Holder for holographic testing of aspherical lenses with spherical and flat reflective surfaces

The present invention provides a lens holder (14) for a lens under test (26) in an in-line hologram interferometric test apparatus (10) which includes a flat reflective surface (34) for rotationally positioning the lens under test and a spherical reflective surface (32) for translationally positioning the lens under test (26). According to this configuration, the lens holder (14) is translationally positioned using the spherical reflective surface (32) to reflect radiation back to the interferometer (12) at a precise distance and position using normal interferometric techniques. The lens under test (26) is rotationally positioned by pivoting the lens holder (14), both in tip and in tilt, so that the flat reflective surface (34) is nulled to the interferometer (12). Thereafter, the lens holder (14) is correctly positioned relative to the hologram (22) and interferometer (12) so that when the lens under test (26) is seated on the lens holder (14), its aspherical shape can be tested.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention generally relates to interferometric testing devices 
and, more particularly, to a holder for a lens under test in an in-line 
hologram interferometric test apparatus. 
2. Discussion 
In-line holographic testing employs a hologram interposed in space relation 
to an interferometer and a lens under test. The interferometer projects a 
beam of radiation through the hologram towards the lens under test. As the 
beam propagates through the hologram a wavefront is created which 
converges to a focal point at a pre-selected focal length from the 
hologram. From the focal point, the beam diverges to the lens under test. 
When the lens under test is correctly positioned with respect to the 
interferometer, hologram, and focal point, the wavefront is reflected back 
through the hologram to the interferometer. The hologram converts the 
wavefront back into an aspherical beam suitable for interference 
patterning by the interferometer. As such, any abnormalities or errors 
associated with the lens under test can be detected and analyzed by the 
interferometer's analysis system. 
During this testing, it is critical to correctly position the lens under 
test with respect to the interferometer in order to accurately and 
precisely reflect back the aspherical wavefront. The exact position for 
the lens under test is often dictated by the characteristics of the 
hologram itself. To properly locate to lens, translational positioning of 
the lens must be performed in the X, Y and Z planes and rotational 
positioning of the lens must be performed in tip (about a horizontal axis) 
and tilt (about a vertical axis). 
According to the prior art, the lens under test is secured to a fixture 
which is initially positioned to the interferometer focus. The lens under 
test is then iterated to the desired position using slides, scales, and 
spacers. This technique requires a large degree of optician skill and 
experience to position the lens under test quickly and accurately. 
Furthermore, rotational positioning of the lens under test according to 
the prior art is achieved largely through trial and error. 
Accordingly, there is a need in the art for an apparatus and method for 
quickly and accurately positioning a lens under test relative to an 
interferometer and hologram in an in-line hologram interferometric test 
apparatus. 
SUMMARY OF THE INVENTION 
The above and other objects are provided by a lens holder for a lens under 
test in an in-line hologram interferometric test apparatus which includes 
a flat reflective surface for rotationally positioning the lens under test 
and a spherical reflective surface for translationally positioning the 
lens under test. According to this configuration, the lens holder is 
translationally positioned using the spherical reflective surface to 
reflect radiation back to the interferometer at a precise distance and 
position using normal interferometric techniques. The lens under test is 
rotationally positioned by pivoting the lens holder, both in tip and in 
tilt, so that the flat reflective surface is nulled to the interferometer. 
Thereafter, the lens holder is correctly positioned relative to the 
hologram and interferometer so that when the lens under test is seated on 
the lens holder, its aspherical shape can be tested. 
In an alternate embodiment of the present invention, the lens under test 
may be angled relative to the hologram and interferometer. To accomplish 
this, the lens seats of the lens holder are oriented at predetermined 
angles with respect to the flat reflective surface. As such, the lens is 
appropriately angled relative to the hologram and interferometer when the 
flat reflective surface is nulled to the interferometer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed towards a lens holder for supporting and 
positioning a lens under test in an in-line hologram interferometric test 
apparatus. The lens holder enables the lens under test to be quickly and 
easily translated (i.e., in the X, Y and Z planes) and rotated (i.e., 
about the horizontal and vertical axes) relative to the hologram and the 
interferometer so that errors in the lens under test can be detected and 
analyzed by the interferometer. To accomplish this, the lens holder 
includes a spherically shaped reflective surface which is used in 
conjunction with the interferometer to translationally position the lens 
under test with respect to the interferometer and hologram in the X, Y and 
Z planes. The lens holder also includes a flat (i.e., planar) shaped 
reflective surface for rotationally positioning the lens under test in 
conjunction with the interferometer with respect to the hologram and 
interferometer about the vertical and horizontal axes. The present 
invention overcomes the shortcomings of the prior art by alleviating the 
need to initially position the lens under test at the focus of the 
interferometer and then iterating to position the lens under test at the 
appropriate location for testing. 
