From: Peter L. Peres

> The mechanical construction must be such that the device does not have
> 'play' larger than about lambda/2 if you need precision. Not your average
> ruler, and I don't think that the average laser diode will work here ;-)
>
> Peter
>
> Yup.  plenty of cheap digital calipers around in the import tool
> catalogs.  Up to 6" span, they are cheap, and accurate to around
> 0.001".
>
> ------------
> Barry King, KA1NLH

The catalogues I've looked in show inside, outside, depth, height but
not length per se and definitely nowhere near 500mm. Extensometers come
close, but no cigar. What I was thinking of was a quick and easy way to
measure a line/distance on paper/photograph to 0.1mm or better and then
perform a scaling calculation. Once I get a read head working the length
of the ruler is immaterial, it's only printing

The idea was to use a simple slider, not a screw mechanism, and
optically measure the distance by counting the fixed divisions from a
zero point, like you say, by transmission or reflection and then finding
the best coincidence between the fixed and the sliding vernier markings
to get the decimal place. I figured with 1.0mm fixed and 0.9mm Vernier
markings a reading to .1mm would be doable. As for interferometry, thanks
for the mechanical advice, my Rubbish Workshop is not in that league at
all at all. I think I'll have to fall back on Plan B, which involves some
degree of precision. The PIC would look at 9 miniature interrupters, one
across each Vernier mark, possibly using lenses to magnify the effect, to
determine which has the lowest output. "Might" ;-) be tricky knocking up
a prototype but it'll be fun trying.