OPTITEST
Applications
Autocollimators
The applications of autocollimators are mainly
related to detection and measurement of small angular
displacements.
Angle measurement of Optical
Components
Wedge and deflection angle
The parallel beam emerging from autocollimator is
reflected from both surfaces of the wedge. The wedge
angle is given by:
= d / 2 n f
where:
d = displacement of the reflected
image
n = refractive index of glass
f = focal length of the
autocollimator
For fast measurement in optical manufacturing, the
displacement d for a given angle
tolerance and focal length f
can be calculated and transfered to the illuminated
reticle in form of a pinhole, so that the acertainement
of the componenent can be made on a "go" and
"no go" basis:
 Wedge out of tolerance
 Wedge at the tolerance limit
 Wedge in tolerance
The deflection angle through the wedge is
given for small angles by:
= d (n
 1) / 2 n f
Prism angles
The range of applications in this field is very wide,
only some fundamental setups are presented:
90°Angle of Prisms
The 90° prism is put on a
accurate flat surface. The emerging beam of
autocollimator is reflected on the prism side and flat
and returns along the original path if the angle is
exactly 90°. No displacement appears in the eyepiece.
Deviations from 90° can be measured in the eyepiece. The
error size:
= d / 4 f
where f = focal length of
autocollimator. The sign /+ of the error is determined
by defocusing the eyepiece: moving the focal plane of the
eyepiece towards objective lens, a negative error results
if the distance d becomes smaller.
45°Angle of Prisms
The Autocollimator is directed on one side
of the 90°angle. Two images will be produced from both
sides of the prism.
The internal reflection within the prism will produce
a displacement d depending on the error
of the 45° angle :
= ( d / 4
n f ) ± ( / 2)
where is the error of 90° angle.
Deviaton angle through prisms
The autocollimator is mounted on an
adjustable stand and can be tilted at any angle.
A master prism is used to align the autocollimator to
the mirror. The master prism is replaced by the prism
under test and the angle difference is read off through
the eyepiece.
Other Applications
Other applications includes measurement of geometrical
parameters of mechanical parts. This is a typical
application in machine construction, machine tools and
aerospace industry. The measurement can be done for:
 straightness
 squareness
 parallelism
 flatness
Measurement of optical parameters
Radius of curvature
Aditional achromats are mounted on the one end of the
autocollimator tube. The illuminated image of the
autocollimator will be projected into the focal plane of
the achromat. This image is reflected back from the
vertex of the lens and the center of curvature of
lens surface. The linear displacement between these two
position  where a sharp image is seen in the eyepiece 
gives the radius of curvature.
Both concave and convex surfaces can be
measured. Spherical and cylindric surfaces
can be measured as well. For convex surfaces the back
focal length of the achromat must be longer than the
radius under test.
Flange Focus
The Flange Focus known also as Flange Focal Length
(FFL) or Flange Focal Distance (FFD) is the distance
between the locating surface of the lens mount and the
image plane. Checking and setting of this distance is
important especially for camera lenses. The film plane is
replaced by a mirror mounted on an adjustable jig.
For checking the FFD when the lens under test is set
to infinity a standars autocollimator is used. For
testing the lens set at other distances as infinity is
recommended:
 achromats attached to the autocollimator for
short distances
 a focusing autocollimator for infinity and long
distances
The reticle normally used is a Siemens Star. When the
reticle is sharply seen at the given distance, the camera
lens is correctly set. Deviations can be measured with an
adjustable jig and the lens correspondingly adjusted.
Centration errors
The autocollimators can be used for measurement of
centration errors in transmission (see collimators
applications) or in reflection.
For measurement in reflection additional achromats are
attached to autocollimator. The precision rotary holder
is equiped with a chuck runing true to the rotation axis.
The spherical surface under test is located on the front
surface of the chuck and held in contact by means of a
small vacuum device. When rotated the surface under test
will reflect the image reticle. This image describes a
circle with a diameter depending on the decentration
size.
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