Referring now to the drawings, an apparatus 10 for testing aspherical 
lenses according to the in-line hologram interferometric method is 
illustrated. The apparatus 10 includes an interferometer 12 of a known 
type located in spaced relation to a lens holder 14. As depicted, the lens 
holder 14 is located in radiation receiving relation to the interferometer 
12. However, it is to be understood that the present invention is directed 
towards positioning the lens holder 14 to this location. 
A conventional interferometric reference sphere 16 is coupled to the 
interferometer 12 for shaping the wavefront of the radiation 18 emitted 
from the interferometer 12. In this case, the reference sphere 16 causes 
the radiation 18 emitted from the interferometer 12 to be shaped as a 
spherical beam 20. The spherical beam 20 passes through an in-line 
hologram 22 interposed between the interferometer 12 and the lens holder 
14 and propagates towards the lens holder 14. 
The hologram 22 is positioned at a pre-selected location with respect to 
the interferometer 12 as dictated by the test set-up using the hologram's 
own conventional positioning system. After passing through the hologram 
22, the beam 20 converges to a focal point 24. From the focal point 24, 
the beam 20 diverges towards the lens holder 14 and a lens under test 26. 
The hologram 22 creates a wavefront which, when the lens under test 26 is 
correctly positioned, reflects back through the hologram 22 and is 
converted back into an aspherical wavefront with errors associated with 
the lens under test 26 which can be analyzed by the analysis system of the 
interferometer 12. 
The lens holder 14 includes a first lens seat 28 and a second lens seat 30 
for supporting the lens under test 26. As described in greater detail 
below, the second lens seat 30 may also be used to angle the lens under 
test 26 relative to the spherical beam 20 and interferometer 12. To 
correctly position the lens under test 26, the lens holder 14 also 
includes a spherical reflective surface 32 and a flat reflective surface 
34. Although it is presently preferred to form the lens holder 14 from 
brass or other reflective metallic material and then to grind in the 
reflective surfaces 32 and 34, the skilled artisan will appreciate that 
other techniques such as silvering may also be utilized for this purpose. 
The spherical reflective surface 32 includes a radius of curvature R which 
is equal to the distance from the focal point 24 required by the 
holographic test set-up for properly positioning the lens under test 26 
relative to the hologram 22 and interferometer 12. One skilled in the art 
will recognize that this distance varies from test set-up to test set-up 
as dictated by the hologram employed therein. However, this distance is 
consistent in that the required spacing between the lens under test 26 and 
the hologram 22 is known for each test apparatus depending on the hologram 
used therein and the radius of curvature R is selected such that the lens 
under test 26 is properly positioned when the spherical reflective surface 
32 is properly positioned. As such, when the lens holder 14 is translated 
to the appropriate position relative to the interferometer 12 and hologram 
22, the beam 20 is reflected from the spherical reflective surface 32 
along the same ray path to the interferometer 12. This position is 
indicated by a first known interference pattern displayed on the 
interferometer 12. 
The flat reflective surface 34 is designed such that parallel radiation (to 
be described in greater detail below with reference to FIG. 2) is 
reflected directly back to the interferometer 12 when the lens holder 14 
is rotated to the appropriate angle relative to the interferometer 12 and 
hologram 22. The appropriate angle is indicated by a second known 
interference pattern displayed on the interferometer 12 when the flat 
reflective surface 34 is nulled to the interferometer 12 and hologram 22. 
In other words, the lens holder 14 is correctly angled when the flat 
reflective surface 34 is perpendicular to the parallel radiation. 
