The recent emergence of bright, inexpensive colored LEDs offers several advantages over traditional light sources,
including reduced size and increased portability, low power consumption and heat production, increased durability and
longer life, and high temporal resolution. We assembled a modular array of different Phillips LUMILED LUXEON
LEDs, white and seven colors with peak wavelengths between 450 and 640 nm and bandwidths of 20-30 nm. LED
illumination was fiber-optically coupled to the transmitted light path of an inverted microscope, and digital images of
sectioned human tissue stained with absorbing dyes were acquired using combinations of the white and color LEDs. The
LED array was also coupled to an endoscope and used to image human and mouse tissue in situ. Image contrast was
assessed (1) qualitatively by looking down the microscope and by viewing the digital images, and (2) quantitatively by
using entropy analysis in the real and frequency domains to assess the dependence of contrast enhancement on spatial
frequency. Contrast in image features of a given color range was enhanced by LEDs conjugate to that color, whereas
LED colors spanning a wider range enhanced contrast in the entire image, with white LEDs often maximizing contrast of
tissue. This analysis demonstrates the utility of LED illumination in modulating contrast in light microscopy and
endoscopy, which may facilitate spectral segmentation and classification of image features.
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