In a standard situation, light spreads through the space in a diverging manner: every single part of the beam moves further away from the other parts of the beam. Some light sources (such as lasers) have an (almost) non-diverging light beam by nature: the entire beam has the same width independent of the distance from the source. Nearly all other light sources have diverging beams. A diverging beam can be converted into a non-diverging beam using a parabolic mirror or a (correctly placed) lens and vice versa. Such a lens is often present in laser LEDs and the like. Sunlight (on earth) can also be considered non-diverging, because the sun is at such a great distance.
Distance
A diverging beam means that the distance from the light source to the surface illuminated is an important factor to the amount of energy received on the surface. If you ever walk outside in the dark with a flashlight, you know that you can illuminate the path in front of you well enough, but an object at greater distance is only vaguely lit (unless you have a very good flashlight).
The relation of the distance to the area illuminated is “squared” (if the distance is doubled, the area is a factor 4 larger). Inversely, the amount of energy per square meter (or other surface area) is a factor ¼ related to the distance. This means that the amount of energy emitted at the light source, and/or the sensitivity of the detector, must be increased a lot even with an small increase in distance to be covered.
This has a number of implications if a diverging light source is used:
- A beam breaker setup using a reflector (or other mirroring device) needs at least a 4 times more powerful light source than an opposing setup (because the distance is traversed twice, and some power is lost in the reflector)
- A light source / detector pair may work well at a chosen test distance, but may not work in a real situation where a race official quickly sets up the timing system near the track. 10% more distance means more than 20% less energy received in the detector.
It is clear that the main advantage of a non-diverging light source is that the distance between source and destination is not a factor in the system design; all of the energy in the light beam reaches the destination. There is however a (much) higher cost for light sources with non-diverging beams (lenses and lasers etc. are not cheap).
Reliability
For a beam breaker to be reliable, it must detect its light source when the line is not crossed, and it must not detect its light source when the line is crossed. This sounds simple, but there are 2 situations that can occur (which are bad):
- Invalid "crossing" detected: line is not crossed but detector detects no light. Reasons may be that the light source may be too weak, or there is something else blocking the path of light (rain, fog, leaves dust, etc.)
- Invalid "no crossing" detected: line is crossed but detector still detects light. Reasons may be that another matching light source sends light to the detector, or light from the real source is reflected on other surfaces (e.g., the track, shoes, tires) or deflected by rain.
We're looking for a reliable detector that ideally signals the correct status (line is crossed or not) at all times, in a fast manner (within milliseconds). And does not cost much …
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