13. Brooks' states the following with respect to phase
masks, distinguishing phase masks from amplitude masks:
The diffracted light beam 35 eventually
impinges on an opaque filter 38. This may, for
example, be an opaque plate such as a metallic
plate having a suitable pattern cut out. Such a
plate may form an amplitude mask which masks the
diffracted light beam in accordance with its
amplitude. Alternatively a phase mask may be
used, this may, for example, consist of glass
plate having a varying thickness so as to delay
the wave front by different amounts depending on
the thickness of the glass. (Column 3, lines 59
65).
14. In the involved Hill patent the "phase grating mask" is
shown as a transparent mask, for example, in Fig. la with a
surface relief pattern 5 approximating a square-wave as shown.
FIG. la
2A
T5A
6
15. In the involved Anderson patent the "phase grating
mask" is shown as a transparent mask, for example, in Fig. 2 with
a surface relief pattern 40 approximating a square-wave as shown.
7 Brooks et al. (Brooks), U.S. Patent 3,668,405, granted 6 June 1972, based
on application 04/791,597, filed 16 June 1969. This reference was not relied upon
during the prosecution of the involved Hill or Anderson patents or the Snitzer
involved application.
5
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