Thermal, SWIR & UV lenses

How to choose a thermal, SWIR or UV lens

Outside the visible band, ordinary optical glass stops working. It is opaque to long-wave infrared, absorbs deep ultraviolet, and its coatings and colour correction are designed for 400-700 nm. Thermal (LWIR), short-wave infrared (SWIR) and ultraviolet (UV) cameras therefore need lenses made from other materials and corrected for their own band. Start from the camera's waveband and detector, then choose focal length, aperture and the mechanical details.

1. Waveband: match lens, camera and light

Thermal lenses use infrared materials such as germanium and carry hard coatings on the front element. SWIR lenses are corrected and coated for the InGaAs band, so focus and contrast hold where a visible lens would lose both. UV lenses use UV-transmitting materials; choose the range that matches the light source, for example 254 nm or 365 nm.

2. Detector size and image circle

The image circle must cover the detector diagonal, which depends on pixel count and pitch:

The thermal lenses here list the detector they were designed for, from 160 x 120 to 1024 x 768. SWIR and UV lenses are listed by sensor format (for example 2/3" or 1"), like visible lenses.

3. Focal length and spatial resolution

Thermal lenses here run from under 2 mm (very wide angle) to 180 mm fixed focal length, plus zoom lenses such as 30-150 mm and 36-180 mm for long-range monitoring. SWIR lenses include the usual machine vision steps from 12 to 50 mm. For thermal work, compute the pixel footprint: instantaneous field of view (IFOV) = pixel pitch / focal length. With a 12 µm detector and a 25 mm lens, IFOV is 0.48 mrad, so one pixel covers about 4.8 mm at 10 m. A hot spot needs several pixels across it for a reliable temperature reading, not just one. For field of view, the focal length calculator works for any sensor size.

4. Aperture

Microbolometers need a lot of radiation, so thermal lenses are fast: mostly F1.0-F1.2 here, with a few faster. A slower lens makes the image noisier, since thermal sensitivity worsens roughly with the square of the F-number. SWIR lenses here start at F0.9; UV lenses range from about F1.4 to F3.8.

5. Temperature, focus and sealing

The refractive index of germanium changes strongly with temperature, so a thermal lens drifts out of focus as it warms or cools. Athermalized lenses hold focus over their rated range, typically -40 to +80 °C for the lenses here; others are marked non-athermalized and suit stable indoor temperatures. Many are fixed-focus; others have manual or motorized focus. For outdoor or dirty locations, look for an IP67 front seal and a hard front coating (DLC). Check the mount: thermal lenses use threads such as M17 x 0.75, M20 x 0.5, M25 x 0.5, M34 or M45 x 1, or a flange, rather than C-mount.

Typical applications

• LWIR: hot spots in electrical cabinets, heat seal and glue bead checks on packaging, process and kiln monitoring, fire and smoke detection.
• SWIR: moisture and bruise detection in food, sorting plastics, looking through silicon wafers (silicon transmits above about 1.1 µm), laser spot checks at 1064 and 1550 nm.
• UV: fluorescent adhesives, coatings and markings under 365 nm light; UV-C imaging at 240-280 nm, for example corona detection.

Common mistakes

• Fitting a visible lens to a thermal camera: glass is opaque at 8-12 µm, so the image shows only the lens itself.
• Putting a thermal camera behind a glass or acrylic window, which also blocks LWIR; use an infrared window.
• Choosing a lens for a 12 µm detector on a 17 µm detector with the same pixel count: the image circle is too small.
• Using a non-athermalized lens outdoors and refocusing every season.
• Using a SWIR lens on a camera that only sees visible light, or a visible lens on an InGaAs camera, and losing contrast.

Questions

Can a thermal camera see through glass?
No. Window glass blocks long-wave infrared; the camera sees the glass surface temperature. Infrared windows (for example germanium) are used where a housing needs a window.

What does athermalized mean?
The lens design compensates for temperature so the focus stays put over the rated range, without refocusing.

Which cameras do these lenses fit?
The LWIR lenses fit the LWIR models among the infrared (thermal) cameras (MWIR cameras need MWIR optics). SWIR lenses fit InGaAs area and line scan cameras and 900-1700 nm hyperspectral cameras with the same mount. Check the mount and detector size on both product pages, or match them in the configurator.

How do I combine a thermal image with a normal camera image?
See thermal and RGB image fusion; it explains how to overlay the two without losing the temperature values.