AT HOME IN THE DARK - THERMAL IMAGING
Being able to see clearly in the dark is a powerful asset, whether you are in the military, law enforcement, security, out hunting or on pest control.
The technology in devices allowing you to do so have made huge leaps in recent years, especially in the thermal imaging category. Next to professional users, more and more consumers are sold on the idea of thermal imaging and have come to see it as a piece of gear as essential, or even more, as their daylight optics. With the thermal imaging market maturing, technology is ever more developed while users are growing more discerning. Which is why, in this month’s Great Gear, we do a deep dive into the crucial parameters of thermal imaging devices.
Whereas tube-based image intensifiers and digital night vision function by gathering ambient light, supported - or not - by an infrared illuminator, thermal imagers detect thermal radiation. Better known as heat, thermal radiation is still part of the electro-magnetic spectrum, yet of a completely different infrared wavelength than the one used for tube-based and digital devices. The thermal imaging device focuses these infrared waves on a sensor called a microbolometer. With incredible precision, thermal imagers spot temperature differences and translate these differences into an image on a digital display. This description makes the whole process look rather insignificant. Obviously this is not the case. In order to judge quality in thermal imagers, it is important to get to grips with key specifications like sensor resolution, frame rate, pixel pitch and thermal sensitivity. Only when familiar with these, you can compare apples with apples.
The thermal sensor - microbolometer - resolution is an important parameter for assessing the quality of a thermal imaging device. This is measured as the width and height of the thermal sensor in pixels. It is the number of sensitive elements, pixels, constituting the sensor. Sensors with a larger number of pixels can produce a more detailed image of an object. A display with a resolution of 320×240 will be 320 pixels wide by 240 pixels high. A high resolution sensor will imply a crisp and clear definition of the image in your display. Lower resolution sensors will generate more distorted images, with greater pixelation of the resulting image. In the low-budget entry level range sensor resolutions of 160×120 exist. In the mid-range you will find 320×240 among others. In the high-end, resolutions of 640×480 and more are common. Although a resolution of at least 320 pixels is recommended for general hunting use, the need for higher end models with a 640 pixel display depends on the range you will want to bridge. The longer the anticipated range, the more resolution you will need to have a really crisp image.
Another factor linked to sensor resolution is pixel pitch. This is the distance between the centres of two adjacent pixels of a microbolometer. In thermal imaging sensors, this distance is measured in microns (µm). This also gives an indication of pixel size. Pixel size is responsible for level of detail. The smaller the pitch, the more detail you’ll see. Nowadays, 17 micron is an industry standard with 12 micron pixel pitch being top notch on the consumer market.
The greater the number of pixels and smaller their size on the thermal sensor, the better the resolution will be. This statement is true when compared thermal sensors are of the same physical size. Thermal sensors with a greater pixel density will have a better resolution.
Another important topic concerning thermal imaging devices is refresh rate. This is a variable that shows how quickly the device can render an image and create a visual output. This refresh rate shows how often an image is refreshed and is measured in cycles per second or hertz (Hz), ranging from 9 to 60 in most devices. A refresh rate of 9Hz implies that an image will be refreshed 9 times per second while with a rate of 60Hz an image will be refreshed 60 times per second. A higher refresh rate means a smoother image, especially when tracking a moving object or when you yourself are tracking from a moving vehicle. With lower refresh rates the image, especially with a moving object, will start to look more like a laggy slideshow than ‘live’ images.
One of the most important parameters characterising the quality of a thermal imaging device is the Noise Equivalent Temperature Difference (NETD) which describes its thermal sensitivity. This parameter is measured in millikelvin (mK) and denotes when the temperature value signal is equal to the noise signal. When in use, a thermal imaging sensor not only registers useful signals but also noise; which interferes and prevents it from producing a clear, quality image. In an instance where the noise signal is equal to the smallest temperature difference that can be perceived by the unit, the thermal detector is not able to discern a useful thermal signal to resolve an image of the object. The higher the noise level, the higher the NETD of the thermal detector and the worse it can discern small temperature differences.
In practice, this means that when the temperature difference of an object drops down low enough, the heat signature it is radiating may merge with noise to such an extent that the thermal imager can fail to distinguish the object’s signal from noise. When this happens the object in the scene merges with the background or other objects in the scene and becomes practically indiscernible. Therefore, the lower the NETD value (specified in millikelvin (mK)) the better the sensor can register small temperature differences. NETD values below 40 mK are considered excellent, between 40 and 50 mK is good, between 50 and 60mk acceptable. Meanwhile a NETD between 60 and 80 mK is satisfactory at best. In conditions when the temperature differences of observed objects are minimal - for example; cold weather, rain or fog - the thermal imager with the lower NETD value will show a richer, better quality image. In other words, the unit with the lower NETD value will show object details with more contrast and be more visible.
As thermal imaging doesn’t need visible light to function, these devices can be used in every situation imaginable, from bright daylight to complete and utter darkness underground. Thermal imaging will also detect heat from an object that is camouflaged or hidden in a bush. With regular day or night vision this object will most probably go unnoticed. Thermal imaging also allows you to scan an area far more quickly for game, intruders… then you would be able to use regular night vision (at night) or daylight optics (during the day). No wonder hunters have become extremely fond of their thermal devices. With the key parameters we described in mind, you’ll be even better prepared to advise and support your customers in making even better choices.