It is demonstrated experimentally, as well as analytically, that when
the polarization of the light incident upon the first Nomarski-Wollast
on prism in a differential interference contrast (DIG) light microscop
e is switched by 90 degrees, image highlights are changed into shadows
and vice versa, Using an inexpensive ferroelectric liquid-crystal mod
ulator, which is easily installed in the microscope, this switching ca
n be done at 30 frames s(-1), synchronized to the camera. Subtraction
of alternate digitized frames generates a stream of images in which co
ntrast is doubled, compared with conventional video-enhanced DIG, whil
e image defects and noise tend to cancel. Subtraction of alternate ima
ges is carried out efficiently by frame buffer operations and amounts
to massively parallel synchronous detection, The new method eliminates
the problems inherent in obtaining a separate background image, as re
quired by current video-enhanced DIC practice, without loss of resolut
ion.