An optical flat must be more accurate (flatter) than the surface of the work or piece being tested. For example, if the surface being tested is flat to 1/4 wave or 1/2 of a light band, the optical flat used to determine if the work or test piece is flat should be 1/10 wave or 1/5 of a light band. The optical flat should be larger than the part being tested so that an accurate reading of the entire work surface is obtained.

The chart below offers the best substitute for popular allen wrench sizes from both Imperial to Metric and Metric to Imperial. Be aware that using a smaller size key will increase the chance of stripping the hardware. For some applications, you may want to try using a larger size key than the one recommended in the chart to see if it fits. When substituting sizes, there isn't always a perfect counterpart and it's best to have both Metric and Imperial allen wrench sets available.

Advanced Optics offers personalized service; saving you time and money by helping you select the best option for your application.

The accuracy of each optical flat is traceable to the National Institute of Standards and Technology. Fused Silica Technical Data Sheet from Corning Incorporated (pdf)

A: An Allen wrench is a tool that's used for installing and removing socket or hex head fasteners, such as set screws and socket head cap screws. They are usually L-shaped, however there are other styles available.

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Optical flats can be made of several low expansion materials such as fused silica, fused quartz, Zerodur© or Clearceram©. Fused silica and fused quartz provide low thermal expansion and are ideal for high wear applications. Zerodur© and Clearceram© are glass-ceramic materials with extremely low thermal expansion and are an ideal choice for applications where temperature fluctuation is a concern. BOROFLOAT©33 may also be considered as a less expensive alternative for test plates.

Dual Surface Optical Flats/Optical Windows are an excellent choice when an optical system requires separation between two environments while passing a specific wavelength(s) of light or for demanding interferometry requirements.

To determine the surface flatness of a test piece (work surface) using the interference pattern formed by an optical flat, gently place the optical flat on top of the work piece. The working surface of the test piece and the optical flat should be clean and dust-free. Care should be taken not to slide the optical flat across the surface of the part being tested as it may cause damage (scratches) to the optical flat. Scratches or damage to the surface of the optical flat can cause it to become less flat and therefore affect the results of the test. Next use a soft object, such as the new eraser of a pencil, to apply pressure to various points across the surface of the optical flat. Applying gentle pressure will reduce the air gap between the optical flat and the working surface of the part being tested. A fringe pattern (light and dark bands) will appear as the air gap is reduced. Continue to apply light pressure to spread the fringes so that only a few are visible. Then imagine two straight and parallel lines, one between the ends of any one fringe and the other at the top of that same fringe. The number and curvature of fringes located between the two lines can be used to determine flatness.

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Whether you require custom optical fabrication or one of our many off-the-shelf flats, call today for a competitive quote!

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Whether you require one of our many catalog flats or need a custom optical flat, let Advanced Optics be your trusted partner - You can depend on our 100% outgoing inspection, on-time delivery, and unmatched customer service, all at competitive prices.

In addition to a large selection of in-stock optical flats, Advanced Optics offers custom optical flats, as well as free engineering services. Our capabilities allow us:

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Optical flats become worn and abraded from use and accuracy is lost. This degradation in performance is due to a decrease in the distinctness of the interference pattern, and to a change in figure caused by scratches on either side of the optical flat. Resurfacing or re-polishing the surface of an optical flat back to its original accuracy can be less costly than replacement. Advanced Optics maintains an optical flat refinishing service with a fast turnaround time.

