Re: Curious about correcting exposure times for time-series imagesets

Posted by Michael Schmid on Feb 16, 2010; 10:28am
URL: http://imagej.273.s1.nabble.com/Curious-about-correcting-exposure-times-for-time-series-imagesets-tp3689371p3689373.html

Hi Chris,

apart form the nonlinearity problem (which is really important!):

Most Canon cameras have a long exposure mode (night mode, typically 1  
second to 15 seconds) where you can set the exposure manually. In  
this mode, the camera also takes a dark frame and subtracts it from  
the image. It might work without overexposure if you choose a low ISO  
value.

For the shorter exposure times, it might help to make test exposures  
of two light sources, one with variable brightness to force the  
camera change the exposure, and in the same image one light source of  
fixed brightness. Then you will see how it scales with exposure time  
(this will also tell you something about nonlinearity).

Michael
________________________________________________________________

On 16 Feb 2010, at 11:12, Gluender wrote:

> Chris,
>
> besides your core question and because you are using a consumer  
> camera:
>
> Did you compensate for this camera's gamma?
>
> Before you do any scientific image analysis or processing, make  
> sure that the image data doesn't suffer from any nonlinear  
> transduction process.
>
>> Hi...
>> I have several series of time-lapse photos of transport of  
>> fluorescent dye in leaves, 120 pics/series.
>> Wanting to measure increase of dye in a leaf overall by counting  
>> the number of pixels containing
>> beyond a certain threshold of that dye color.
>>
>> We've been driving a Canon A40 camera with remote-control software  
>> that let us set everything
>> but exposure time. Didn't think that there would be much variation  
>> in this, but it turned out that
>> exposure times ranged from 1 sec (darkroom, UV light) to about  
>> 1/20sec. This has pretty much
>> leveled the dye data overall, unfortunately.
>>
>> I've written batch processing code for extracting EXIF data from  
>> each JPEG image for the exposure
>> time. Questions:
>>
>> 1) Assuming that the best approach would be to decrease the image  
>> brightness inverse-proportionally
>> to exposure time, is there a "sanctioned" algorithm for doing  
>> this? If we could do this over again, I
>> would include a reference card (white, 50% gray, black) in each  
>> pic for equalizing levels. Instead,
>> thinking that I'll use a rectangular region in the image that  
>> should be invariant across each series,
>> and equalize with that even though it's not grayscale.
>>
>> 2) It doesn't seem that equalizing histograms would be viable, as  
>> the images get brighter as the
>> dye migrates through each leaf.
>>
>> 2) Is there a better way to do this? Dare I hope that someone has  
>> done a plug-in for this?
>>
>> Regards,
>> Chris
>
> Best
> --
>
>                   Herbie
>
>          ------------------------
>          <http://www.gluender.de>