Light Source Comparison

Introduction
This note is to discuss the physical and optical characteristics of various forms of flashing light sources for hazard warning purposes and to examine the relative merits of these technologies.

Warning beacons are designed to alert a person to the presence of an abnormal situation while not contributing to any escalation of the event. Hence the device must:

• Be attention getting in wide range of lighting and weather conditions
• Not dazzle or confuse the viewer
• Allow the viewer to judge how far they are from the beacon
• Not be inherently dangerous

Light sources

Light sources examined for the Eflare included:

Superbright LED
The new Superbright LEDs form an ideal source from an efficiency and safety perspective. The LED has a high output per watt and does not contain any high temperature elements. As the light generated is monochromatic (pure color) there is no light lost passing through a colored lens or filter.

Incandescent Lamp
Halogen and Krypton lamps form very efficient white light sources which are easy to incorporate into optical designs but have the disadvantage of a very hot filament. Because the lamp generates white light it is necessary to filter the light through a colored lens. This introduces considerable light loss and loss of efficiency.

Xenon Strobe Tube
Xenon strobe tubes can be used for high intensity sources and are also easy to incorporate into optical designs. As the Xenon source produces a very high intensity blue/white light, introducing a color lens significantly reduces the overall efficiency.

Physical Characteristics

LED
The low operating voltage, sealed construction and low temperature rise makes Superbright LEDs an ideal light source. There are some power and beamwidth limitations.

Incandescent Lamp
Halogen lamps are rugged, operate at low voltages and have low surface temperatures allowing them to be incorporated into non-incendive devices.

Xenon Strobe Tube
Xenon strobe tubes require high voltages (>400Volts) to operate, making them expensive to protect to meet the requirements of non-incendive devices.

Physiological Effects

Optical Response
The human eye and brain respond differently to various light sources. Flashing or moving lights are more attention getting than fixed or stationary lights as the brain processes the movement as a possible alarm situation. Research has shown that flash rates of 2 to 7 per second are the most effective at alerting the viewer.

The duration of the flash can affect the binocular response of the viewer. If the duration of the flash is too short the brain can tell that there is a light source but cannot form a judgement of distance. Flash durations of less than a few milliseconds will disable the binocular capability of the brain. It is often very difficult to judge the distance of a Strobe source. Indeed strobes are often pulsed two or three times per cycle to try to overcome this problem.

The apparent brightness is affected by the ratio of light-on to light-off periods. The optimum is an on to off ratio (duty cycle) of between 1 to 3 and 1 to 5.

The difference between the brightness of the light and the background (the contrast ratio) is also important being high enough to get attention and not to high that the eye is saturated.

The intensity of the very short duration pulses emitted by a strobe can affect the night vision of pilots and other dark adapted viewers to such a point their night vision may be affected for a few minutes if there is a strobe light in their vicinity.