This is exactly my point. If you could see it, you wouldn't need the timing in the first place.If someone is seeing their system being susceptible to such light influences
It is not hard to imagine a video camera light illuminating the sensor at the same time that the car finishes and is blocking the timer's source light. The result is that the car still finishes and the timer still reports a time. But the time is wrong. The reported finish is different from the actual finish, undetectable by humans who assume that if the computer shows a time, the time must be correct. In the extreme it is of course possible for there to be no finish at all but that's unusual. It's more likely that the car's reported finish is an inch or two later than it actually was.
This error would not be there for a car that crossed without the video camera present. This error would be different if the video camera were held in a different position. For a forum that often deals with incredible nit picking when it comes to cars and their construction, this unquestioned acceptance of an enormous error source truly baffles me.
In some poorly designed circuits, a flash can over-illuminate the detector and the 'darkness' caused by the sudden loss of the flash illumination can cause the sensor to report an early finish.
Properly designed mechanical sensors don't have either of these problems. Even mechanical sensors that (as reported elsewhere in this forum) appear 'flimsy' or 'can bend' are far more reliable both because the potential error from the 'bend' is much smaller and because the software used with them runs all cars on all lanes and averages the times to find a winner. The result is that even if the sensor were 'bent' several inches (or even located several feet up the track), all cars would use that lane and sensor once and the error would disappear in the final calculation. This error elimination does not occur with optical sensors because the error is not constant.