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| It's all in
the way you look at it!
Superficially, if you look
at a piece of ordinary window glass, it looks flat and seems smooth.
However, our eye is not very good ad determining the quality of any surface.
Under a microscope, such
a smooth surface shows numerous scratches and other small imperfections.
Under an interferometer,
we see that the surface is not flat; indeed it may be very wavy and irregular. |
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How
can a surface be flat and yet rough?
There is considerable confusion
over the meaning of flatness and roughness. Really, it is all a matter
of scale. The figures at the left attempt to explain the difference.
After we have defined some
surface types, we will look at how they are measured. We will also
see that any definition of flatness or roughness depends upon which type
of tool we use to measure it -- and, of course, it is all very relative!
A
surface that is both flat and smooth is a special case -- the ideal type
of surface that is so often sought in the world of optics.
Example: a high quality
optical flat. |
|
Most optical
surfaces fall short of the ideal. Many are relatively flat but with some
roughness-- usually in the form of scratches and digs or pits.
Example: most flat optical
surfaces |
|
| Then there
is a general class of surfaces that may have a relatively smooth surface,
but with subtle curves or "waviness" that can not be considered flat. Example:
a piece of high quality float glass |
|
Some surfaces
are really beyond classification with no particular surface figure -- flat
or otherwise, and have a rough surface texture.
Example: most surfaces
including optical glass during the grinding process. |
|
 |
One
way of visualizing the properties of a surface that is both flat and smooth
is to imagine such a surface being inclined slightly as shown at the left.
Now a smaller block also with a smooth flat surface is placed on the inclined
plane and observe that it easily slides along. No bumps and little
friction. |
 |
A similar
experiment with a surface that is flat but a little rough shows that the
small block with a smooth flat surface likewise has little problem sliding
along the inclined surface -- provided that the block is not small enough
to drop into one of the pits! |
 |
When we
take the same surface and allow a small ball to roll down, it may or may
not make it all the way. The image at the left shows such an experiment
where the ball ultimately gets caught in one of the pits. Note that the
results of this experiment depend upon the size of the ball! |
|
|
This short treatise scarcely
covers such a complex subject, but perhaps it will set the stage for further
understanding. There is such variation in surface texture, from very
smooth to very rough, with semi-matte and matte surfaces in between.
Smooth, polished surfaces are readily measured using a standard Fizeau
Interferometer, while semi-matte and matte finished surfaces can be
measured using grazing incidence interferometers. We invite
you to explore the capabilities of these interferometers which ar described
in detail throughout this website. If you still need assistance with
your measurement problem, don't hesitate to contact Gordon Graham: Phone
(818) 700-1263 E-mail: techinfo@grahamoptical.com |
