automatic brightness correction images

[SDmanc]

Hello all,

Ive a problem with my prosillica EC640c, basically im using a rs232 monochromator and a digital camera to make a spectrometer- i take a set of reference images and then a set of sample images and compare the brightness of certain regions of the two pairs of images (point one on the ref and the same point on the sample, then take a ratio etc...).

the problem is that the there seems to be some sort of automatic brightness correction applied, in that when i take subsequent sets of sample images the brightest region of the second set has a higher intensity than the darkest of the previous... even though it should be less as the samples grow more concentrated. it seems as if the other regions of the image are scaled down in brightness from the brightest point..

ive turned off the gain, gamma reduced the shutter speed, played (a very small bit) with the white balance(s) and still cant get the 'correct/expected' intensity.

any ideas or suggestions would be much appreciated.

regards,

David 

[muks]

What is the lighting source you are using. Can it be ambience? What is aperture level of your lens?

[SDmanc]

its a newport monochromator, as for ambient light it could be.. but i try to do it all in as complete darkness as possible.

as for the lens its a 70mm telecentric lens with a mechanical iris- is this what you mean?

[muks]

There is one golden rule in machine vision.That is to operate the camera with minimum lens aperture and with a bright illumination.Try this to make sure that ambience is not the problem

[BruceAmmons]

Make sure your camera is not set to automatically adjust the gain or shutter speed.  Keeping them both fixed is critical for repeatability.

Bruce

[muks]

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Make sure your camera is not set to automatically adjust the gain or shutter speed.  Keeping them both fixed is critical for repeatability.

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Couldnt agree more