>From what I've been able to gather of the requirements
>so far, you have 10 or so plasticized pages, in a binder,
>that flip down/up much like a clip board would. You also
>need very low cost, ($3 in quantity) and presumably have
>a PIC/LCD so you have some sort of power for the sensors.

OK, on this revised statement of requirements, I would suggest the
following.

Two sensors, such as LDR (fast response no needed), but photodiodes may
provide smaller sensors.

Each page has a varying size of mark printed on the edge - the sort of big
black mark that is used to index the edge of pages in telephone directories,
catalogues, and the like. By making each page have a different sized mark,
the reflectivity of the each page in this area will be different according
to how much black there is. The second sensor is used to compare the "full
white" somewhere else on the page, and the ratio of the two takes care of
varying light levels. You may want to have an IR LED to illuminate the page
to get rid of other possible problems, like human shadow on the white sense
area, but not on the black.

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