Use of light-emitting diode (LED) in interference microscopy

We present a systematic investigation of the use of LED as light sources for interference microscopy, in comparison with more standard halogen illumination. For translation height mode (also known as vertical scanning or low coherence microscopy), five white LED-based illuminations setup have been tested, including the use of filters to remove the shoulder in the blue region often encountered in such LED. For the six white light illuminations (five LED plus halogen), we have measured the irradiance spectra and calculated and measured the corresponding correlograms. The influence of the combined effect of the illumination spectra and a dispersive phase shift on the calculated height reconstruction is shown for a center-of mass algorithm. In phase shift mode, both monochromatic LED and white LED with inteference filters have been used. Blue LED illumination improves the lateral resolution compared to red illumination, a task which can be done with halogen lamp only with very reflecting sample due to its low power in the blue wavelengths. All measurements have been performed with. our home-made interference microscope, which is described in Proc. SPIE 6188,61880T (2006).