Compared to night vision goggles, which rely on detecting reflected (residual) infrared light that is imperceptible to the human eye, thermal goggles are based solely on the detection of heat emitted by an object or body with a temperature above zero degrees. In summary, thermal cameras utilize the infrared light emitted by objects through heat, while night vision goggles rely on the infrared light reflected by those same objects.
On the market, you can find two main types of thermal goggles: those that display the heat aura emitted by the body and scanning goggles equipped with an infrared ray detector. Information is acquired by a system that scans the captured scene. The thermal signal is then converted into an electronic signal and sent to the digital display through a processor.
THE TECHNICAL SPECIFICATIONS OF A THERMAL IMAGER
Sensor Resolution (pixels): The resolution of the sensor affects the level of detail in thermal images. In the hunting industry, thermal imagers are available in different sensor resolutions, which can be divided into three main categories:
- Low resolution: ≤ 160×120 (19,200 pixels)
- Medium resolution: 320×240 (76,800 pixels) or 384×288 or 400×300
- High resolution: 640×480 (307,200 pixels) or 640×512 (327.680 pixels)
In comparison to photography, where cameras with sensors of over fifty million pixels exist (often unnecessary for amateur printing), the performance differences between low-end and high-end thermal imagers are significantly noticeable. Considering that the most advanced thermal imagers provide a resolution of 640×480 pixels, it is a fraction of space that is easily manageable not only on a monitor display but also on a home printer.
PIXEL PITCH
The most commonly used term by manufacturers is “pixel pitch.” But what does it mean? The core of a thermal imager is the IR detector, which consists of an array of thermal pixels arranged in a regular matrix. The distance in microns between the centers of two adjacent pixels is the pixel pitch and is measured in microns. The level of detail provided by a thermal imager also depends on the size of the pixels. Smaller pixel size corresponds to a smaller pixel pitch and allows for greater detail to be visible. Therefore, a higher pixel density produces a more detailed image, although, similar to photographic sensors, thermal noise needs to be managed and limited. Having excellent resolution is also crucial for perceiving details in adverse conditions such as rain, fog, or smoke.
Another benefit that is highlighted when using a high-resolution sensor is the ability to zoom in optically without losing sharpness in the details. Most thermal cameras are equipped with a lens that shows approximately 25° to 30° field of view. Therefore, in general, a thermal camera with a 640×480 pixel sensor with 2x digital zoom will provide similar performance to a thermal viewer with a 320×240 sensor but with a more expensive lens of greater focal length, which captures, for example, a 12° field of view.
THERMAL SENSITIVITY or NETD
The second characteristic that affects the image quality is thermal sensitivity. This value is not standardized as there are different tests to quantify its performance. The thermal sensitivity varies with the temperature of the object, and the signal-to-noise ratio (SNR) will improve when viewing very hot objects. Unfortunately, thermal viewers are primarily used to analyze low thermal temperatures and their differentials (ambient, animals, and humans). For these reasons, the thermal contrast will never be very high.
This characteristic is sometimes highlighted in manufacturers’ catalogs with the acronym “NETD” (Noise Equivalent Temperature Difference). It expresses the ability of a thermal imaging detector to perceive very small differences in the thermal radiation of the observed scene. When the thermal noise is equivalent to the minimum temperature difference that can be measured, a thermal viewer has reached its capacity to resolve a useful thermal signal. Therefore, the higher the noise, the higher the NETD value of the viewer. Here is a simulation to illustrate the hypothetical difference in noise between an image obtained with a viewer with 200 milliKelvin and an image with a viewer with a NETD of 100 milliKelvin.
THE THERMAL NOISE
Infrared radiation is absorbed by each pixel in a simple yet effective manner. The electrical circuit of an infrared sensor is very basic and allows converting the temperature changes from pixel to pixel into a digital value. For this reason, all analog signals also carry a certain level of “noise” generated by the sensor. It is the ratio between signal and noise that negatively affects the quality of an image, and that’s why with less advanced thermal imagers, the “noise,” often referred to as “snow” in the images, will be more visible.
