It does have a problem a 2,500 feet elevation.
O2 and N2 scatter blue sunlight and make a background for the bullet to have a shadow.
There is not much O2 or N2 above me to make a shadow.
So I turn the pro chrono sideways and get a bullet shadow on a box.
Clark,
I think you’re misunderstanding the optical mechanism a little bit. While it’s true that the atmosphere scatters blue wavelengths more than other wavelengths in white light, which is why the sky “looks” blue, there is plenty of white light that reaches the Earth from the sun in a “line-of-sight” path.
My Pro Chrono works just fine at 5000 feet ASL, with or without diffusors.
I may be confused, there is always the danger of hidden variables. And I know you are scary smart. But here in Seattle at 300 feet, the sky is a bright blue and the chrono triggers. In MT at 2500 feet, a clear sky [looking straight up] looks black and the chrono will not trigger. But with a little cloud cover at the same spot at 2500 feet, it will trigger.
I can think of a hidden variable. Your chrono could have a trigger set to a more sensitive threshold than mine.
I don't doubt your experiences, and it is true that at lower elevations there is more atmospheric scattering causing a greater amount of anisotropic blue light, but there is also a lower intensity of direct, isotropic white sunlight reaching the ground exactly because there is less atmosphere scattering the incoming light.
You're right that the sensitivity of the sensors in my ProChrono unit may differ from the sensitivity of the sensors in your unit. It would be interesting to know what type of light sensors the ProChrono uses, as the sensitivity of different optical sensors to various wavelengths of light varies. It may well be that the sensors that the ProChrono uses are most sensitive to blue wavelengths, in which case they would work well at high elevation where the incident angle of white sunlight causes sufficient shadow when the bullet passes by, but poorly where angle of the sun is such that the shadow is mainly caused by scattered atmospheric blue light. It makes sense that your unit triggers at higher elevation under cloud cover, as the clouds act as a diffuser increasing the anisotropy of the white light.
