  Optical detection and spectroscopy of single molecules has become an
indispensable tool in biological imaging and sensing. Its success is based on
fluorescence of organic dye molecules under carefully engineered laser
illumination. In this paper we demonstrate optical detection of single
molecules on a wide-field microscope with an illumination based on a
commercially available, green light-emitting diode. The results are directly
compared with laser illumination in the same experimental configuration. The
setup and the limiting factors, such as light transfer to the sample, spectral
filtering and the resulting signal-to-noise ratio are discussed. A theoretical
and an experimental approach to estimate these parameters are presented. The
results can be adapted to other single emitter and illumination schemes.
