Sunday, November 14, 2010

E Ink Color Display - handicapped because of a fundamental color mistake?

E Ink Inc. whose Pearl greyscale displays are used in all the major e-reader devices have introduced a color e-paper display. If successful it may also compete against the new class of tablet computers such as Apple's iPad.

But is the color display handicapped because its engineers made a fundamental mistake in color technology?

E Ink's new Triton display will come in 5-, 7-, and 10-inch varieties, and E Ink seems to think that it will be instrumental in establishing digital newspapers and periodicals.It is claimed to be able to display “thousands of colors,” specifically 4096. However the color is extremely desaturated even in their carefully presented marketing materials. The display's lack of color saturation may actually reveal the cause of the problem.

How the display works

E Ink is short for “electrophoretic ink”. Technically speaking, charged pigments suspended in a clear liquid micro-capsule respond to a voltage that moves black or white pigments to the screen’s foreground.The technology differs from traditional displays because electrophoretic displays reflect light, rather than emitting it. Whereas computer displays and mobile phone screens rely on a backlight to illuminate pixels of different colors, E Ink technology leverages ambient light just like ink on paper.

With the E Ink Triton color configuration, a thin transparent colored filter array (CFA) is added in front of the black and white display. Now the display can also reflect color.The CFA consists four sub-pixels – red, green, blue, and
white – that are combined to create a full-color pixel. The result? A low-power, direct-sunlight, readable color ePaper display.

So what's the problem?

Emissive color displays like those used in LCD computer displays and televisions are based on the additive color model and use red, green, and blue light to produce the other colors. Combining one of these additive primary colors with another in equal amounts produces the additive secondary colors cyan, magenta, and yellow. Combining all three primary lights (colors) in equal intensities produces white. Varying the luminosity of each light (color) eventually reveals the full gamut of those three lights (colors).
The additive color model used by emissive color displays uses combinations of red, green, and blue primaries.

On the other hand, reflective color displays like newspapers and magazines use the subtractive color model which starts with light, presumably white light. Colored inks, or filters, between the viewer and the reflective surface subtract wavelengths from the light, giving it color. In most color printing, the primary ink colors used are cyan, magenta, and yellow.
The subtractive color model used by reflective color displays uses combinations of cyan, magenta, and yellow primaries.

E Ink's Triton color display, although a reflective device, uses the primary colors of emissive devices - red, green, and blue rather than cyan, magenta, and yellow. The result is very poor color saturation and very much reduced color gamut.
Triton color display screen captures - note the absence of Yellow - a color that cannot be achieved with combinations of red, green, and blue filters in a reflective, subtractive color-based display system.

video
An E Ink video explaining the RGB(!) display technology.

Did the engineers made a fundamental mistake in their choice of color technology?

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