Why Grazing Incidence?


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The Model 2GRP LAZER GRAZER is a non-contacting Grazing Incidence Interferometer. . . the first modestly priced Interferometer for measuring flatness of semi-matte or matte-finished surfaces. The Model 2GRP also permits the measurement of parts which are much larger than its 2-inch aperture because is measures flatness across a 2 inch by 8 inch elliptical beam path as illustrated below.

Principle of Operation
When a rough surface is examined at a low grazing angle, it appears to be highly reflective. 

The LAZER GRAZER utilizes this principle to produce high contrast interferograms of unpolished surfaces.  A grazing angle of 14.48 degrees was selected since at that angle, each interference fringe equals one wavelength.  With a normal incidence Fizeau Interferometer, each fringe equals one half wavelength. Clicking on the link below will provide further information on the subject of 
Grazing Incidence Interferometers

With a normal incidence interferometer, 1 fringe = 0.5 wave = 0.3164 micron
With a grazing Incidence interferometer (grazing angle of 14.48 degrees) 1 fringe = 1 wave. = 0.6328 micron
A typical LAZER GRAZER screen as produced by Durango Software.
Note that the live image shows interference fringes on the surface of the test sample while providing a "3-D" image of the part. This unique display will only be found with GRAHAM's LAZER GRAZER Interferometers!  This capability is of enormous value in the interpretation of test pieces with complex geometry.
Comparing the interferogram of a normal incidence Fizeau with a Grazing Incidence FizeauComparison of Normal Incidence Interferogram
(1 fringe = 0.5 wave)

and Grazing Incidence Interferograms 
( 1 fringe = 1 wave)

Upper Drawing shows the interferogram of a part using a normal incidence interferometer.  8 circular fringes which implies part is concave or convex by:
8 X 0.3164 microns = 2.53 microns

The Lower Drawing shows the interferogram of the same part using a grazing incidence interferometer.. The interferogram shows 4 circular fringes which implies the part is concave or convex by:
4 X .6328 micron = 2.53 microns

Note that the live image shows interference fringes on the surface of the test sample while providing a 3-D image of the part. This is a unique display is a feature available only with GRAHAM's unique Grazing Incidence Interferometers!  This capability is of enormous value in the interpretation of test pieces with complex geometry.
Ray Trace the sample area of the Lazer Grazer
Red arrows show light coming in from the left at a "Grazing Angle" of 14.48 degrees

Again, the Ray Trace in drawing formatThis drawing illustrates the ray path shown in the photograph.  The collimated beam passes through the interferometer's reference surface, strikes the test sample, bounces up to the right and reflects from the return mirror, again bounces off the sample and reenters the interferometer,  and interferes with the reference beam. 
Measure flatness 
                   ...before polishing!
The ability to measure flatness of unpolished parts opens the door to examining surfaces which would be difficult if not impossible for a normal incidence interferometer.  This permits the operator to check parts for flatness while still in the grinding stage. For some operations, this is all that is needed since the finished parts do not require polishing. GRAHAM's LAZER GRAZER  puts an end to all of those “tricks” using wax and china markers to increase reflectivity of the surface.

Difficult substrates easily tested
With the LAZER GRAZER, matte, semi-matte, or polished surfaces can be checked for flatness of virtually any material. Very difficult substrates such as ceramics, graphite, tungsten carbide and silicon carbide can be evaluated with ease. Even machined metal surfaces can be tested.  The high contrast fringes produced by the LAZER GRAZER will assist in fine-tuning the production process and increasing both quality and through-put.

Because of the wide variation in sample thickness encountered, it is essential to make provision for the sample surface to be totally covered by the incident laser beam. By providing a height adjustment for the tilt table, samples of various thickness can be examined.  After the appropriate tip-tilt adjustment has been made, the tilt-table may be raised or lowered as required for the sample to fully encounter the beam.

Field of View covered by the 2-inch Lazer Grazer
Because of the low grazing angle of the incident light, the illuminated area of the tilt-table is much elongated with the result that a long ellipse of light, in this case 4 times the interferometer's aperture is available for evaluating  long narrow parts.  It can also be used to look at parts substantially larger than the interferometer's aperture by examining a narrow swath across the substrate and then rotating the sample to look at another swath.
Special advantages offered by LAZER GRAZER Interferometers
  • Ability to measure flatness of semi-matte and matte surfaces
  • Measure narrow swaths across surfaces much larger than the interferometer aperture
  • Simultaneous oblique view of test piece and interferogram of surface.
GRAHAM OPTICAL SYSTEMS, 9530 Topanga Canyon Blvd.,  Chatsworth, CA 91311
Phone (818) 700-1263   FAX   (818) 700-1627   E-mail: techinfo@grahamoptical.com
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This page last updated JUly 7, 2008