Short Arm SAE Allen Wrench Dimensions (Hex Key) Hex Allen Key Sizes Click on the size to shop products Width Across Flats Length of Long Arm Length of Short Arm Max Min 0.028" 0.028" 0.0275" 1.219" 0.219" 0.035" 0.035" 0.0345" 1.219" 0.344" 0.050" 0.050" 0.049" 1.656" 0.531" 1/16" 0.0625" 0.0615" 1.75" 0.562" 5/64" 0.0781" 0.0771" 1.875" 0.609" 3/32" 0.0937" 0.0927" 2" 0.656" 7/64" 0.1093" 0.1077" 2.125" 0.703" 1/8" 0.1250" 0.1235" 2.25" 0.75" 9/64" 0.1406" 0.1391" 2.375" 0.796" 5/32" 0.1562" 0.1547" 2.5" 0.844" 3/16" 0.1875" 0.1860" 2.75" 0.938" 7/32" 0.2187" 0.2172" 3" 1.031" 1/4" 0.250" 0.248" 3.25" 1.125" 5/16" 0.3125" 0.3110" 3.75" 1.25" 3/8" 0.375" 0.373" 4.25" 1.375" 7/16" 0.4375" 0.4355" 4.75" 1.5" 1/2" 0.50" 0.04975" 5.25" 1.625" 9/16" 0.5625" 0.56" 5.75" 1.75" 5/8" 0.625" 0.6225" 6.25" 1.875" 3/4" 0.75" 0.747" 7.25" 2.125" 7/8" 0.875" 0.872" 8.25" 2.375" 1" 0.050" 0.049" 1.656" 0.531"  * Additional Imperial sizes are available on our website. Short Arm Metric Allen Wrench Dimensions (Hex Key) Hex Allen Key Sizes Click on the size to shop products Width Across Flats Length of Long Arm Length of Short Arm Max Min Max Min Max Min 0.9 0.899 0.876 34 28 11 6 1.3 1.270 1.244 44 39 16 11 1.5 1.5 1.47 45 43 14 13 2 2.00 1.97 50 48 16 15 2.5 2.50 2.47 56 53 18 17 3 3.00 2.96 63 60 20 18 4 4.00 3.96 70 66 25 23 5 5.00 4.96 80 76 28 26 6 6.00 5.95 90 86 32 30 8 8.00 7.95 100 95 36 34 10 10.00 9.95 102 106 40 38 12 12.00 11.95 125 119 45 43 14 14.00 13.95 140 133 56 53 17 17.00 16.93 160 152 63 60 19 19.00 18.93 280 271 70 67 * Additional Metric sizes are available on our website. Long Arm SAE Hex Key Dimensions (Hex Key) Hex Allen Key Sizes Click on the size to shop products Width Across Flats Length of Long Arm Length of Short Arm Max Min 0.050" 0.050" 0.049" 2.844" 0.531" 1/16" 0.0625" 0.0615" 3" 0.562" 5/64" 0.0781" 0.0771" 3.188" 0.609" 3/32" 0.0937" 0.0927" 3.375" 0.656" 7/64" 0.1093" 0.1077" 3.562" 0.703" 1/8" 0.1250" 0.1235" 3.75" 0.75" 9/64" 0.1406" 0.1391" 3.937" 0.796" 5/32" 0.1562" 0.1547" 4.125" 0.844" 3/16" 0.1875" 0.1860" 4.5" 0.938" 7/32" 0.2187" 0.2172" 4.875" 1.031" 1/4" 0.250" 0.248" 5.25" 1.125" 5/16" 0.3125" 0.3110" 6" 1.25" 3/8" 0.375" 0.373" 6.75" 1.375" 7/16" 0.4375" 0.4355" 7.5" 1.5" 1/2" 0.50" 0.04975" 8.25" 1.625" 9/16" 0.5625" 0.56" 9" 1.75" 5/8" 0.625" 0.6225" 9.75" 1.875" 3/4" 0.75" 0.747" 11.25" 2.125" 7/8" 0.875" 0.872" 12.75" 2.375" 1" 0.050" 0.049" 14.25" 0.531" * Additional Metric sizes are available on our website.   Long Arm Metric Allen Wrench Dimensions (Hex Key) Hex Allen Key Sizes Click on the size to shop products Width Across Flats Length of Long Arm Length of Short Arm Max Min Max Min Max Min 1.5 1.5 1.47 78 76 14 13 2 2.00 1.97 83 81 16 15 2.5 2.50 2.47 90 87 18 17 3 3.00 2.96 100 97 20 18 4 4.00 3.96 106 102 25 23 5 5.00 4.96 118 114 28 26 6 6.00 5.95 140 136 32 30 7 7.00 6.97 145 140 36 34 8 8.00 7.95 160 155 36 34 10 10.00 9.95 170 164 40 38     * Additional Metric sizes are available on our website.   Torx/Star Tamper Proof L Key Dimensions Drive Size Click on the size to shop products Length of Short Arm Length of Long Arm T-6 0.5" 1.65" T-7 0.55" 1.75" T-8 0.6" 1.85" T-9 0.65" 2" T-10 0.7" 2.1" T-15 0.75" 2.2" T-20 0.8" 2.35" T-25 0.85" 2.5" T-27 0.95" 2.75" T-30 1.05" 3" T-45 1.25" 3.75" T-50 1.35" 4.25" T-55 1.5" 4.75"   Allen Wrench / Hex Key Conversions The chart below offers the best substitute for popular allen wrench sizes from both Imperial to Metric and Metric to Imperial. Be aware that using a smaller size key will increase the chance of stripping the hardware. For some applications, you may want to try using a larger size key than the one recommended in the chart to see if it fits. When substituting sizes, there isn't always a perfect counterpart and it's best to have both Metric and Imperial allen wrench sets available. Hex Key / Allen Wrench Conversion Chart SAE Conversion for MM MM Conversion for SAE 5/64" 2 mm 2 mm 3/32" 3/32" 2.5 mm 2.5 mm 7/64" 7/64" 3 mm 3 mm 1/8" 1/8" 3.5 mm 3.5 mm 9/64" 5/32" 4 mm 4 mm 5/32" 3/16" 5 mm 5.5 mm 7/32" 7/32" 5.5 mm & 6 mm 6 mm 1/4" 1/4" 7 mm 7 mm 9/32" 5/16" 8 mm 8 mm 5/16" 3/8" 10 mm 9 mm 3/8" Frequently Asked Questions  Q: What is an Allen Wrench used for?  A: An Allen wrench is a tool that's used for installing and removing socket or hex head fasteners, such as set screws and socket head cap screws. They are usually L-shaped, however there are other styles available. Q: Is an Allen wrench the same as a hex key? A: Yes, an Allen wrench is the same as a hex key. They are also called L-Keys, L-Wrenches, Allen Keys Q: Why are they called Allen Wrenches? A: In 1909-1910, William G. Allen patented a method of cold-forming screw heads around a hexagonal die. The first trademarked hex key in the US was made by Allen Manufacturing and the name has been used ever since. Q: Are Allen wrenches the same as Torx wrenches? A: No, allen wrenches have a hexagon shape while Torx wrenches have a star shape. Q: What are the types of Allen wrenches? A: L-shaped, T-handle, P-handle, folding hex, ratcheting hex, torque wrench, Allen screwdriver, ball hex Additional Resources Download Allen Wrenches PDF Shop Related Products Allen Wrench Individual KeysAllen Wrench Key Sets Allen Wrench Key Cases

