Study and design of colour correction optical filters

Abstract

The aim of this work is to assess the potential use of frequency selective optical filters as wearable devices for colour correction in human visual perception, establishing computational design methods based on numerical optimization for the design of these devices, and fabricating some of these using well established deposition techniques.Computational colour appearance models based on physiological cone sensitivity functions were implemented, and red-green anomalous trichromacy was simulated according to the photopigment replacement model. Using numerical optimization, a set of ideal optical properties was computed by minimizing the average total colour and chroma differences between the perception of normal and anomalous trichromats provided with a frequency selective filter. It was found that while some enhancement in colour perception can be achieved by blocking light in the overlap region between the cone sensitivity functions, the difference is imperceptible to most observers. Optimizing for the average square, and higher powers, it was found that wider stopbands in the cone overlap region can improve the discrimination of colours whose anomalous perception is further away from the normal response (such as red, green, and orange). While the total number of discernible colours is reduced by the effect of the designed optical filters, caused majorly by the reduction in the luminance value, there is a significant increase in colour saturation. Optical filter design methods based on refinement of an initial solution, needle synthesis (NS) methodology, and genetic algorithm (GA) synthesis. While NS can provide high precision and computation efficiency, GA is a flexible and robust alternative for optical filter design. Silica/silicon nitride multilayers designed by these methods were fabricated using plasma enhanced chemical vapour deposition (PECVD), and compared to well-known optical devices, such as notch filters, and dielectric mirrors. Spectral characterization of the fabricated devices is in good agreement with expectations, both for normal and oblique incidence. Other photonic structures, such as colloidal crystals and inverse opals, fabricated by colloidal self-assembly, were also considered for this application. The results from this work support the idea of using optical filters for passive assistance in redgreen anomalous trichromacy. The trade-off between the improvement and detriment in the perception of different colours, and the correlation between optical power and manufacturing costs, makes the design of these types of optical filters an open-ended question