasing lamp size and intensifying the reflectivity at specific points across the reflector surface. This also optically dilutes the hot spot.
A headlight is usually an efficient ellipsoid or parabolic reflector, but most “thinner” general-purpose automotive driving lights do not include efficient reflectors.
In other vehicle lights the addition of a fresnel condenser lens between the lamp and the viewer will refract and focus the light into a parallel beam, preventing further losses of light splaying off-axis, thus maintaining beam intensity over a narrow angle. Most automotive lights use a simple plastic Fresnel lens and/or diffuser to partially refract (bend) light.
Fresnel lenses are easily molded in plastic, but without a proper reflector behind the lamp they collimate only a portion of the light, diffusing the remainder. A fresnel lens used without a reflector is satisfactory for general purpose automotive use, but a quality reflector and lens combination is crucial for any specialised purpose.
In many light designs coloured filtersare placed between the source and the viewer to alter the colour composition of the light. They do this by transmitting some wavelengths and absorbing others, thus a yellow filter will pass red and green light, but not blue light.
Light losses occur when filters are used, the loss being greatest for darker colours - even the latest LED designs have variations in light output for different colours. As the distance increases, the filtered and coloured light strength decreases until the intensity may be below the threshold for colour vision. The colour is not perceived by the eye and the distant light is now visualised as a simple white or off-white brightness.
The different sources of artificial light
There are a number of different forms of artificial light source available. These are incandescent lamps, halogen lamps, gas discharge or strobe lamps and Light Emitting Diodes (LED’s). Lamps are usually mounted in a metal, glass or plastic body that houses the electrical connections, supports the lamp against breakage, holds the coloured filter and keeps the entire assembly weather tight.
These lamps are very common, using a filament surrounded by a partial vacuum or an inert gas, all within a glass capsule. When the filament gets hot, it glows and emits light. The filament takes time to heat up and then cool after use. High flash rates cannot be achieved due to this heat/cool cycle and the lamp life is shortened because of it.
Progressively, the tungsten filament is degraded, evaporating over time to blacken the glass.
The advantages of incandescent lamps are their cheap price, reasonable lamp life and ease of servicing.
The colour temperature of incandescent lamps is 2,600o K.
These lamps are similar to incandescent lamps in design.
Halogen lamps are filled with one of the halogen family of gases, thus pressurising the glass container.
The halogen gas reduces the blackening of the glass by redepositing the evaporated tungsten back onto the filament at the very high working temperature.
This produces an efficient light source with even distribution of light and subsequently, a whiter and brighter light output.
Heat output at very high levels is a by-product of halogen light generation. Halogen lamps are very common in the transport industry for all types of lighting and are the major conventional lamp used in warning lights. Halogen lamps are more expensive to replace than incandescent lamps and are on par with the cost of strobe tubes. Replacement and servicing of these lights is straightforward.
The colour temperature of these lamps is 3200o K.
Gas Discharge or Strobe Lamps
Originally developed for high speed flash and stop motion photography the strobe, unlike ordinary lamps, has no filament.
The light is generated by a spark arcing between two electrodes. A gap separates the electrodes and the space between is filled with xenon gas, allowing rapid and high intensity electrical discharges of extremely brief duration. Strobes require a separate power supply with large discharge capacitors to store the electrical energy.
They are able to produce extremely high output to multiple flash heads (which can be very compact) while using
less power than conventional lights.
Flash tubes can be fitted inside headlights and parking light housings.
Flash rates can be varied from 1/500 second to 1/50,000 second or produce a steady light source when the fusion frequency is reached (Neobe).
The short duration discharge produces extremely high output, as it must, for the short flash to be visible. Manufacturers can program the strobe power supply to flash in many different rapid bursts or patterns.
Relatively, a strobe radiates less heat than a conventional light source.
These lamps are vibration resistant and extremely reliable as there is no fragile filament to break. Many strobe lights and lightbars use thick diffusers around the tube to increase the strobe’s visual diameter and diffuse the intense light. The power supply units are not user serviceable and must be returned to the manufacturer for repair.
The colour temperature for strobes is 6,000o K to 7,000o K.
Light Emitting Diode (LED) technology.
Lights using multicoloured LED's are now available commercially and the first emergency beacon style light was produced in the mid nineties. Light emitting diodes are a miniaturised lamp that is encapsulated in a weatherproof plastic lens.
The tiny lamps are used singly or combined into groups, each containing as many lamps as required for a given situation or orientation. The lamps emit coloured light which is focused by the lens, increasing its luminous intensity. When the lights are combined in groups the total luminosity increases proportionally. LED lighting uses only about 10% of the power consumed by the equivalent conventional lamps, has a fast off/on cycle, are more compact for a given output and last ten time longer than normal lamps. The LED is vibration resistant and weatherproof. New developments mean that LED light strips can follow curved vehicle panels and illuminate areas under car doors.