To understand how a sensory input leads to a particular behavioral output, we are studying prey capture behavior in zebrafish larvae. First, we would like to describe the behavior in a quantitative manner using several parameters (position, swimming speed, approach to prey, eye convergence, etc.). These parameters may be obtained automatically by image processing. For a successful image processing, lighting condition is a critical issue.
Here are some tricks that make it possible to isolate zebrafish larval eyes and paramecia from the background.
I came to this solution after a bit of trial and error.
This is a darkfield illumination but the background is not too dark.
So, the paramecia appear in white and the zebrafish larval eyes appear in black on the gray background.
Uniform background and enough contrast for both the zebrafish larva and paramecia.