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Appendix H DIF Camera 101
Tips and Tricks
Lab Illumination
In DIF measurements, it is necessary to remain mindful of the possibility of laboratory
light affecting the images. Because the first image can be timed with precision, laboratory
light that reaches the camera would generally not be a problem in the first image,
particularly if the capture time is short (few microseconds). The second image, on the
other hand, is much more susceptible to degradation from laboratory illumination
because, even though the second image time may be set to just a few microseconds, the
time to close the shutter, ~8 ms, must be added to that value. Light impinging on the
photosensors during that time will be integrated with the second image. Unless the
experiment is arranged so that background light can’t reach the camera, or unless the
signal is quite bright, the possibility of the second image becoming degraded must be
considered. Where this source of degradation is a problem, the solution may be to sharply
reduce the laboratory illumination. It should be noted though, that the signal from many
strobed measurements will be sufficiently bright to allow normal laboratory illumination
to be maintained.
Background Subtraction
In any of the double imaging modes, a good idea would be to block both of your light
sources and go ahead and take two images in the same DIF mode and with the same
settings as will be used for the real measurements. The result will be two background
images that can later be subtracted from the experimental data images.
Background subtraction allows you to automatically subtract any constant background in
your signal. This includes both constant offsets caused by the amplifier system in the
controller as well as time-dependent (but constant for a fixed integration time) buildup of
dark charge. The background subtract equation is:
(Raw image data – Background) = Corrected image data.
When background and flatfield operations are both performed, background subtraction is
always performed first.
Flatfield Correction
Flatfield correction allows the user to divide out small nonuniformities in gain from pixel
to pixel. Flatfield correction is done before the images are saved to RAM or disk.
Directions for doing Flatfield correction are provided in the WinView/32 software
manual.
Mask Bleed-through Correction
As described previously, the first image is stored under the mask while the second image
is being acquired. Although the mask is basically opaque (light attenuation is on the order
of 4000:1), a small amount of illumination does get through and could influence some
measurements. One solution would be to establish a correction file by taking the first
image with the light source dark, and the second image with the light source on. Any
bleed through the mask during the second image will appear in the first image. This data
could then be stored and used later to correct "real" first images in a post-processing math
operation.