  An optical microscope is described that reveals contrast in the Mueller
matrix images of a thin, transparent or semi-transparent specimen located
within an anisotropic object plane (anisotropic filter). The specimen changes
the anisotropy of the filter and thereby produces contrast within the Mueller
matrix images. Here we use an anisotropic filter composed of a
semi-transparent, nanostructured thin film with sub-wavelength thickness placed
within the object plane. The sample is illuminated as in common optical
microscopy but the light is modulated in its polarization using combinations of
linear polarizers and phase plate (compensator) to control and analyze the
state of polarization. Direct generalized ellipsometry data analysis approaches
permit extraction of fundamental Mueller matrix object plane images dispensing
with the need of Fourier expansion methods. Generalized ellipsometry model
approaches are used for quantitative image analyses. We demonstrate the
anisotropic contrast optical microscope by measuring lithographically defined
micro-patterned anisotropic filters, and we quantify the adsorption of an
organic self-assembled monolayer film onto the anisotropic filter. In our
current instrument we estimate the limit of detection for organic volumetric
mass within the object plane of $\approx$ 49 fg within $\approx$
7$\times$7~$\mu$m$^2$ object surface area. Compared to a quartz crystal
microbalance with dissipation instrumentation, where contemporary limits
require a total load of $\approx$ 500~pg for detection, the instrumentation
demonstrated here improves sensitivity to a total mass required for detection
by 4 orders of magnitude. We present further applications to detection of
nanoparticles, to novel approaches for imaging chromatography, and to new
contrast modalities for observations on living cells.
