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40 cm is good for hand held reading of 10 point type . When the reading distance is decoupled from the need to hold something in your hand -- and the person is looking more in the forward direction than down, the average measured chosen reading distance is greater than 50 cm for the equivalent of ten point type.
See: For normative data on reading 10 point type from a hand held document: SOME NORMATIVE DATA ON READING DISTANCE
DONALD H. SCHUSTER, GEORGE G. KARAS DOUGLAS C. ANTONELLI
Iowa Stare University IBM Systems Development
Division, Rochester, Minn. Perceptual and Motor Skills, 1969, 28, 202.
Hand held reading distance for ten point type is slightly less for young -- high school graduate -- women that for hs grad. young men -- possibly because the women have arms that are about 10-12% shorter.
For normative data on reading 12 point type from printed document taped to a vertical screen and the same document held in the hands. See: "Working distance and eye and head movements during near work in myopes and non-myopes", Andreas Hartwig, et al, Clinical and Experimental Optometry 94.6 November 2011
and "Analysis of head position used by myopes and emmetropes when performing a near-vision reading task"Andreas Hartwig, et al,Vision Research 51 (2011) 1712–1717
The weight, bulk and cost of the system varies in some respects as the cube of the angular image size. The shorter the reading distance , the greater the angular image size. We hope to come out with a basic size and a smaller and larger size to accommodate the great majority of book readers.
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