Let’s say you’re a prospective buyer touring an older home that you suspect has some weatherization issues. What if you could verify your hunch by literally seeing cold air seeping under doors or cooling walls where insulation is missing? And what if you could do this on the spot using a smartphone?

Figure 1: The FLIR ONE attaches to the back of the iPhone 5 and 5s. (Image: FLIR Systems)
Long associated with million-dollar, airborne cameras that look like upside-down droids mounted on helicopters, airplanes, and ships, thermal cameras are currently small enough to fit inside a cell phone. Now, consumers can utilize lower-resolution thermal imagers in a variety of practical applications, from home repairs and electrical inspections to looking for a lost child at night.

Earlier this year, FLIR Systems released a pocket-sized thermal imager called FLIR One that attaches like a case to the iPhone 5 and 5s (see Figure 1). Using the iPhone’s LCD screen, the imager measures and displays invisible heat energy (also called thermal energy) instead of visible light. With standard settings, warmer areas (like the inside of a house) will appear brighter on screen, while cooler areas (cold air sneaking in) will appear dark.

In this article, we will explore the emergence of compact, low-cost thermal imagers and their current impact on the development of commercial products equipped with thermal imaging.

Early Thermal Imagers

Thermal technology has been around for a century, but the first thermal imaging applications did not emerge until shortly after World War II, when the US Air Force developed a rudimentary reconnaissance thermal camera mounted in the belly of cargo planes and bombers. The first commercial application of a thermal imager appeared in 1965, when the Swedish company Agema built the first infrared scanning camera for power line inspection.

In order to sense faint thermal signatures, the early thermal imagers cooled their internal detectors to cryogenic temperatures with liquid nitrogen. Later models achieved the same cooling with internal cryo-coolers. Today’s long-range thermal cameras use highly advanced versions of these coolers to maximize range performance. The thermal cameras are mostly used by the traditional government customers: the military and law enforcement agencies.

Uncooled thermal cameras operate without the need of additional cooling. A common detector design uses a microbolometer — a tiny vanadium oxide resistor with a large temperature coefficient. Changes in scene temperature cause changes in the bolometer temperature that are converted to electrical signals and processed into an image. Uncooled sensors work in the longwave infrared (LWIR) band, from 7 to 14 microns in wavelength, where terrestrial temperature targets emit most of their infrared energy.

The uncooled cameras are generally much less expensive to produce. The sensors can be manufactured in fewer steps, with higher yield and less expensive vacuum packaging. Because they are far less expensive than their cooled counterparts, uncooled thermal cameras have enabled broad commercialization of thermal imaging.

Wafer-Level Technology

Figure 2: The MTC produces a longwave infrared image with a resolution of 80 by 60 pixels, at a frame rate of <9 Hz. (Image: FLIR Systems)
In the thermal camera world, resolution is the most significant element to improving image quality. The higher the thermal resolution, the more detail the camera captures. There are generally three resolution standards: 160 x 120 (low), 320 x 240 (medium), and 640 x 480 (high). Which resolution is most appropriate is really a matter of the final application. A soldier using a thermal rifle scope in combat, for example, is going to require exceptional detail, while a thermal imager with a lower resolution will be perfectly suitable for predictive maintenance in a factory.

The micro thermal camera (MTC) inside FLIR One demonstrates a miniaturization of uncooled thermal camera cores. Named Lepton®, the uncooled imager is produced using wafer-level technology, where the thermal sensor, lens, and supporting electronics package all fit on a single chip (see Figure 2).

Lepton has a resolution of 80 x 60 pixels, far less than its more powerful cooled siblings. Nonetheless, just as a smartphone can now accomplish what used to take a room full of mainframe computers, MTCs can be manufactured at a high volume and equip new mobile devices and camera systems with thermal imaging capabilities.

Creating the compact thermal camera required several proprietary technologies, including a wafer-level detector packaging, wafer-level micro-optics, and a custom integrated circuit that supports all camera functions on a single, integrated, low-power chip.

