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Autocollimators



Applications of autocollimators
 
The applications of autocollimators are mainly related to detection and measurement of small angular displacements.
 
Angle measurement of Optical Components
 
Measurement of wedge and deflection angle
 
Measurement of wedge and deviation angle with autocollimators
 

 
The parallel beam emerging from autocollimator is reflected from both surfaces of the wedge. The wedge angle d is given by:
d = d / 2 n
where: d = displacement of the reflected image, n = refractive index of glass, = focal length of the autocollimator

For fast measurement in optical manufacturing, the displacement "d" for a given angle tolerance and focal length "" can be calculated and transfered to the illuminated reticle in form of a pinhole, so that the ascertainment of the component 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 g is given for small angles by:
    g = d (n-1) respectively: g = d / 2 n (n-1)
  • parallel beam emerging from autocollimator
     

     
    Measurement of prisms angles
     
    The range of applications in this field is very wide, only some fundamental set ups 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:
     
    a = d / 4
     
    where = 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.
    Testing external 90°-angle of prisms

     
    45-Angle of Prisms
     
    testing deviation angle through pentaprisms Measurement of 45°-angle

     
    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 "a":
     
  • a = d / 4 n f d / 2
    where d is the error of 90 angle.
     
    Measurement of 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. Example: Measurement of deviation angle through pentaprisms.
     
    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.
    radius measurement with autocollimators

     

     
    Measurement and Testing of 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 standards 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.
  • Testing flange focal distance

     
    Measurement of 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 equipped with a chuck running 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.
    measuring centration in transmission

    Read more about MTF measurement and autocollimators.
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