Saturation & Demodulation

Working with one of our openSFDI systems, we were getting some strange results related to demodulation of the AC images. Sometimes the demodulation would work well, other times there would be stripes in the image at the same spatial frequency as the projection.

On examination of the data, we found that one of the AC images had a higher DC offset than the other two, which is what was causing stripes in the demodulated images.. What seems to be happening is that if any of the images have regions of saturation (pixel counts of 65530) none of the wavelengths would demodulate properly. Reducing the exposure time of the offending wavelength such that there was no saturation in any of the images solved this problem.

A more permanent solution might be to change the order that images are acquired. The way the control code is currently designed, it displays a pattern on the DMD and collects images for each wavelength before switching patterns. Changing it so all the patterns for a wavelength are collected before the wavelength changes, might prevent a single image with saturation from corrupting the entire dataset.

I’m not sure if this is an issue with the camera, the software, or something else. Let me know if you have thoughts in the comments (or via e-mail).

Condenser Lenses

The original version of the guide recommended using condenser lenses with built in diffusers. The idea was that the diffusers would prevent the LED emitter from being imaged onto the DMD leading to inhomogeneities in illumination. What we found was that the built in diffusers don’t work very well for the long beam paths associated with openSFDI. Too much light is lost around the edges which can lead to low light throughput and long exposure times.

We now recommend using aspheric condenser lenses without diffusers. Specifically the Thorlabs 16 mm focal length aspheric condensing lens ( ACL25416U-B ).

To align this lens first image the emitter on the DMD surface. You should see a series of stripes. Then move the lens toward or away from the LED until the image of the emitter is blurred and the illumination on the DMD is uniform. This should increase throughput and allow for shorter exposure times and faster imaging.