Illumination intensity control for video application

Thread Starter

Sara Selvig

Joined Jun 29, 2017
3
I am thinking about photo-finish application (linear array) involving exposure times less than 0.0005 seconds (2000 frames/second). To minimize variation in brightness so that significant changes in frame content can be recognized reliably (and "duplicate frames" can be compressed on the fly), I think that I need to avoid the flicker associated with lighting powered directly by 60 Hz AC.

I think that I need to supply constant intensity illumination sufficient to mask variation due to ambient light sources. To do this, I suspect that I need to be able to control the sensitivity of the photo sensor AND to control the intensity of supplementary illumination.

Secondly, since the system may need to be "tuned" efficiently (quickly) to operate in new environments, I need "setup" diagnostic outputs to register (1) absence of flicker in the exposed images and (2) "middle of the curve" image exposure. (The linear images will be combined to display time-domain "photos".)

Questions:
1. Am I "overthinking" this?
2. Are LED's powered by well filtered DC a good choice for illumination?
3. Is it most reasonable to include a "setup mode" in the support software to guide setting the illumination and sensitivity?
Bonus question:
4. Are there color linear photo arrays available (at reasonable cost) in which the color sensitive pixels are not offset? (I ask because all of the color linear photo arrays that I've looked at are really rectangular rather than linear and depend on constant speed of the subject to form a true image of the subject.)
 

Thread Starter

Sara Selvig

Joined Jun 29, 2017
3
How about setup mode operated as follows:
1. Adjust illumination and sensitivity to "center".
2. Increase or decrease illumination for "middle of the scale" exposure.
3. Increase or decrease sensitivity to acceptable exposure variation. Stop if no adjustment necessary, else repeat from step 2.
 

panic mode

Joined Oct 10, 2011
1,794
i would take a note from designs of those that make such optical sensors (beam-through).
they all use modulated light source (not constant output). This allows to check for received signal both with and without own light source and then make a decision...
this makes it immune to ambient light
 

Thread Starter

Sara Selvig

Joined Jun 29, 2017
3
Hmmm ... I think that the light intensity needed for such short exposure times will cause the ambient light to be swamped. That should go most of the way toward eliminating 60 Hz "buzz" from ambiance, but I think I need to be able to adjust to circumstances.

I'd also like to have the option to increase the frame rate by 2 or 4 times, which will affect the necessary supplementary illumination intensity. So control over intensity will be needed, at least, initially. (Not sure how much freedom the microprocessor speed will give me, though.)
 
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