Manipulation of image gray levels by variable transmittance in methyl red-doped E7

Abstract

Image processing with optical components is a topic of recent interest, mostly due to the advent of numerous applications associated with nonlinear optical materials. All optical techniques also represent real-time information processing not available with electronics-based technology. Amplification and contrast enhancement of gray-scale images using a photorefractive crystal have been reported, but the technique involved the application of a large external field and a relatively bulky set-up.
Liquid crystals are anisotropic, organic substances which exhibit an ordered arrangement of molecules. E7 is a nematic liquid crystal (NLC), implying a long-range orientational order of the molecular axes. Methyl red is a dichroic dye present in the LC cell in small concentrations. The addition of a dye modifies the optical properties of the liquid crystal, due to light-induced orientational or molecular changes in the dye molecules.
Consider light polarized along the z-axis. The probability of absorption is proportional to cos²θ, where θ is the angle between the absorption dipole of the dye molecule and the z-axis. The dye molecules are mostly aligned along the director of the NLC. It thus becomes possible to control the absorption of a dye-doped NLC by rotating it about the propagation axis of a polarized laser beam. The change in transmittance results in the intensity modulation of gray-scale images made to pass through the cell.
In this paper, a simple image processing set-up based on a dye-doped nematic liquid crystal is presented.