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Optical Flat Inspection
19/04/25

 

optical flat

 

An optical flat is a precisely polished flat surface, used as a reference against which the flatness of an unknown surface may be compared. The flatness of an optical flat is measured in fractions of a reference wavelength, 632.8nm. A λ/20 flat will have a maximum peak to valley deviation of 632.8/20 or 31.64nm. Honour Optics offers several levels of flatness, such as λ/4, λ/10, and λ/20.

WHAT AN OPTICAL FLAT SHOWS
When an optical flat polished surface is placed in contact with a surface to be tested, dark and light bands will be formed when viewed with monochromatic light. These bands are known as interference fringes and their shape gives a visual representation of the flatness of the surface being tested. The surface flatness is indicated by the amount of curve and spacing between the interference fringes. Straight, parallel, and evenly spaced interference fringes indicate that the work surface flatness is equal to or higher than that of the reference surface.

APPLICATIONS
Measurement of the surface flatness of polished surfaces can be determined visually by comparing the variations between a work surface and the surface of an optical flat. Optical flats are versatile optical components used in many applications, such as: inspection of gauge blocks for wear and accuracy, as well as the testing of various components including windows, prisms, filters, mirrors, etc. They can also be used as extremely flat optical windows for demanding interferometry requirements.

 

HOW AN OPTICAL FLAT WORKS
optical flat inspection

 

interferogram

An optical flat utilizes the property of interference to exhibit the flatness on a desired surface. When an optical flat, also known as a test plate, and a work surface are placed in contact, an air wedge is formed. Areas between the flat and the work surface that are not in contact form this air wedge. The change in thickness of the air wedge will dictate the shape and orientation of the interference bands. The amount of curvature that is shown by the interference bands can be used to determine the flatness of the surface. If the air wedge is too large, then many closely spaced lines can appear, making it difficult to analyze the pattern formed. Simply applying pressure to the top of the optical flat alleviates the problem.

The determination of the flatness of any particular region of a surface is done by making two parallel imaginary lines; one between the ends of any one fringe, and the other at the top of that same fringe. The number of fringes located between the lines can be used to determine the flatness. Monochromatic light is used to create sharp contrast for viewing and in order to specify the flatness as a function of a single wavelength.

WHICH ACCURACY SHOULD BE USED
This is a commonly asked question and the answer is dependent on what is being tested. If the surface that is being tested is flatter than λ/4, then a more precise flat will be required to show a change in the interference pattern. In this case, a λ/4 flat would exhibit straight parallel lines, but λ/10 or λ/20 flats would show enough curvature in the fringes to measure the surface accurately.

RECALIBRATION
Please note that over time and with repeated use, optical flats can lose their degree of precision.  Please contact our Sales Department via e-mail: [email protected]