AlexP,
I want to know if the physical units of exposure is available in comparison with the binary values from 0 to 511. I’m trying to cross-compare other cameras with PS3 eye at 1/250s exposure time. Knowing physical units like 1/60s is also quite useful in other places so could you post a correspondence chart or a formula if you have one? Thanks.

liudr,

Here is how it works. First of all this timing depends on the capture resolution. Assuming that you are capturing at 640x480 the exposure is as following:

  Texposure = ExposureValue * Trow

There are total of 510 rows between two VSync periods. So for example if you are capturing at 30fps this time amounts to:

  Trow = (1/30) / 510 = 6.5359e-5 s

So for your desired exposure time of 1/250, the ExposureValue would be:

  ExposureValue = (1/250) / Trow = 61.2 ~ 61

Therefore if you capture at 30fps at 640x480 resolution, in order to get exposure time of 1/250 s you would set the sensor exposure parameter value ExposureValue=61.

The thing to remember is that at:

1. 640x480 there are total of 510*Trow between two consecutive Vsync periods.
2. 320x480 there are total of 278*Trow between two consecutive Vsync periods.

AlexP

AlexP,

Thanks for your quick response. So for 640*480, this means full value 511 means the exposed time equals the time of each frame, and the fraction exposureValue/511 means the lines are exposed a fraction of the full frame time. A full frame time would be 1/30s at 30FPS or 1/50s at 50FPS. Is this right? What about 320*240, is the full value of exposureValue still 511 or something like 556?

liudr - 29 December 2009 10:53 AM

AlexP,

Thanks for your quick response. So for 640*480, this means full value 511 means the exposed time equals the time of each frame, and the fraction exposureValue/511 means the lines are exposed a fraction of the full frame time. A full frame time would be 1/30s at 30FPS or 1/50s at 50FPS. Is this right? What about 320*240, is the full value of exposureValue still 511 or something like 556?

You are correct, at 640x480 the exposureValue of 511 gives you the 100% exposure time per frame. So for frame rate of 30FPS the resulting 100% exposure time is 1/30s. At 320x240 resolution the 100% exposure time is when exposureValue is 278.

AlexP

Thank you AlexP, that was crystal clear.

Alex, are you sure that 511 would give 100%? There isn’t any time between exposing the last row and exposing the first row? And all the pixel readout is happening in parallel with the exposure?

Yes, that seems right according to the sensor datasheet. For VGA there are total of 510 Trow intervals between two consecutive VSYNC pulses.