Thermal imagers: myths and reality.

Thermal imagers: myths and reality.

– Why does a thermal imager have an unusual lens?
– Because it’s not made of glass, but of… well, not glass.
– Of germanium?
– Yes, of Germany, they make good optics, believe me.
From a conversation between a buyer and a seller

The dialogue given in the epigraph took place in real life.

After hearing this, you want to exclaim: “I don’t believe it!”

After all, how can you trust a person who is trying to sell you something that he himself has no idea about? a very vague idea? Will such a person be able to help you solve problems?

Hardly, so let's figure out together what lenses in thermal imagers should be like… and not only them.

First, let's answer the question raised in the epigraph.

Yes, indeed, lenses for thermal imagers are not made of glass. The choice of materials is influenced by a fairly large number of factors, among which the main ones are the ability to transmit infrared radiation and the refractive index of the material.

The cheapest and most common material, optical glass, is transparent only in a limited spectral range (wavelength less than 2 microns). Most thermal imaging systems have to use other materials — special glasses and various compounds. One of the most common materials for lenses in thermal imaging is germanium.

Germanium is used to make most lenses operating in the 3..5 and 8..14 micron ranges. However, it should be noted that at high temperatures, germanium parts lose transparency — twice as much when the temperature rises to 100° C and almost completely at 160-200° C.

Let's take a closer look at the physical properties of germanium.

Germanium is a fairly durable material, as hard as silver. But this does not mean that a thermal imager lens is a durable metal product that will not fail under any conditions. Since germanium lenses are much more expensive than glass or plastic ones, they must be handled with care.

Here are some tips on how to maintain thermal imager lenses.

As with video cameras, they get dusty, and sometimes even fingerprints. And although germanium is as hard as silver, it is better to literally blow off dust particles from it. You can use special nitrogen cylinders for this.

Unfortunately, it is often impossible to remove the thermal imager from the object and take it to a service center, so in «field» conditions, a damp cloth can be used to remove dust. It should be remembered that when cleaning the lens, you should not apply much force. Also, do not wipe the lens for a long time, do not wipe it in a circular motion — one S-shaped movement is enough. Fingerprints and other greasy contaminants can be removed with acetone or ethyl alcohol.

It is not recommended to use cologne, strong alcoholic drinks and cosmetic nail polish removers, since the absorption spectrum is often unknown to us — and instead of cleaning the lens, you can simply ruin it.

Germanium is not the only material used to manufacture optical components of thermal imagers. Zinc sulfide (for example, for hermetic housing illuminators), alkali metal fluorides and other materials are also currently used.

The relatively high cost of these materials prevents their widespread use, although in some military applications their use is indeed justified due to specific operating conditions.

Some «respected experts» use the term «motorized lens» in their articles.

Being interested, we decided to type this phrase into an Internet search engine. But after the word «motorized» we were offered the following options: Wehrmacht division and armor.

Does that mean there are no such lenses?

Unfortunately, it doesn't happen. This phrase is a computer translation of the term «motorized lens», which in English-speaking countries refers to a lens with motorized focus adjustment. What are such lenses for? After all, the presence of an electric motor does not have a positive effect on the reliability of the equipment, or, to be more precise, does not increase fault tolerance. Everything becomes clear when we pay attention to the specifics of using thermal imagers in security systems.

In the previous article “Thermal Imagers: It’s Not That Simple,” we pointed out that due to the laws of physics, lenses for uncooled thermal imagers (which are most often used in security systems) must be high-aperture, which means they will have a small depth of field.

Let’s remember how to calculate the depth of field for a lens. The depth of field limits depend on the aperture F, the focal length of the lens f, the distance at which the lens is focused x, and the pixel size of the sensitive element of the thermal imager d. The far limit of the depth of field is calculated using the formula: (1)

   (1)

If the denominator of the expression is less than or equal to zero, then the far boundary of the depth of field corresponds to infinitely distant objects. The near boundary of the depth of field is calculated using a similar formula: (2)

    (2)

Using formulas (1), (2), we calculate the depth of field for a thermal imager with a pixel size of 25 microns, with an f=75 mm (F#=1) lens, focused at a distance of 500 m. The far boundary will correspond to infinitely distant objects, the near one will be located at a distance of 345 meters.

When the intruder goes beyond the limits in which the image has the required clarity, a quick adjustment of the thermal imager lens is required. Indeed, it is not necessary to adjust it manually every time? Therefore, high-tech thermal imagers for security systems are equipped with lenses with motorized focus adjustment.

