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Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The construction makes me think it is an optical sensor in the base that detects the darkening that the bullet causes when it passes between the detector and the white diffuser at the top.
I have never seen one up close but I am pretty sure it works like that.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The link you put leads to pages that describe the 'sails' as "sky screen' I guess to screen the sky from the optical pickup.
Since (in general) photo-diodes have a much faster rise time (dark-to-light response) than they do fall time (light-to-dark) I ASSUME theres a light source pointing up and the bullet reflects a small portion back to the photo sensor. The optical signal is quite small so 'shielding' the sensor from the sky light is a good idea.
This is all guesswork on my part, others here probably know better!
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
With some searching I found some information.
As a bullet is darker than the sky above, the skyscreen sensors detect a slight light drop as the bullet passes over them thus allowing the chronograph to record the velocity. When there is not enough light on the diffuser or the bullet is too bright, there is the potential for problems. Additionally, when there is snow or water on the ground, the reflection which is cast back up on the bottom side of the bullet can prevent the sensors from seeing the bullet causing erratic performance.
My guess is that it was done like that since WWII and it has been done like that ever since without anyone questioning the design.
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optical speed trap / chronograph sensor question
