Digital BW, The Print

A CCD will work as an integrator. That's the way they are used in 
wideband optical signal processing systems where you are using a laser 
to illuminate a (normally ) an accusto-optics device and the results 
focused on a detector array. Increased integration time will allow an 
expansion of the output voltage range (dynamic range) since if one area 
of the array is related to a area of very low transmission rate and 
another is a very high transmission rate the number of photons counted 
will between areas will expand with the integration time.

This will get the output A/D's more signal to work with. In many cases 
coherent optical process can result into upwards to 90 dB spur free 
dymanic range and non-coherent up to 60 dB. I would expect the same 
thing could be realized in scanners, HOWEVER, there is a DC shift in the 
noise floor with this integration and it has to be accounted for in the 
process, and it probably isn't.

Truman

Austin Franklin wrote:

>Hi Truman,
>
>  
>
>>A CCD counts photons and produces a voltage as a function of the number
>>of photons detected over a time window (integration time). So while it
>>is probably a misnomer to call anything associated with CCD's analogue,
>>the effect is similar if it is done correctly.
>>    
>>
>
>Hum...  Gain is a multiplication factor, increasing exposure is a shift
>(called an offset).  One will expand the range, the other will simply shift
>it.  Right?  Basically, there are three things you can adjust in a signal.
>Gain, offset and linearity.  Gain is a multiply, offset is an add, and
>linearity is an add as well, but is on an individual value, not an overall
>value, like a LUT (Look Up Table).
>
>  
>
>>I would much prefer an integration time based on the film and then gain
>>adjustments of the output voltage prior to the analog to digital
>>converters - with some care taken as not to increase the background
>>noise level.
>>    
>>
>
>Correct.  That is exactly how I have done it, and noise is the operative
>problem.
>
>Regards,
>
>Austin
>
>
>
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