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Well variation is not that big actually. There's some variability in black density, and some variation in the amount of residual black on general pageturns, but the base white is pretty consistent across Vizplex, and the black never really gets dark (I don't have any real numbers to cite a variation range, but probably less than +/-10%). On some readers, the black density can be tuned a little, which I believe is sometimes done for static display models, but usually comes with serious side-effects (more extreme ghosting for example) and still never reaches anything close to "black".
What most people actually see are lighting effects and reflections. If the device is reflecting a dark surface, it'll show both a darker black and a darker white, but the overall contrast will appear higher. If a white surface is being reflected, the white will appear whiter but the text will seem grayer and more washed out.
Another thing is that oblique point lighting combines with the silver reader's surface in particular to create an interesting optical effect. When held at certain angles, the sandblasted metal casing will scatter or reflect light away from the eyes (thus appearing darker), while the Vizplex will retain its reflectivity. Since the brain "knows" the case is a silverlike metallic material, it interprets the white state of the e-ink to be much brighter.
No e-ink device really has high contrast, a white white, or a black black. The eyes can be cheated into thinking so under some conditions, but the screens never really are white and black, nor have they ever been.
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