MWIR Achromatic Doublets Optimized for 3 - 5 µm

  • AR Coating for 3 - 5 µm
  • Design Wavelengths at 3 µm, 4 µm, and 5 µm
  • 1" Outer Diameter


f = 100 mm


f = 200 mm

Each mount includes an engraved arrow
indicating the direction of light propagation
required to collimate a point source.

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Common Specifications
Diameter Ø1" (Ø25.4 mm)
Dimension Tolerance +0.00/-0.10 mm
Centration <3 arcmin
Clear Aperture  Ø20 mm
Spot Size Diffraction Limited On Axis
Design Wavelengths 3 µm, 4 µm, and 5 µm
Antireflection Coating Ravg < 1.5% from 3 to 5 µm, 0° AOI
Substrates Silicon and Germanium
Focal Length Tolerance ±1% at 4 µm
RMS WFE <λ/10 @ 4 µm
Surface Quality 60-40 Scratch-Dig


  • Air-Spaced Doublet Design Provides Superior Aberration Performance than Singlet Lenses
  • AR Coated for the 3 - 5 µm Range
  • Designs Incorporate Silicon (Si) and Germanium (Ge) Substrates
  • Mounted in Engraved Ø1" Aluminum Housings

These AR-coated, air-spaced achromatic doublet lenses provide excellent transmission in the 3 - 5 µm range (surface reflectance for each coating can be seen in the graph to the right). They incorporate two AR-coated lenses, one with a germanium substrate and another with a silicon substrate.

Each achromatic doublet on this page has three design wavelengths: 3 µm, 4 µm, and 5 µm. By optimizing the design for these three wavelengths, the focal shift across the 3 - 5 µm range will be minimized (see the graphs below for details), providing excellent on-axis performance.

Thorlabs' MWIR achromatic doublets are pre-mounted in engraved SM1-compatible mounts, making it easy to incorporate these optics into your setup. Each engraving includes the part number, AR coating range, focal length, an arrow indicating the direction of light propagation to collimate a point source, and an infinity symbol denoting that this lens has an infinite conjugate ratio (i.e., if a diverging light source is placed one focal length away from the flat side of the lens, the light rays emerging from the curved side will be collimated).

Note: We do not recommend disassembly of these lenses, as this may affect performance.

Optic Cleaning Tutorial
Optical Coatings and Substrates
Zemax Files
Click on the red Document icon next to the item numbers below to access the Zemax file download. Our entire Zemax Catalog is also available.

Posted Comments:
Emmanuel Lassalle  (posted 2020-11-05 02:52:19.437)
Hi, In the top panel showing the page: we can see the focal lens shift as a function of the wavelength for the lens AC254-100-E, showing a focal lens shift from -0.6 to about 0.8 microns. However, in the graph located in the table below ("3 - 5 µm Air-Spaced Achromatic Doublets"), one can see the same trend but the shift is mulitiplied by 10! (from -6 microns to 8 microns). Is there an error in the first graph? Thanks!
YLohia  (posted 2020-11-10 03:59:35.0)
Hello, thank you for your feedback and bringing this discrepancy to our attention. The graph in the table is indeed correct and the one on the banner image is incorrect. We are working on fixing this issue.
Florian Herbst  (posted 2020-05-04 05:45:28.193)
Dear Sir or Madam, I, unfortunately, seem not to be able to find information about the material thicknesses in AC254 lenses. Could you please provide me with the center thicknesses and the edge thicknesses of the silicon and the germanium lenses in AC254-100-E, please? Thank you very much in advance! Kind regards, Florian Herbst
YLohia  (posted 2020-05-04 12:52:08.0)
Hello Florian, thank you for contacting Thorlabs. This information is given in our Zemax files for this lens and we are working on adding this information to the Specs table for ease of access. For the AC254-100-E, Silicon lens center thickness is 2.15 mm and the edge thickness is 1.67 mm. The Germanium lens center thickness is 5.00 mm and the edge thickness is 5.20 mm.
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3 - 5 µm Air-Spaced Achromatic Doublets

Item # f
Reflectance Plot Left Lens Materiala Right Lens Materiala Graphsb
AC254-050-E 50 10.0 40.4 Air-Spaced Doublet Drawing Plot Silicon Germanium
AC254-075-E 75 12.5 64.2
AC254-100-E 100 12.5 89.4
AC254-200-E 200 15.5 192.6
  • "Left" and "Right" refer to the orientations of the lenses in the drawing found in the Reference Drawing column.
  • Click on for plots and downloadable data of the focal length shift and transmission for the lens.
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
AC254-050-E Support Documentation
AC254-050-ESi-Ge Air-Spaced Achromatic Doublet, AR Coating: 3-5 µm, f=50 mm
AC254-075-E Support Documentation
AC254-075-ESi-Ge Air-Spaced Achromatic Doublet, AR Coating: 3-5 µm, f=75 mm
AC254-100-E Support Documentation
AC254-100-ESi-Ge Air-Spaced Achromatic Doublet, AR Coating: 3-5 µm, f=100 mm
AC254-200-E Support Documentation
AC254-200-ESi-Ge Air-Spaced Achromatic Doublet, AR Coating: 3-5 µm, f=200 mm