A ROTATABLE PARALLEL GLASS PLATE PERISCOPE FOR EFFECTIVE APERTURE MAGNIFICATION IN METROLOGY

Optical instruments like autocollimator or interferometers have a limi ted aperture. Here we describe a simple device that effectively enlarg es the viewing aperture, based on parallel shifting of the beam in a t ilted parallel glass plate. This periscopic action is done with two ad ditional degrees of freedom: variable radial displacement adjusted by the plate tilt angle and azimuthal movement controlled by plate rotati on around an axis parallel to the optical axis of the device. For exam ple, a 1.8 refractive index glass plate of 80 mm thickness can be tilt ed up to 60 degrees, enlarging a 40 mm diameter aperture up to 130 mm effective aperture. A rotatable parallel glass plate periscope was con structed to explore the inside aperture of a ceramic tube, using a 1 m m diameter HeNe laser beam as a probe, obtaining 1 mm accuracy. To eas e the plate rotation, the glass plate was chosen to have a trapezoid s hape and a balancing weight was designed and constructed to evenly bal ance the plate weight through the operation range of the device. (C) 1 995 American Institute of Physics.