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FIS Photocell Spec

J jacunskimMember
Hi everyone.

I was recently reading the latest version of The Good Book (that would be the FIS Timing Booklet), and I noticed something odd. The technical specs for photo cells are listed on page 33. It states the following:

Triggering Object:

An 8 mm object moving with a speed of 10 km/h is not allowed to trigger the photocell (measured at a distance of 2 m from lens of the receiver).

A 100 mm object moving with a speed of 200 km/h must trigger the photocell (measured at a distance of 2 m from lens of the receiver).

Of course, the photocell doesn’t know anything about the object’s size or speed. It just knows for how long the beam was interrupted. But here's the thing:

an 8 mm object moving at 10 km/h will break the beam for 2.9 ms;

a 100 mm object moving at 200 km/h will break the beam for 1.8 ms.

How can a 1.8 ms break trigger the photocell, but a 2.9 ms break not? Am I missing something? Has FIS created an impossible spec?

Thanks for the insight.

-- Mark
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themightyskunkthemightyskunkMember
Ah. Math.

If you think only about the math and not at all about the practicalities of ski racing, this is a gotcha.

This spec mandates not only the jitter rate of the beam, it also mandates the diameter of the beam.

In the first example, that 8mm object (about the size of two pencils) represents a ski pole. That part of the spec isn't about jitter rate, it's about requiring a beam wide enough so that a ski pole will not totally obscure the receiver to the point where the cell generates an impulse.

The second example is about capturing an impulse from a racer's shin (100mm = about 4 inches) at a speed well beyond that which an alpine racer will go.

In abstract terms, a beam the diameter of a human hair would flunk this spec because it could be tripped by a ski pole. Obversely, a beam the diameter of a trash can lid would also flunk because a racer's shin wouldn't cause an impulse.
J jacunskimMember
I see.

One minor point, the beam diameter is actually not mandated in the spec. At least it's not noted in the FIS Timing Booklet. However, it does state "the technical concept of the photocell is not restricted by the FIS." I believe that means that manufacturers can achieve the requirements how ever they'd like (within reason), and I see that adjusting the diameter is one way to do it.

Thanks for the info.

-- Mark

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