Best practice when camera exposure must be changed between each grab?

Question

Hi all,
I'm acquiring two images in fairly quick succession from a GigE camera. In an initialization state I use 'Open' then 'Configure Grab'. Then, in my acquire state I read in the exposure required via a property node and use a 'Grab'. Then I loop around and do a second 'Grab'. The first image has a long exposure, the second image needs a shorter exposure.
Mostly this works well but occasionally my second image is too bright (meaning the exposure is too long.) Is this a problem with how I'm acquiring the image? (Is 'Grab' the best vi to use?) Or, do I need to give the camera more time to make the change? (Add in a delay?)
All thoughts must appreciated.

Answer 1

The problem is that you never really know when your new exposure is received and when it is applied.
Moreover when you call the grab, you do not really acquire an image (the acquisition process in your case seems to be continuous), you just read an image in the computer memory.
So your Grab/change exposure time/grab sequence does not include the "real" acquisition process (exposing the camera sensor to generate an image).
There are many possibilities, most of them require triggering. For most of them I would recommend the use the "low level" Vis instead the "Configure Grab/Grab" ones :
 1-  For every image, stop acquisition, set exposure then start acquisition and get the image.
 2- One technique you could try, is to use software trigger to control the image acquisition process :
    - configure the camera to trigger mode and Software trigger as the trigger source
    - for each image :
           - set the exposure time,
           - if needed, add a short delay
           - use a property node to generate a sw trigger
           - read the image.
 3- Most of cameras include a pulse width trigger mode : the width of the exposure pulse defines the exposure time for the image. So, if you're able to generate a pulse train with alternate pulse widths (long pulse for the first image, short pulse for the second one), it is probably the most efficient way as you can use the camera at maximum frame rate if your trigger pulse train is well defined.
 4- Some cameras have special features dedicated to the task you want to implement : for instance if you use a Sony camera, you may check if it includes a feature called bulk mode.
Photonfocus cameras have a powerful I/O controller that you can use to create sophisticated tasks. However, this will probably require a good understanding if the hardware and maybe some tricky programming.
Hope this helps,
Sami
Sami Fathallah
ALLIANCE VISION
HW and SW Components for Scientific Imaging and Machine Vision
http://www.alliancevision.com