and focusing of light from source 11 into reference and sample beams r s transmitted along axes r s is achieved in the demonstration unit using off axis parabolic mirrors 32 33 rather than spherical mirrors parabolic mirrors being preferable in order to minimize image blur caused by mirror aberrations and to minimize image size at detector 17 image preservation along both sample and reference optics 19 21 is therefore substantially improved u02dc 6 db without using beamsplitters lenses or rotating mirrors chromatic aberration light absorption and background emission in optics 19 21 are substantially eliminated signal levels of beams r s are maximized system reliability is optimized and optics 19 21 may therefore be used at substantially any wavelength from the uv to the far ir light chopper 35 is disposed along axes r s near sample 20 for selectively modulating beams r s to reduce background electronic and thermal noise and so that densitometer 10 can be operated substantially immune from drift on long sec time scales chopper 35 may comprise a rotatable polished aluminum blade disposed along axes r s near sample 20 as suggested in fig1 to take advantage the folding of beams r s provided by mirrors 30 31 so that blade size is minimized blade surface may be unanodized to exhibit low.