(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)
Though invisible to the eye, thermal radiation can be detected by thermal imaging cameras, also called thermographic or infrared cameras. Heat is emitted by any object with a temperature above absolute zero. This radiation increases as an object’s temperature rises. Thermal cameras typically provide screen images, called thermograms that display variations in the heat their sensors detected. Thus, different levels of reflection can affect what the visualization looks like.
Visible light cameras work in the 0.45 – 0.75 µm (450-750 nm) wavelength range. Thermal imaging cameras detect wavelengths up to 14 µm (14,000 nm). For engineering use, these cameras allow one to view, pinpoint and analyze differing thermal patterns. They can detect heat dissipation and leakage from target objects or over an expanse. When used with electronic devices, thermal imaging cameras show if and where a device is running hot, or if it is running below its normal operating temperature.
Figure 1. Keysight Technologies Thermal Imager for electronics design and troubleshooting. [1]
These cameras calculate surface temperature based on a combination of emitted, reflected and transmitted heat. Today’s cameras are capable of making temperature measurements from -80°C to +3000°C. Some cameras can detect temperature differences as small as 0.02°C.
Professional quality cameras integrate extremely high imaging performance and accurate temperature measurements with integral software for analyzing and reporting. They are suitable for research, testing, troubleshooting and product validation applications. [2] A wide range of thermal imaging cameras is available for general and recreational use. Most recently, new mobile apps allow phones and other portable devices to be used for basic but useful thermal imaging.
Cooled and Capable
In broad terms, thermal imaging cameras fall into two categories: those that are cooled and those are not.
Higher performance thermal imaging cameras have integral cooling systems to adjust their sensor temperatures to cryogenic levels, typically starting at -150°C. Most modern cooled detectors operate in the 60 to 100 K range (-215 to -175°C), depending on type and performance level. Cooled cameras typically are vacuum-sealed and feature cryocoolers or other systems to lower sensor temps. One cooling method uses pressurized gas, which is expanded via a micro-sized orifice and passed over a miniature heat exchanger resulting in regenerative cooling via the Joule–Thomson effect. [3]
Very cold sensor temperatures are needed to prevent these high-performance cameras’ own thermal radiation or any other away from target radiation – referred to as thermal noise – from interfering with the heat signal from whatever is being imaged.
Cooled cameras typically have greater magnification capabilities than uncooled cameras because they can detect shorter infrared wavelengths. Because cooled cameras are more sensitive they can employ lenses with more optical elements, or thicker elements can be used without degrading the signal to noise ratio. This results in better magnification performance. [4]
Highly sensitive cooled cameras can detect the slightest temperature differences. They can be designed to enable imaging in the mid-wave infrared band, 3-8 µm, and the long-wave infrared band, 8-15 µm, of the wavelengths spectrum where there is higher thermal contrast. The more thermal contrast, the easier it is to determine objects against a background not hotter or colder than the object.
Many applications of cooled thermal imaging cameras are found in space, typically in orbiting satellites. In those environments, cryocoolers help cool thermal imaging devices that need to reach as low as -195°C (-320°F) to detect what they’re designed to capture. Lockheed Martin’s new microcryocooler allows the use of large and highly sensitive IR sensors capturing very high-resolution images. [5]
Figure 2. Lockheed Martin’s microcryocooler cools satellite-based thermal imaging cameras, providing continuous operation for 10 or more years. [5]
Among the drawbacks of cooled infrared cameras are their price and operating costs. Cooling is both energy-intensive and time-consuming. A camera’s electronics may need several minutes to cool down before it can begin working. But while their cooling mechanisms are comparatively bulky and expensive, cooled infrared cameras provide superior image quality compared to uncooled ones.
Uncooled and User-Friendly
The image sensors in uncooled thermal cameras work at or near ambient temperature. No cryogenic cooling is needed. The sensors work by detecting changes in resistance, voltage or current when heated by infrared radiation. These changes are measured and compared to the values at the operating temperature of the sensor.
The primary type of uncooled detector today is the microbolometer, a device based on microelectromechanical (MEMS) technology. A microbolometer is a tiny vanadium oxide (VOx) resistor with a large temperature coefficient on a silicon element with large surface area, low heat capacity and good thermal isolation. When infrared radiation in wavelengths between 7-13μm strikes a microbolometer’s detector material and is absorbed, it heats up and the resulting change in its electrical resistance is the basic sensing technique.