In order to correctly position the lens under test 26 for interferometric 
testing, the hologram 22 is placed on the spherical beam 20 emitted from 
the interferometer 12 at a precisely determined place using the 
positioning system of the hologram 22. Next, the lens holder 14 is 
translationally and rotationally positioned relative to the hologram 22 
and interferometer 12 as dictated by the holographic test set-up. To 
accomplish this, the spherical and flat reflective surfaces 32 and 34 are 
employed to create known interference patterns in the interferometer. 
More particularly, the lens holder 14 is translationally positioned in the 
X, Y and Z planes relative to the interferometer 12 and hologram 22 by 
using the spherical reflective surface 32 to reflect the wavefront 20' 
from the interferometer 12 and hologram 22 back to the interferometer 12. 
Due to its radius of curvature R, the spherical reflective surface 32 
creates a first known interference pattern by reflecting the wavefront 20' 
back to the interferometer 12 when located at a precise distance and 
position. This location is found by using normal interferometric 
techniques while translationally adjusting the lens holder 14. 
Referring now to FIG. 2, after the lens holder 14 is appropriately 
translated, the reference sphere 16 is removed from the interferometer 12 
and replaced with a reference flat 36. The reference flat 36 causes the 
radiation 18 from the interferometer 12 to be shaped as a parallel (i.e., 
non-spherical) beam 38. The beam 38 passes through the hologram 22 without 
converging and impinges upon the flat reflective surface 34 of the lens 
holder 14. When the lens holder 14 is properly angled relative to the 
interferometer 12 and hologram 22, the flat reflective surface 34 reflects 
the beam 38' back through the hologram 22 to the interferometer 12. By 
using normal interferometric techniques while rotating the lens holder 14 
about the vertical axis 40 and the horizontal axis 42, the flat reflective 
surface 34 is positioned so as to be nulled to the interferometer 12. 
After translating and rotating the lens holder 12 as described, the lens 
holder 14 is correctly positioned for supporting the lens under test 26 at 
the appropriate testing point relative to the hologram 22 and 
interferometer 12. Accordingly, the lens under test 26 may be placed on 
the lens seats 28 and 30 and tested for its aspherical shape. An 
additional benefit of the present invention is that the aspherical shape 
of the lens under test 26 can be referenced to the mounting features 
(i.e., lens seats 28 and 30) of the lens holder 14 since their optical 
characteristics are known from the positioning sequence. 
If desired, the reference flat 36 may be removed from the interferometer 12 
after rotational positioning and replaced with the reference sphere 16. 
The translational position of the lens holder 14 can then be verified by 
using the spherical reflective surface 32 and re-translating in the in X, 
Y and Z planes if necessary. In order to facilitate the translation and 
rotation of the lens holder 14, it is preferred that the lens holder 14 be 
mounted to a standard five axis interferometric holder (not shown). 
Referring now to FIG. 3, a second embodiment of the lens holder 14a is 
illustrated. In some instances it may be desirable to angle the lens under 
test 26 with respect to the hologram 22 (FIGS. 1 and 2) for holographic 
testing. To accomplish this, the second lens seat 30a of the lens holder 
14a is oriented at a preselected angle with respect to the flat reflective 
surface 34. As such, when the flat reflective surface 34 is nulled 
relative to the interferometer 12, the second lens seat 30a is 
appropriately positioned for holding the lens under test 26 at the correct 
angle relative to the spherical beam 20, interferometer 12 and hologram 22 
(FIGS. 1 and 2). 
Accordingly, the present invention provides an in-line hologram 
interferometric test apparatus for testing aspherical lenses. The test 
apparatus includes a lens holder having optical references for positioning 
the lens under test precisely with respect to the in-line hologram and the 
interferometer. The lens holder advantageously allows the lens under test 
to be positioned at the correct distance from the interferometer as well 
as at the correct angle with respect to the hologram plane without 
requiring a translation or trial and error iterations from the 
interferometer's focal point. 
Those skilled in the art can now appreciate from the foregoing description 
that the broad teachings of the present invention can be implemented in a 
variety of forms. Therefore, while this invention has been described in 
connection with particular examples thereof, the true scope of the 
invention should not be so limited since other modifications will become 
apparent to the skilled practitioner upon a study of the drawings, 
specification, and following claims.