Are thermal imagers inherently more expensive than visible-light digital cameras?

Question

We're finally starting to see practical thermal imaging sensors (microbolometers) entering the consumer market. However, they are still vastly more expensive than comparable visible imaging sensors. 384x288 17µm pixel (i.e., 32mm²) thermal imagers with a fixed manual-focus lens run about $500, whereas $500 will get a 6000x4000 2µm pixel (i.e., 96mm²) CMOS sensor ... plus 5-axis sensor stabilization and a nice zoom lens.

My question: Are there physical constraints that would prevent large-scale production of thermal imagers from achieving price levels comparable to visible-light cameras?

I think there are two significant differences that need consideration: Sensors and lenses.

First is the sensor: Thermal imaging looks for radiation with wavelengths between 7-14µm, whereas visible light is in the range 0.4-0.7µm. Based on the physics alone, at the diffraction limit microbolometer pixels will have an order of magnitude greater surface area. Apparently commercial sensors are at the diffraction limit for both visible light (at 1 micron pixels) and thermal light (at 17 micron pixels). So, to make it fair, we would compare a 1" 24Mpx visible sensor with a 1" 300kpx thermal sensor. Microbolometers can be made from silicon using a CMOS process. Their structure looks a little trickier than state-of-the-art visible spectrum CMOS sensors, requiring a thermal bridge for each pixel as well as vacuum encapsulation of the sensor. But I know little of large-scale manufacturing processes, so are these variables significant in the limit on a per-unit basis? Update: This question now answered here.

Second is the lens: Thermal radiation requires lenses of different materials – typically silicon or germanium. Because of its longer wavelength, I would imagine thermal lens systems would be less sensitive to flaws than are visible optics, but maybe thermal attenuation is so high that it's just not possible to put a significant number of elements in front of the sensor, and maybe these lenses are intrinsically more expensive to manufacture?

Answer 1

Currently there are techniques for processing silicon to produce huge arrays of components through photolithography. Typical IR camera sensors are narrow-band-gap semiconductors such as HgCdTe, which are much more difficult to mass-produce. In addition, lenses for longer wavelengths are made of expensive, difficult to polish materials such as Ge and Al2O3. Even more expensive are bolometer and superconducting sensors, which require a cooling mechanism (e.g. liquid He in the Webb Space telescope).

Answer 2

Answer on state of the art in commercial thermal lenses found here:

Precision glass molding (PGM) of chalcogenide glass (ChG), combined with the recent reduction in the cost of microbolometers, has enabled a new generation of low-cost thermal imaging devices.... Consumer-based handheld thermal imagers, commercial thermal weapon sights for hunters, and thermal camera add-ons for cell phones have all become readily available to the average consumer in the last few years because of this technology.

Alan Symmons, co-author of the Field Guide to Molded Optics, and EVP at LightPath Technologies, says:

Chalcogenide glass enabled these advances because it is an amorphous material that can be molded, unlike its more traditional competitors: germanium (Ge), zinc sulfide, zinc selenide, and so on. Chalcogenide glasses such as LightPath's BD6 (As40Se60) provide a much lower-cost material that can be manufactured using PGM, a more suitable process for high-volume manufacturing required for consumer products. LightPath's ChGs also provide technical advantages over Ge.... When compared to Ge, ChGs are lighter in weight, have improved transmission at elevated temperatures (Ge is opaque at temperatures higher than 65°C, while ChGs continue to transmit), and are inherently athermal—an important consideration in thermal imaging.

Answer 3

No, I can't see any physical constraints for mass production of IR sensors.

In fact, most sensors in use in DLSRs are IR sensitive and have an IR filter, that means they are sensitive to wavelengths invisible to humans above 800nm and have to be "corrected" to yield natural looking colors. "Wildlife" type cameras exploit this at very reasonable prices. "Modified" cameras, i.e. with all filters removed, are used by hobby astronomers for maximum sensitivity and sometimes even for infrared only pictures.

Sensors of commercial offerings of IR cameras for mid-IR are produced in very similar ways like normal sensors and are mass-produced like the normal ones. They just consist of expensive or difficult to handle substances (PtSi, HdCdTe) which increases prices.

A physical limitation of longer wavelength IR radiation is that glass does not transfer it, so that pictures of humans wearing glasses will show them like wearing sunglasses and it is impossible to take a picture through a glass window. This will probably not help making these cameras attractive for wider audiences. But again these are economical and not physical considerations.