Image Engineering
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Note: Please use caution when positioning the camera. The DOE is made of glass and has a
thickness of only 1.5 mm.
Ensure the calibration is performed in a dark environment to avoid stray light, reflections, or
similar interfering factors.
3.3.2
Exposure
For best results, the exposure should be selected so that the individual light points are not saturated (i.e.,
no white pixels or a minimal amount of white pixels in the center of each point) with the exception of the
0
th
diffraction order. The 0
th
order will always be much brighter than the points surrounding it. See Figure
5 below for an example.
Depending on your camera, you might encounter different problems:
The exposure time of my camera cannot be set short enough to avoid saturation.
Possible solutions:
-
Set the ISO to the lowest native value available (ISO 100 for most cameras)
-
Stop down the aperture if possible
-
If the image is still too bright, you could use an ND-Filter to reduce the amount of light
reaching the image sensor
The intensity dropoff is too steep towards the edges of the image, and no more points are visible.
This problem may occur for ultra-wide-angle lenses. If the images can not be adequately analyzed, try the
following.
Generate an HDR image by taking multiple images of the point grid using a range of exposure times (do
not change aperture settings in this case) and combine them into a high dynamic range image to achieve
more or less uniform exposure across the entire sensor. Do not change the camera's orientation between
images because the points would no longer be aligned to one another through multiple images.
Figure 5: Suitable exposure (dark backgroumd, small points) Exposure too bright (red background, larger, saturated points)