Figure 3. Depiction of a microbolometer for use in uncooled thermal imaging cameras. [6]
These changes are processed by separate core electronics to create a thermal image. The detectors are quite sensitive and are able to sense heat radiated from objects depending on their temperature. [7]
Uncooled cameras are smaller and easier to use. However, their resolution and image quality tend to be lower than cooled detectors. This is due to difference in their fabrication processes, limited by currently available technology. Uncooled cameras can be used without cool-down intervals and require less power than cooled thermal imaging cameras.
Uncooled cameras are generally much less expensive than cooled infrared cameras and they have fewer moving parts, so they tend to have much longer service lives than cooled cameras under similar operating conditions. Uncooled image sensors are manufactured in fewer steps, and with higher yields, compared to cooled sensors, and uncooled cameras do not require cryocoolers, which are very costly devices. [7]
Thermal Imaging by Smartphones and Tablets
The FLIR ONE, from FLIR Systems, transforms a mobile device into a thermal imager that sees heat and measures temperature. The device provides users with the ability to see temperature variations smaller than a tenth of a degree. FLIR’s technology enables a host of practical applications, from identifying energy inefficiencies and water leaks in a home, to enabling safe and enjoyable outdoor exploration. Versions are available for both iOS and Android device platforms. [8]
Figure 5. The pocket-sized FLIR ONE transforms a mobile device into a thermal imager that sees heat and measures temperature. [8]
The FLIR ONE uses a micro-USB connector for Android devices and a Lightning connector for iOS devices to offer a compact accessory that connects to smartphones and tablets. Its power comes from an internal battery. Images are enhanced with FLIR’s multi-spectral dynamic imaging (MSX) technology, which embosses the edge details from FLIR ONE’s visible camera onto the thermal image producing high fidelity images.
Seek Thermal, in conjunction with defense and aerospace company Raytheon, developed a 32,000 thermal pixel imaging chip — with a resolution of 206 x 156 array. The Seek thermal camera, which weighs about half an ounce and measures 2.75 x 84 x .84 inches, plugs into the device’s Lightning connector. It is based on a proprietary 12-micron sensor chip and custom software. For temperature readings, the sensor can detect values from -40°C to 330°C. [9]
Figure 6. Seek Thermal’s app features four thermography modes with nine color overlays to apply to the temperature measurements. [9]
Seek Thermal’s app features four thermography modes with different kinds of information and gives a choice of nine color overlays to apply to the temperature measurements that will create an image.
The Seek Thermal camera works with Apple devices that have Lightning connectors, such as the iPhone 5, 5c, 5s, and 6 running iOS7 or iOS 8. It also connects to Android devices with a microUSB connector running Android 4.4.2 (KitKat) or later. Aside from the iPhone, the app has been tested with the Samsung Galaxy S4 and S5 and the Moto G and X.
Figure 7. The Therm-APP TH camera leverages modern smartphones’ high-definition and touchscreens. [10]
Thermal imaging cameras from Therm-App are designed for Android smartphones. Different models of Therm-App cameras feature long-range night vision along with video and sound recording. Their high-performance Hz model features a 384 x 288, 17µ thermal detector and 25 Hz frame rate that provides better image quality when tracking objects on the move. [10]
Thermal imaging cameras are valuable tools for detecting problems in electronics, and have myriad uses in everyday life. High end cooled thermal cameras provide impressive performance at premium costs and somewhat restricted use. Uncooled thermal imaging cameras can be used in many professions and by hobbyists. Among the newest developments for uncooled cameras are their use with smartphones and tablets.
References
1. Keysight Technologies, http://www.keysight.com/en/pc-2386399/trueir-series-thermal-imagers?nid=-32903.0&cc=US&lc=eng
2. Quality Magazine, http://www.qualitymag.com/articles/92609-high-speed-thermal-imaging-for-automation-applications
3. http://www.flirmedia.com/MMC/CVS/Tech_Notes/TN_0005_EN.pdf
4. FLIR Systems, http://www.flir.com/science/display/?id=65982
5. Lockheed Martin, http://www.lockheedmartin.com/us/news/press-releases/2015/august/space-Microcryocooler-payloads.html
6. Ipi-Infrared, http://www.ipi-infrared.com.au/how-do-infrared-cameras-work/
7. Sofradir-EC, http://www.sofradir-ec.com/wp-uncooled-detectors-achieve.asp
8. FLIR Systems, https://www.flir.com/news-center/press-releases/flir-systems-announces-availability-of-third-generation-flir-one-thermal-imaging-cameras-for-smartphones-and-tablets/
9. Seek Thermal, http://thenextweb.com/creativity/2014/09/25/seek-thermals-new-mobile-camera-attachments-let-see-heat
10. Therm-App, https://therm-app.com/
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.