Differential interference contrast microscopy using light-emitting diode illumination in conjunction with dual optical traps

Differential interference contrast (DIC) microscopy is a common mode of biological light microscopy used to achieve maximal resolution and contrast with label-free, weakly absorbing specimens such as cells. Maintaining the polarization state of the illuminating light is essential for the technique, and this requirement can conflict with optical trapping. We describe how to optimize DIC imaging using a light-emitting diode illumination source in a microscope while integrating a dual optical trap into the set up. Every time a polarized light beam reflects off or transmits through a dichroic mirror in the beam path, its polarization state will change if it is not polarized exactly parallel (p) or perpendicular (s) to the plane of incidence. We observe wavelength-dependent optical rotation and depolarization effects in our illumination light upon reflection from/transmission through dichroic mirrors in the beam path, resulting in significant degradation of image quality. We describe a method to compensate for these effects by introducing quarter-waveplates and a laser clean-up filter into the imaging pathway. We show that this approach achieves a full recovery of image quality. (C) 2013 AIP Publishing LLC.