Subject: How does a time registration with photofinish exactly works?
From:
[email protected] (Ilse Backers)
Date: 9/23/04 12:32 PM Central Daylight Time
Message-id: <
[email protected]>
Hello,
Can somebody tell me more about how does a time registration with
photofinish (e.g. in a cycling race, in a speedskating race,...)
exactly work?
What is the accuracy of it? And what do you have to do if you want it
as accurate as possible? Are there other influences on it???
Can somebody tell me also how it, electronically seen, works?
Thank you!!
A good block diagram of a modern race timing system is shown on this website
http://www.splitsecond.com.au/greyhound.htm
Timings to the nearest millisecond are typically obtainable with this method.
Longer distance LED photosensors are typically modulated at an intermediate
frequency for better receptivity. Faster response times can be obtained by
increasing the modulating frequency of the LED emitter and phototransistor
receptor bandpass filter, making response times less than 1 ms. Photosensor
manufacturers catalogs have all kinds of information on this method.
http://www.bannerengineering.com/literature_resources/tutorial/old/1_sensi
ng_intro.html
While accuracy better than 1 ms really isn't necessary (and the backup of a
camera, typically CCD, provides a photo record in case of a "tie", anyway), you
can improve on this by dropping the modulating frequency and just going off
dramatic changes in photosensor output, which decreases your sensitivity to
electrical noise. Another possible here is using a low power laser diode, and
providing a point-to-point aim for sensing. The limiting factor then becomes
phototransistor response times. These can be improved again by using a
photodiode and amplifier. But at this point, we're way beyond any reasonable
accuracy for race times (better than than 10 microsecond, or .01 millisecond
resolution).
I hope this has been of help.
Chris
