Astigmatic illuminating system in an internal reflection spectometer

Abstract

In a spectrometer, an internal reflectance crystal having opposed internally reflecting surfaces extending between light-entrance and light-exit end surfaces is illuminated by a light source through a slit. The beam of light from the slit is reflected from a spherical mirror illuminated off-axis to form first and second astigmatic images of the slit. The tangential image, having a distorted (elongated or extended) height, is formed at the entrance surface of the internal reflectance crystal. The width of the tangential image matches the width of the entrance surface. The sagittal image is formed at the exit surface of the internal reflectance crystal. The width of the sagittal image, which would be distorted (extended) absent the internal reflectance crystal, is limited by the internally reflecting surfaces of the crystal. Thus, there is substantially no loss of radiant energy through the length of the crystal. In one embodiment, the exit surface is curved along its height to make the diverging beam leaving the internal reflectance crystal less divergent and thus more compact at the light detector lens.