Appeal No. 96-2953
Application 08/258,565
The invention relates to an apparatus, such as an
optical encoder, for optical measurement of a displacement of
a moving body. In particular, referring to Figure 12, a light
source 11 and a collimator lens 12 illuminate a fixed
diffraction plate 13. Diffracted light from plate 13 passes
through movable diffraction plate 14 which has the same
grating pitch as plate 13. The step difference “d” of the
height between tops and bottoms of the gratings of the fixed
and movable diffraction plates 13 and 14 is related to the
wavelength 8. A condenser lens 15 condenses light transmitted
through the movable diffraction plate 14 onto photosensor 16.
The movable diffraction plate 14 is usually fixed to a body to
be measured (not shown) and moves in the same way as the body
to be measured. A feature of the invention is that the
entrance pupil of the condenser lens 15 is limited to be
within D-2g8/p where “D” is the size of the beam collimated by
the collimator lens 12, “g” is a distance between the fixed
diffraction plate 13 and the movable diffraction plate 14, 8
is the wavelength of the light and “p” is the pitch of the
gratings formed on the diffraction plates. The distance “g”
between the diffraction plates is set to satisfy the
2
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