Fig. 1. Graph of the dependence of the location of the near boundary of the depth of field on the focal length

In Figure 1The dependence of the near boundary of the depth of field (in meters) on the focal length for lenses focused to infinity is given. As we can see, with an increase in focal length, the near boundary of the sharply depicted space moves away from the camera. And if when using short-focus lenses (up to 35 mm) this can be neglected, then when using long-focus lenses (50 mm and more), it is highly desirable to use focus adjustment — otherwise, objects close by will be poorly distinguishable.

One of the myths about the exploits of thermal imagers is the huge range of target detection. Manufacturers and distributors do not hesitate to write that an uncooled thermal imager with a lens with a focal length of 100 mm detects an intruder at a distance of 2 km. True, then they make a footnote and indicate in small print that this is «an approximate value obtained on the basis of the Johnson criterion.» As you know, the most interesting things are always written in small print in advertising.

Let's look at this nuance more closely. First, it should be said that under certain conditions this statement may be true. If we neglect weather conditions and background radiation, we will get an image of a person. Its dimensions will be approximately 4×2 pixels. If we assume that the operator is looking at a 17-inch monitor, which displays 4 cameras, including our thermal imager, the height of the person on the screen will be approximately 2 mm. Theoretically, detection is possible. But in addition to optics, a number of other factors should be taken into account, which undoubtedly contribute to the probability of detecting a target.

? Characteristics of the detected object (contrast with the background, brightness, size, complexity of the contour, position on the screen, speed, etc.).
? Characteristics of the scene (background): brightness, noise intensity.
? Weather conditions — the rate and nature of precipitation, relative humidity, dustiness.
? Characteristics of the observer: training, motivation for action, fatigue, age, visual acuity, etc.
? Tactical requirements: area of ​​the search zone, permissible search time, illumination in the room, various distractions, etc.

As we can see, the probability of detection depends not only on the focal length of the lens. But since it is difficult, and often impossible, to take into account all the above factors (after all, the designer does not know how incorruptible the guard is or how fast the intruder will move), designers of thermal imaging systems often use the Johnson criterion. For example, according to the Johnson criterion, two raster elements are enough to solve the problem of detecting a target with a 50% probability, six for discrimination, etc. Also, «advertising specialists» in pursuit of the detection range forget that we are talking about a 50% probability. Yes, it is much easier to look at the table and substitute the values ​​than to make a calculation, but practice shows that it is necessary to adjust these values. For example, the UK Home Office recommends having at least 10% of the raster for the height of the person being detected — and this is much more plausible, taking into account all the above factors.

It should be remembered that no matter what lens you use, detecting a person against a background heated to a temperature of 36-37° C is a virtually impossible task. It is believed that Malevich's painting «Black Square» was originally called «Negroes Steal Coal at Night». As with negroes, so with thermal imagers: if the contrast of the detected target and the background is less than the sensitivity limit of the human eye, detection, discrimination, and recognition are impossible.

When designing a thermal imaging system, it is also worth remembering that it will most likely be used outdoors. Therefore, we must not forget about weather conditions. And this is not only about protecting the thermal imager from rain, snow, dust and maintaining the operating temperature. It is also necessary to remember that atmospheric phenomena and dust have a negative effect on the target detection range. In the book «Infrared and Electro Optical Systems Handbook» the authors provide the following dependence of the attenuation coefficient on the rate of precipitation (rain).

Fig. 2. Recommended object size for detection

Fig. 3.The boundaries and average value of such a coefficient for the luminosity of bodies heated to a temperature of 36 oC

For example, in summer rain, the detection range can drop three to four times. Dense fog, snow, and dust also affect the propagation of radiation. If you ignore the influence of the weather, the funds invested in creating a surveillance system will not allow you to solve the problems set before it. Of course, modern thermal imagers are equipped with electronic means of contrast enhancement, which allows you to slightly increase the distance at which you can detect an intruder, but you should remember: neglecting the influence of the atmosphere on the operation of an infrared system is a very serious mistake!

In conclusion: we believe that thermal imagers have taken a worthy place in the security systems market. The scope of thermal imagers is not limited to security television surveillance — with the development of technology, new, more effective solutions to old problems appear. But you should not rush to buy thermal imagers without taking into account how, where and for what they will be used. In the next article, we will present a method for selecting a thermal imaging camera, suitable optics and other peripheral devices — protective covers, analytics devices, etc. After all, a thermal imager is a tool. And any tool requires precise adjustment and professional use.

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S. Nikitin, «SKN»