SM1 Thread

SM1 is a specialized thread standard predominantly used in the field of optomechanics. Originally developed by Thorlabs, a major manufacturer of optical equipment, the SM1 thread has become an industry-wide standard for mounting, housing, and aligning circular optical components, particularly those with a 1-inch (25.4 mm) diameter.

Thread Specifications

The "SM" in SM1 stands for "Sewing Machine," a historical nod to the highly precise, fine-pitch threads traditionally used in the sewing machine industry, which were adopted by early optomechanical designers for their fine adjustment capabilities. The "1" roughly corresponds to its compatibility with 1-inch optics.

The exact specification for an SM1 thread is 1.035"-40.

  • Major Diameter: 1.035 inches (approx.() 26.29 mm)
  • Thread Pitch: 40 threads per inch (TPI)

The extremely fine 40 TPI pitch provides high-resolution positioning and prevents components from easily vibrating loose, which is critical in precision optical setups.

Applications in Photonics and Optics

SM1 threads are foundational to modular optomechanical systems. Because the internal diameter of an SM1-threaded tube is just wide enough to accommodate a standard 1-inch optic, it is extensively used to build enclosed, light-tight optical paths.

Common applications include:

  • Lens Tubes: Stacking multiple threaded tubes to hold lenses at specific focal distances.
  • Filter Mounts: Securing optical bandpass, shortpass, or longpass filters. The thread allows retaining rings to be screwed down flush against the filter to hold it securely without inducing mechanical stress that could cause birefringence or warp the thin-film coatings.
  • Detector and Source Housings: Mounting SWIR cameras, photodiodes, or light sources to an enclosed optical path.
  • Substrate Holders: Securing bare UV fused silica substrates, dichroic mirrors, or waveplates at specific Angles of Incidence (AOI) when integrated with kinetic mounts.

Comparison to Related Thread Standards

SM1 is part of a larger family of "SM" threads designed for different optic sizes:

  • SM05 (0.535"-40): Designed for 0.5-inch (12.7 mm) optics.
  • SM2 (2.035"-40): Designed for 2.0-inch (50.8 mm) optics.
  • SM3 (3.035"-40): Designed for 3.0-inch (76.2 mm) optics.

SM1 vs. C-Mount: SM1 is frequently compared to, and sometimes confused with, the C-mount standard widely used in machine vision cameras. While both are fine threads of similar size, they are not directly compatible. C-mount is a 1.000"-32 thread (1-inch diameter, 32 threads per inch). Thread adapters are heavily utilized in laboratories to interface SM1 lens tubes with C-mount cameras.

Practical Example

The Components

  1. SWIR Camera: Typically features a standard C-mount (1.000"-32 thread) on the front.
  2. 1550nm Bandpass Filter: A standard 1-inch diameter optical component.
  3. SM1 Lens Tube: A hollow cylinder with internal SM1 threads (1.035"-40).
  4. SM1 Retaining Ring: A thin, threaded metal ring designed to screw into the SM1 lens tube.
  5. SM1-to-C-Mount Adapter: A crucial transition piece with external SM1 threads on one side and internal C-mount threads on the other.

The Assembly Process

  1. Seating the Optic: You drop the 1-inch 1550nm bandpass filter flat into the SM1 lens tube. Because the tube is precisely machined for 1-inch optics, the filter fits perfectly without rattling side-to-side.
  2. Securing the Filter: You thread the SM1 retaining ring down into the lens tube using a specialized spanner wrench. The extremely fine 40 TPI (threads per inch) pitch of the SM1 standard is vital here. It allows you to thread the ring down until it just barely kisses the surface of the filter. You can secure the substrate tightly enough that it won't move if the machine vibrates, but gently enough that it won't induce mechanical stress, which could warp the thin-film interference coatings and degrade the filter's performance.
  3. Adapting the Thread: You screw the SM1-to-C-Mount adapter onto the back end of the SM1 lens tube.
  4. Final Integration: You thread the entire light-tight SM1 assembly directly onto the front of the SWIR camera.

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