What do you use to test the flatness of glass or metal surfaces? If you want extremely precise measurements, you need to use an optical flat.
In order to achieve maximum accuracy, large optical flats should be utilized when a large part is being tested so that the whole work surface fits. In other words, the optical flat should be the right size for the test object.
Are you curious about optical flats? Keep reading to learn everything you need to know in this handy guide.
What are Optical Flats?
An optical flat is usually a disc-shaped object that people use as a reference against which they compare the flatness of a variety of optical surfaces.
Optical flats are made of optical-grade glass or quartz and are polished so that they’re extremely flat on both surfaces.
They are constructed of materials that have a very small coefficient of thermal expansion. They are sometimes coated on one side so that they’re highly reflective.
How Do Optical Flats Work?
Optical flats determine the flatness of surfaces by using the property of interference. When the test plate (optical flat) is put in contact with a work surface, an air wedge may develop. This air wedge forms in the areas between the flat and the work where there is no contact.
When there is a change in thickness of the air wedge, this dictates the shape and orientation of the interference bands. These interference bands show the amount of curvature and this determines the flatness of the surface of the test object.
How to Measure Flatness With Optical Flats
When determining the flatness of an area of the surface, you make two parallel imaginary lines. One line goes between the ends of any fringe (light and dark bands) and then the other at the top of that same fringe. How many fringes are there between the lines? This number tells you the flatness.
When measuring the flatness of polished surfaces, you compare the variations between a work surface and the surface of the optical flats. Monochromatic light creates sharp contrast so you can see it and specify the flatness as a function of a single wavelength.
Looking for flat optics in a multitude of shapes, sizes, and specifications? Learn more here.
Optical Flat Uses
Optical flats have a number of useful applications. Some of the primary applications include the testing of filters, mirrors, and prisms. They are also used for the inspection of gauge blocks.
Do you need to calibrate the flatness of a variety of optical surfaces? Optical flats are what you would use. They are also used for spectrophotometry or the quantitative measurement technique scientists use to investigate the optical properties of materials.
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