when you the light at a given intensity the LED light flickers at that given frequency.
Of course this makes sense, otherwise there would be constant high intensity light delivery outside of frame acquisition for no reason.
So to test this, I placed the miniscope LED in front of a photodiode while changing the frame rate (and simultaneously recording the frame acq. TTL).
This gave me the above values. To explain it more specifically for one of these settings:
For 30 fps, at the moment of a frame acquisition (as defined by a change in the TTL signal), there is an increase in the light intensity that lasts 18 ms. Then light goes down (not completely off, but I guess to the 0% leakage current) for 15 ms and then the next frame ttl comes and simultaneously light intensity increases again.
Now I do not know how 'Exposure' relates to this, but I guess a setting of 255 (i.e. 100%) would imply that the sensor collects light for the full 18 ms of light on (but not for the 15 ms of light off, but perhaps this doesn't matter).
The point though is that from the above values I would expect that either the duty cycle would be the same for all the frame rates (i.e. for 75% duty cycle, for 10 fps there is 75 ms light on, for 60 fps 12.75 ms light on) or the light on duration would be the same for all settings (e.g. 10 ms) and the only thing that changes is the repetition rate (frame rate).
Now in the ideal case, these two settings are independent (and independently controlled), since they affect different aspects of the process.
- Light on time affects how bright the image is
- Frame rate affects how often you care to measure the given signal
Of course the brightness you can control by reducing the exposure, but this is not quite the same as controlling the light time, since with the exposure you have to use higher light intensity and you can reduce the light received (but this has negative consequences for the bleaching of the fluorescence), while with controlling the light time, you can optimize this value and you can have 100% exposure (full frame duration).
Am I understanding this process correctly?
|Posted by Nikolas on 25 June 2016 at 11:52.|