The MTC operates on low voltages found in standard mobile phones, fits into a standard 32-pin Molex socket, and uses control and serial video interfaces that are compatible with phone standards. The serial video interface is MIPI compatible and features a D-PHY transmitter to send serial video data and clocks to the host. The video is accessible either via the MIPI interface or as packetized video using SPI. The control interface is similar to the I2C protocol. Designers supply standard mobile industry voltages and a clock to get thermal images through mobile industry standard interfaces.

Consumer and Industrial Applications

Figure 3: Formula 1 racing teams can use thermal imaging to examine tire wear. (Image: FLIR Systems)
A smartphone-sized thermal imager is a versatile tool for consumers, both in terms of night vision and thermography. Consumers can use the device for personal safety while walking at night, finding lost pets, or observing wildlife in the outdoors. For thermography, the imager identifies temperature differences that indicate overheated circuits and water leaks within buildings or homes.

An MTC can also measure temperatures as a spotmeter, which consists of a 3x3 pixel area on the imager that is converted from digital counts to Celsius or Fahrenheit temperature units. Spotmeters typically measure small areas on the surface of mechanical equipment or electrical components to ensure there are no issues with overheating. The technology also performs diagnostics with HVAC equipment, automotive parts, and motors used in manufacturing (see Figure 3).

The thermal spotmeter is calibrated in absolute temperature units. The temperature calibration also takes into account the emissivity of the material being measured, since that directly affects temperature measurement accuracy.

In addition to consumer applications, micro thermal technology stands to have a major impact on commercial industries, where investment in thermal imaging has traditionally lagged due to cost. In many situations, a low-resolution thermal imager is more than adequate for the job. A small security camera containing a MTC, for example, can easily capture human activity within a medium-sized room or hallway. And as prices drop, budgets can stretch farther, with more imagers available for wider coverage within a facility.

MTCs are also affordable options for manufacturers of miniature unmanned airborne vehicles (UAVs) and even smaller nano unmanned aerial systems (NUAS) used for surveillance and reconnaissance. For years, UAV makers have used larger uncooled imagers because they are capable of higher resolutions. Even a low resolution from a lightweight and mobile MTC, however, can provide valuable data.

A thermal scan of a bedroom reveals sections of wall that are cooler due to missing insulation. (Image: FLIR Systems)
Beyond its use as an imager, an MTC’s prowess may actually be best realized as a detector. Thermal detectors are already widely used as motion sensors. Inexpensive passive infrared sensors use small thermopile arrays with resolutions as low as 8 x 8 pixels. Their job is simply to trigger an alarm when there is a sudden change in temperature within a scene. From a safety perspective, an MTC can recognize the presence of a human inside a hot car and trigger the windows to lower, or it can be programmed to adjust the thermostat in a home when it no longer detects humans present.

The low cost of MTCs will make it possible for manufacturers to create even more effective and affordable commercial and consumer-based security systems. Thermal cameras create contrast based on heat, instead of visible light or color. The heat-based measurements are important additions to visible security camera systems, whose images can be obscured by rain, snow, and sun glare — not to mention darkness itself.

Because MTCs create such reliable images, they are also a natural fit for video analytics — a computer-based method of studying video streams to identify ranges of activity, from pedestrians standing on the sidewalk to cars stopped at an intersection. Video analytics is a critical technology used in traffic monitoring. Like many industries, the implementation of thermal cameras into Intelligent Traffic Systems has been slow due to cost. Cheaper MTCs have an opportunity to change that.

As with any breakthrough technology, how manufacturers utilize the power of micro thermal technology remains unclear, but with the technology now a reality — and available on a large scale — the days of thermal imaging being out of reach are over.

This article was written by Dr. Austin Richards, Ph.D, a physicist working in IR imaging systems development at FLIR Systems (Santa Barbara, CA). For more information, Click Here .

Imaging Technology Magazine

This article first appeared in the December, 2014 issue of Imaging Technology Magazine.

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