Putting a thermal core behind a window: why glass fails, what germanium needs, and how the window changes your readings
Most products built around a thermal core need something between the lens and the outside world: rain, dust, cleaning cloths, fingers. The window is easy to get wrong, because the material you would use on a visible camera does not work, and on a radiometric product the window becomes part of the measurement. Here is what to check. Core figures come from the 640 × 512 radiometric module we sell; the window physics applies to any long-wave core.
Why glass and most plastics do not work
Uncooled cores see the long-wave infrared band, 8–14 μm. Ordinary glass, polycarbonate and acrylic absorb strongly there, so behind a glass window the core sees the glass, not the scene. The window has to be an infrared material. Germanium is the most common choice for this band; zinc selenide and chalcogenide glasses are also used.
Why the coatings matter as much as the material
Germanium has a refractive index of about 4, so each uncoated surface reflects about 36% and an uncoated plate passes under 50% (calculated). Anti-reflection (AR) coatings for the long-wave band fix that: one optics catalogue lists 90% or more transmission from 8 to 11.5 μm for an AR-coated germanium window (published figure). On an exposed outer face, a hard diamond-like carbon (DLC) coating protects against abrasion and cleaning. Ask for the transmission curve across 8–14 μm, not one peak value: some coatings are specified only up to 11.5 or 12 μm.
How the window changes a reading
Through a window the core sees three things at once: the target, reduced by the window’s transmission; the window’s own emission at the window’s temperature; and reflections of whatever is behind the window, such as the inside of the housing. Without correction, the reading is pulled towards the window and housing temperature.
A simple example, calculated for a black target, the 8–14 μm band and a window that transmits 90% and absorbs the rest:
- Window at 25°C: a 60°C target reads about 57°C, 100°C about 94°C, 150°C about 141°C
- Window warmed by the sun to 45°C: a 25°C target reads about 27°C
The error grows with the difference between target and window temperature, and it shifts whenever the window temperature shifts.
Correct it, or calibrate through it
Two approaches work:
- Correct for the window in your measurement chain. You need its transmission and reflectance in your band, and its temperature. If sun or a heater changes the window temperature, measure it instead of assuming it.
- Check the finished assembly against a reference with the window fitted: a blackbody if you have one, or a matte black surface measured with a good contact probe, at the temperatures your product measures.
Do one of these before you trust absolute numbers. The accuracy figure of the core in this example, ±2°C or ±2% (typical, lab conditions), applies to the core alone, not to the core behind your window.
Size and position
The window must not clip the field of view. Its clear aperture has to cover the front of the lens plus 2 × spacing × tan(half the diagonal field). For every millimetre between lens and window, the aperture grows by about 1.15 mm for a 9.1 mm lens on a 640 × 512 core (48.7° × 38.6°) and by about 1.6 mm for a 4 mm lens on a 384 × 288 core (64.1° × 48.8°) (calculated). Wide lenses therefore need the window very close. Tilting the window a few degrees keeps reflections of the lens and the housing interior out of the image; allow for the extra aperture this needs.
Outdoors
- A bare core has no ingress rating; the housing and its window seal provide it. Fixed outdoor thermal cameras on the market are commonly rated IP66 (published ratings).
- Water, ice and dirt on the window absorb long-wave infrared. A wet window gives wrong readings.
- Measurement is specified for a narrower ambient range than imaging: −10°C to +70°C against −40°C to +80°C on this core. In cold climates plan a heater, and remember that it changes the window temperature too.
- The manual for this core says not to point it at the sun. Mount the camera so the sun stays out of the field of view.
Before you order windows
- Material, and an AR coating band that covers 8–14 μm
- Transmission and reflectance curves at your angle of incidence
- Clear aperture at your lens-to-window spacing
- Outer coating for your environment and cleaning method
- How you will correct for the window or calibrate through it, and how you will know its temperature
Planning a window for your product?
Send the lens, the window spacing and what the product measures. We reply within 24 hours on working days.