Collimation technique and testing applied to finite size polychromatic sources

Abstract

Highly collimated beams are required in numerous applications and techniques. Different methods have been proposed for collimating monochromatic point light sources during the recent years. In this work, we analyze how a finite size and polychromatic light source can be collimated using only one diffraction grating and a CMOS camera placed after the source and the collimating lens. For this, we determine the period of the fringes diffracted by the grating and compare it with the period of the grating. Analytical equations are obtained to predict the amplitude of the fringes and their period. Since self-images disappear for finite size polychromatic sources at long distances from the grating, the period has to be measured close to the grating. In addition, we give an analytical equation to determine the error in the positioning of the source in terms of the source size and the setup parameters. Finally, we experimentally corroborate the obtained analytical formalism using a white LED of size s = 0.6 mm collimated by a lens with focal length f = 25 mm, and a Ronchi binary grating of period d = 250 mu m. In this case, we achieve an experimental error in the positioning of the source with respect to the focal plane of the lens of delta z(exp) = 92 mu m.