Red Light Answer

[Jason Oldfield]

I'm not an electro-optic engineer, but I have worked in the field of optics and electronics for 20+ years, and I currently work for a company that makes optical sensors (our sensor works with a VCSEL in the IR-A range at about 800nm). In short, you CANNOT say WITH CERTAINTY that the photographers flash did not effect the receiving sensor. Optical sensors are pretty simple devices really, and as Pat pointed out in his original post the receiving sensor (detector) is a semiconductor device that is "tuned" to a specific wavelength of light. This helps increase the signal to noise ratio, and improve overall signal quality.

However, the match between the detector and wavelength of light that it is trying to detect is not exact, i.e. a sensor that is looking for a wavelength of light predominantly at 800nm doesn't usually reject wavelengths at 799nm and 801nm (it would have to be a REALLY high-dollar sensor). Optical sensors are typically open to a range of wavelengths, with their peak wavelength being the specified wavelength, or the wavelength at the center of a gaussian curve, like so:



As you can see from the above graph, there is a rolloff that would happen with the wavelengths of light seen by the detector. The minimum and maximum wavelengths would be determined by the sensor manufacturer from the type of detector technology used, and the tolerances in manufacturing (which in the end is reflected in the cost).

The same is true for the emitter. Again, the output of wavelengths from the emitter is typically a gaussian curve with the predominant wavelength being the specified wavelength. This is even true for lasers, though with lasers the wavelength range is much smaller than non-collimated light. I don't know the exact type of light source that is used for the stage beams in the timing system, but I highly doubt that they are lasers.

So, how would the photographers flash possibly affect the detector? It all depends upon the wavelengths of light that is emits. What the human eye can see is light in the approximate range of 400nm to 750nm. Light with longer (IR, microwaves, etc.) or shorter (X-rays, UV, etc.) wavelengths than that range is invisible to the human eye. What we see from the flash is white light, which is made up of all of the colors of the spectrum over the visible wavelength range. This is why when you shine a white light into a prism you see the entire spectrum of colors (sunlight through water droplets forming a rainbow is the same thing).

So, from this fact alone you can see that the photographers flash does NOT output a finely tuned wavelength of light. What wavelengths does it output? Well, we can say with a high level of confidence that it outputs an approximately wavelength range of 400nm to 750nm, but wavelengths above or below that are unknown. You would only be able to tell ALL the wavelengths by looking at the output with a spectrometer. In addition, is it possible that the flash outputs the predominant wavelength that the timing system's detectors are looking for? In short, the answer is yes, but at what amplitude is unknown (one would expect that it wouldn't be a predominant wavelength, but again you'd need to analyze the output with a spectrometer to know for sure.)

What is also important to note is that just because the detector is mounted inside a box or tube does NOT mean that outside light sources can't impact their performance. Certain materials are invisible to certain wavelengths of light. The X-Rays are typically defined in the 0.1nm to 10nm range, and can pass through many materials. Most optical sensors use either visible light or IR, and IR can pass through materials as well (this is why the military uses IR sensors to image heat sources of bad guys inside buildings). A nice description of IR light can be found at the link below (take notice of the image of the guys arm inside a garbage bag).

http://coolcosmos.ipac.caltech.edu/c..._ir/index.html

Now, does all this mean for certain that the photographers flash caused this event to occur? Absolutely not, but from a scientific standpoint, it seems awfully coincidental that the photographers flash goes off, and immediately afterwards the red light comes on. I'd have to see the original footage frame by frame (you can't do this on YouTube) to see what the timing difference is because optical sensors are fast acting. If the timing difference is long (i.e. more than a few tenths of a second), it would seem likely to me that something else must have caused this to happen. But, if it is literally the next frame as somebody else posted, stray IR light from the photographers flash is a distinct possibility.

Would that mean that we'd all need to demand instant changes to the timing system detectors? No, because IMHO it would need to be a near perfect storm for this to happen. With ALL the rounds of racing, and ALL the pictures taken, how often have you seen this happen? It is obviously NOT a common occurrence. But, at that point we could at least say that we believe that the system is NOT perfect, and try to work on a better, more fool-proof solution (rather than just burying our heads in the sand and saying, "It's not possible for this to occur.")

Science lesson over...