  Many image processing operations involve the modification of the spatial
frequency content of images. Here we demonstrate object-plane spatial frequency
filtering utilizing the angular sensitivity of a commercial spectral bandstop
filter. This approach to all-optical image processing is shown to generate
real-time pseudo-3D images of transparent biological and other samples, such as
human cervical cancer cells. This work demonstrates the potential of non-local,
non-interferometric approaches to image processing for uses in label-free
biological cell imaging and dynamical monitoring.
