  Dichroic filters are used by instrument designers to split a field of view
into different optical paths for simultaneous measurement of different spectral
bands. Quantifying the polarization aberrations of a dichroic is relevant for
predicting the incident polarization states downstream, which could affect the
performance of diffraction limited systems. One important application is the
fore-optics of exoplanet imaging coronagraphs. In this work, the polarization
properties of the Edmund #69-205 650 nm roll-off dichroic are measured using a
rotating retarder Mueller matrix imaging polarimeter. The polarization
properties of this commercial dichroic are compared at normal and 45$^{\circ}$
angle of incidence. The normal incidence measurements verify the instrument
calibration since no polarization aberrations were observed. Transmission
measurements at 680 nm and 45$^{\circ}$ yield a 2.9 rad magnitude of retardance
and 0.95 diattenuation. Effectively, at 630 nm the dichroic is a $\lambda$/4
waveplate with a horizontal fast-axis.