The diameter and aperture of the lens are also essential for improving the sensor’s performance. Typically, a standard zoom camera lens has a focal ratio ranging from F/3.5 to F/5.6, while more expensive fixed lenses can reach up to F/1.2. On the other hand, due to component miniaturization, mobile phone lenses can have focal ratios as high as F/20.
Thermal sensors, however, require very bright optics, such as lenses with an aperture of F/1 or F/1.2. In fact, a thermal imager with a sensitivity of 50mk, when used with an F/1.4 lens, will achieve a sensitivity comparable to 100mk, which will become 200mk when used with an expensive F/1 lens.
CALIBRATION (NUC)
The higher the pixel resolution and thermal sensitivity of the instrument, the greater the ability of the human eye to perceive small differences in image quality.
This process is called NUC, which stands for “Non-Uniformity Correction” or “Calibration,” and it can be perceived whenever a thermal imager freezes the image, emitting a “click.”
When the scene and the environment change, the shutter of the imager lowers between the lens and the detector. Since the heat from the thermal camera can interfere with accurate temperature readings, to improve precision, the camera will measure the IR radiation from its own lens and then adjust the image based on that reading.
Calibration increases the gain and offset in each pixel, resulting in a higher quality image. This is achieved through the use of a shutter block that serves as a flat reference source on which the detector calibrates and thermally stabilizes. This occurs in non-cooled thermal cameras, although it is sometimes also available in cooled models. In some technical specifications, it may be referred to as FFC (Flat Field Correction).
OPERATIONAL SPEED
Initially, the thermal camera will often perform calibration, which reduces as it reaches a stable operational temperature. In practical field use, even though companies advertise operational speed after approximately 20-25 seconds, it will take at least twenty minutes in a stable environment to provide a highly accurate estimation. Some thermal imaging devices used for hunting activities allow users to activate NUC (Non-Uniformity Correction) when they need to carefully verify a reading, although this feature is more commonly found in scientific thermal cameras.
IMAGE MANAGEMENT SOFTWARE
At times, enthusiasts rely solely on the sensor size and the aforementioned characteristics when purchasing their thermal imaging device, without being able to assess the potential of the integrated software. A sensor with 640×480 pixels will often display images on a higher-resolution display, and features such as interpolation, contrast, and sharpness can determine a qualitatively better image, all else being equal in terms of the sensor. For this reason, the best images will be visible in the device that possesses the best capture and management software, and this is not a factor to be disregarded. Personally, over the past few years, I have observed significant differences between devices that, based on technical specifications alone, seemed to offer the same performance but exhibited incredible disparities in practical use. The same applies in the photography industry. With the creation of new and increasingly powerful photo editing programs, it is now possible to enhance images captured with outdated digital cameras, yielding surprising results in terms of interpolation and noise reduction.
SHORT OVERVIEW OF THERMAL CAMERAS
After this extensive introduction, we believe it is possible to divide thermal cameras into five main categories, each of which may have different characteristics. However, given the rapid development of thermal technology, what has been written may be subject to challenge in just a few months.
SMARTPHONE ADD-ONS
These are the most affordable products to enter the world of thermal imaging. They are small, portable accessories, and some of them can be attached to the lens of your smartphone, allowing you to scan a wide panoramic area. They are commonly used by technicians to check for moisture in walls or water leaks. When used outdoors, they may suffer from low light conditions, which can be mitigated by using a larger diameter lens. The sensors in these smartphone add-ons are not very large, typically having a resolution of 80 x 60 pixels.
HANDHELD DEVICES
These are the smallest and most compact thermal imaging devices. At the time of their introduction, some models reached prices close to 16,000 euros, but now they can be purchased for a few hundred euros. Compared to more expensive models, they have a small display with less exceptional quality. Sometimes, the sensor’s pixel resolution does not match the display resolution. As a result, the processor has to interpolate the images, causing delays or motion blur. Typically, these devices have sensors ranging from 80×60 pixels to 60×120 pixels.
