Applied Sciences
LCD fourth filter
Liquid crystal displays are made from two glass sheets. Typically one glass carries the color filter – comprising sub pixels of red, green and blue, separated by a black mask– and the second carries the transistors and electrode structure that drive the liquid crystal. The liquid crystal itself is a very thin layer of fluid, held between the two glass sheets.
In some types of liquid crystal display devices, that utilize in-cell touch detection, the transistor glass faces the user, instead of the color filter glass.
Reflections from the transistor glass are higher than the color filter glass, and the overall reflectivity of the panel can be ~10%.
It’s desirable to mask off these internal reflections, to improve the usability of the display. One method, discussed here, is to use an external filter layer that removes light between the color primaries, thereby reducing the reflectivity to broadband light sources (like sunlight).
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Dye Spectrum Requirement
We propose an RGB-pass dye formulation using Epolight 5699 and 5841.
We optimized the dye concentration for 75% luminous transmission, and color neutrality when the filter is attached to the display. -
Dye Reflectivity
The reflection level is determined using reflectivity of panel and D65 spectrum
Reflectivity from the panel structure under D65 illuminant is reduced to ~0.65 of previous value (theoretical).
The first surface reflection of course remains unchanged ~4%, and overall reflection is color neutral. -
Dye Sample
Epolin provided some samples of plastic film with incorporated dye.
Cosmetic quality was extremely poor, but reduction of reflectivity was verified.
L* (measure of visual magnitude of reflection) is reduced by 15%.
The green-ish color of the panel reflection is partially corrected. -
Panel Emmission Spectra
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Measured Display Emission Color
The display in this case has smaller than sRGB gamut.
The addition of the reflection filter layer does not significantly alter the white point and improves the gamut by reducing the bandwidths of the color primaries. -
Emission Luminance
Emission is significantly reduced leading to ~25% reduction in display luminance overall.
However much of this is bandwidth reduction. The native LCD filter density is not enough to meet sRGB color gamut to start with. -
Conclusion
An anti-RGB dye filter was developed and formulated into film samples.
The reflectivity of the panel was reduced.
The display color gamut was improved.