A: In 1909-1910, William G. Allen patented a method of cold-forming screw heads around a hexagonal die. The first trademarked hex key in the US was made by Allen Manufacturing and the name has been used ever since.

Optical flats come in many different accuracies, but most commonly 1/4 wave, 1/10 wave and 1/20 wave. In addition to describing the accuracy of an optical flat in terms of waves (as above), accuracy may be specified in terms of light bands, microinches, fringes or inches. The table below summarizes how the accuracy of an optical flat may be specified.

Advanced Optics offers in stock fused silica double sided optical flats that may also be used as optical windows. Fused silica offers superior transmission characteristics, a high degree of purity, exceptional environmental durability, and a low coefficient of thermal expansion.

Optical flats are polished to a known flatness and used in conjunction with a monochromatic light source. The optical flat is placed on top of the work to be tested under a monochromatic light of a specific wavelength, typically He-Ne which has a wavelength of 632.8nm and is equivalent to 0.000024 inches per wave. An interference (fringe) pattern of light and dark bands is formed due to the air gap between the work surface and the optical flat. The change in thickness of the air gap or wedge shows the direction and shape of the interference bands. The curvature and distance between bands determine how flat the work surface is compared to the known surface of the optical flat.

Advanced Optics optical flats are measured using either a master test flat traceable to the National Institute of Standards and Technology (NIST) or peak to valley using a Zygo Interferometer.

OpticalFlatfilter

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Keep in mind, understanding your application can help you to select the right optical flat without over specifying your requirements thereby saving you time and money. Choosing the appropriate material, defining the correct accuracy and determining whether the flat should be polished on one or both sides should be considered. If you are still uncertain as to how to specify the correct optical flat, Advanced Optics has over 50 years of experience and can help you define your requirements and wisely choose the correct optical flat for your application.

The term optical flat is also commonly referred to as “test plates” or “reference flats” and may be used as a reference (test plate) against which the flatness of an unknown surface can be compared. An optical flat is a piece of glass that has been precision polished to a specific flatness on one or both sides. The term “Optical Flat” is often used interchangeably with other optical components such as optical windows and as a substrate for optical mirrors.

An optical flat is a piece of glass that has been precision polished to a specific flatness on one or both sides and is used as a reference (test plate) against which the flatness of an unknown surface can be compared. In practice, an optical flat can be used to test and certify metal or glass surfaces that have been lapped or polished such as metal rings or seals, optical mirrors and optical windows, prisms, filters and more. A single sided optical flat (single surface optical flat) is polished on only one side and the opposing side is generally left clear, but not precision polished. A double sided optical flat (dual surface optical flat) is precision polished on both sides and may be used as an optical window.

Opticalflat

Optical flats are commonly round in shape. However, depending on their application, custom shapes and features may be required. Our optic capabilities also include crafting custom shapes including; square, rectangular, beveled, and truncated optical flats. We also have the ability to drill custom holes and can provide coatings to increase the interference (fringe) pattern. In addition, we offer custom wooden storage cases for the protection of the optical flat when not in use.

Optical Flats available in a wide variety of accuracies (λ/4, λ/10, and λ/20). Advanced Optics is a leading manufacturer of optical flats and has been crafting high-quality, affordable optical flats for more than 50 years.