Optimization of stacks of reflective cholesteric films for full color displays

Researchers from Kent Displays recently demonstrated, bright, full color, reflective displays using stacks of red, green and blue reflecting cholesteric films. We examined the factors affecting the brightness of these stacked displays. We measured the reflectivity of the individual films and of two and three layer stacks. We found that the reflectivity depends on the order of the stacks, being maximized when films reflecting at shorter wavelengths are placed on top. It appears that the effect of the order of the stack results from the asymmetric reflection of films; the reflection curves have a long tail on the shorter wavelength side of the reflection maximum. This tail may result in multiple reflection, reducing the overall brightness of the display if films reflecting at longer wavelengths are place closer to the observer and the light source. We have also found that the reflectivity is maximized if the twist sense of the layers is alternated. Maximum reflection in a three-layer stack, required for full color, is produced with a blue reflecting film on top, a green reflecting film in the middle and a red reflecting film on the bottom. Reflection is also optimized when the middle green film has the opposite twist sense of the blue and red films.