Electrophoretic display (EPD) is a paper-like, bi-stable, reflective low power technology that comes in both black & white (B/W) and colored formations. Behind everything is the electronic ink, whose reflectance (grey scale) can be controlled with voltage (16 levels). Resolution of modern EPD can be up to 300ppi. EPDs are outdoor-readable, perfectly suitable for static content with zero power consumption, yet allowing decent browsing capabilities. However, EPD is falling short in high-speed video content due to the rather slow frame rate of about <10Hz for the complete page update. Perhaps the best-known application of an EPD is Amazon Kindle.
The most important property of the B/W EPD is the contrast ratio (CR). From the front lighting viewpoint, on the other hand, the most important aspect is the ability of the system to maintain the CR when the F/L is switched on. This has a lot to do with the light extracting microstructures, their shape and replication accuracy. Namely, any upward scattering/haze introduced by the frontlight operation will decrease the CR. Nanocomp’s frontlight with optimally designed and perfectly replicated microstructures offers superior CR levels for the frontlit display.
Color in EPDs is done either with color filters, or with reflective colored pigments, the actual ink. With color filters, only the black and white ink are needed, which allows for the screen refresh rates to match those of the B/W version. When the colors are realized with ink, the sequential switching between the different colored states (R, G, B) takes more time, but on the other hand, the colors are purer than with the filters.
Most consumer products build on color EPD are implemented with color filters to allow for the most versatile content and various media to be accessed with these devices. Since the filters are of a certain physical distance above the actual ink pixels, there can be leakage of incident light from one pixel to a neighboring one, which causes colors to mix and thus reduce in purity. This means a narrowed color gamut. To compensate for that, the incident light used to illuminate the display should be steep- and narrow angled. This can be implemented with Nanocomp’s frontlight, which indeed can enhance the color gamut of the display, if the F/L is kept operational whenever the display is in state of dynamic use.
In addition to CR and color gamut, one important aspect of Nanocomp’s technical excellence lies in the control of color consistency (stable white balance) across the display area. The plastic materials used to stack up the display modules, including the sealing layers, touch panel, optical adhesives, and protective layer, tend to shift the color of light when it passes through them multiple times. This is especially the case for the front light sources (LEDs), when they are coupled in to the LGP at one edge of the display, and then travel the full length of the display while interacting with these layers multiple times. Nanocomp’s UV replication material, onto which the microstructures are imprinted in the R2R process, can be precisely finetuned by special colorants to compensate for this color shift. This ensures that the frontlight supplied by Nanocomp always stands for the highest color performance and meets the customer´s spec., regardless of the display size.