I was just thinking of something similar, except I was thinking B&W film=20
negative passing through an optical interrupter, with gradual darkening=20
photographed on to it.

Kerry


RussellMc wrote:
>> Gray code encoders, if cheap enough, usually are rotary (I never saw one=
 linear
>> at least). Pretty expensive anyway.
>>    =20
>
> Gray code encoder can be optical or use other sensors
> Gray code can be a SINGLE track with multiple sensors spaced along it
> in such a way that they between them "see" a gray code. Mathematicians
> used to say it was impossible. Nowadays its done. I don't know how
> hard it is to get 6 or 7 bits.
> BUT if you can use and end of track detector then you need simply two
> detectors in quadrature on a single track. Very standard. cf eg
> mechanical wheeled mouse. Each axis ha one "track" and two sensors.
>
> 36mm / 6 bits is about 0.5 mm/bit. Somewhat fine.
>
> An optical "window" with width affected by position would probably be
> easy enough. read light level shining through "window" to get
> position.
>
>
>      Russell
>
>
>
>  =20



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