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The AT1(/M) has an engraved scale for observing beam height, while still allowing scattered light to pass through.

Features

• Diffusive Surface Allows the Beam to be Visible on the Surface of the Tool
• 8-32 (M4) Threaded Metal Insert for Post Mounting
• Engraved Crosshair with 1 mm Scale on Front Surface

Thorlabs' AT1(/M) Post-Mountable Acrylic Alignment Tool is ideal for observing beam height in an optical system. This 9.6 mm (0.38") thick diffusive acrylic alignment tool is 3 cm x 3 cm (1.8" x 1.8"), and the surface is laser engraved with a scale of 1 mm (0.04") graduations. The AT1(/M) is equipped with an 8-32 (M4) tapped metal insert for post mounting; this feature allows it to be easily integrated into optomechanical setups. This alignment tool allows the user to observe the laser spot on the front surface, after which it is scattered diffusely (see the image to the right). 

Note: This tool should not be used as a beam block or as any type of laser safety device. It also should not be used with high power lasers. Its acrylic construction results in a low damage threshold and its diffusive surface causes increased scattering of incident light.

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The AT1 and AT1/M have different colored threaded inserts for easier identification.
The AT1/M (left) has a brass insert and the AT1 (right) has a stainless steel insert.