Partially coherent light-emitting diode illumination for video-rate in-line holographic microscopy

The light of a light emitting diode (LED) or a common thermal source like a tungsten filament lamp is known to be quasi incoherent. We generate partially coherent light of these sources with a volume of coherence in the micrometer range of (5…100 µm)³ by spatial and spectral filtering. The corresponding degree of partial coherence is adapted for microscopic interference setups like a digital in line holographic microscope. The practicability of the sources is determined by the spectral emittance and the resulting signal to noise ratio (SNR) of the detector. The microscale coherence in correlation with the SNR and its resolution for microscopy is analyzed. It is demonstrated how low light level non laser sources enable holographic imaging with video frame rate (25 fps), an intermediate SNR of 8 dB, and a volume of coherence of 3.4*10^4 µm³. Holograms of objects with a lateral resolution of 1 µm are achieved using a microscope lens (50× / NA = 0.7) and a CCD camera featuring a 4…12 bit dynamic range.