THERMAL VIEWERS
This is the widest range in terms of prices and performance. Usually, by investing around 1500 euros, you can purchase high-performing thermal viewers for short to medium distances. You will be able to track subjects in real-time, and with the best products, even spot medium-sized animals up to two hundred meters away. Spending around 2500 euros will allow you to identify animals even at three hundred to four hundred meters. These thermal viewers come with excellent displays and sensors that typically provide an average resolution of 320×240 pixels. They also have outstanding image quality and a high display refresh rate, often at 60 Hz.
At higher costs (from 3000 euros and above), thermal imaging devices are associated with sensors of higher resolution, typically 640×480 (307,200 pixels), and large-diameter lenses, often ranging from fifty to seventy-five millimeters, sometimes even with focal ratios as low as F/1, whereas in the photography sector, lenses with maximum apertures of F/1.4 or F/2.8 are offered. The video recording systems, moreover, are highly reliable, as well as the buffering capacity (data transfer and recording speed in the storage system). The best models are also robust, waterproof, and capable of withstanding strong impacts. Compared to models from previous years, they are starting to be equipped with increasingly powerful and compact batteries. Those who need to use a thermal viewer for many hours and have no specific budget constraints may prefer a model with a binocular system. Of course, such products can cost up to seven thousand euros.
CLIP-ON DEVICES
It is an additional system that attaches to shooting riflescopes. They are usually sold for prices ranging from one thousand to three thousand euros and allow for the utilization of the optical brightness of the used riflescope. I always advise purchasing the most expensive clip-on device that one can afford, and it is normal for its optical performance to depend on both the riflescope used and certain mechanical components (such as the robustness of the riflescope frame, the solidity of the mounting rings, and so on). One downside (in very cheap products) is the potential lower long-term durability compared to a dedicated thermal riflescope and, in some cases, the creation of a bulkier “optical train.”
THERMAL RIFLESCOPES
They function in the same way; however, they must possess specific characteristics to withstand recoil from high-powered calibers. The eye relief of the eyepiece must be at least 40-90 mm, and more sophisticated models may also have an inclinometer to facilitate angle calculation for shooting. Their optical housing is commonly constructed from aluminum instead of the lighter but more fragile polycarbonate.
Investing in higher-priced thermal imaging products can offer several real advantages compared to entry-level options. Here are the summarized points:
- Better image quality: High-quality thermal imagers provide clearer and more detailed images, allowing for easier interpretation and identification of objects.
- Superior ability to detect low-contrast thermal targets: High-end thermal devices excel at distinguishing subtle temperature differences, making it easier to identify targets with low thermal contrast.
- Increased resolution for precise long-range identification: Higher-resolution thermal sensors enable the accurate identification of subjects at greater distances.
- Reduced thermal noise: Premium thermal imagers have advanced noise reduction capabilities, resulting in cleaner and more accurate thermal images.
- Enhanced scanning capabilities for varying distances: High-quality thermal devices can effectively scan and analyze details at different distances, providing better overall situational awareness.
- Compatibility with lower-cost lenses: If the thermal imager’s CMOS sensor is of high quality and resolution, it can utilize lower-cost lenses while maintaining excellent performance.
- Improved anomaly management during temperature readings: High-end thermal imagers offer advanced algorithms and features to handle temperature measurement anomalies and provide more accurate readings.
- Complex and reliable management software: Premium thermal imaging devices come with sophisticated software that offers advanced functionalities, precise controls, and enhanced reliability.
It’s important to note that while it’s not necessary to buy the most expensive thermal imager on the market, it is advisable to invest in the highest-quality option within your budget. The advantages listed above justify the investment as they significantly enhance the user experience and performance in the field.