We love the warm rays of the summer sun, enjoy cooling off with a cold drink or a refreshing dip in the pool. We appreciate the warm touch of another person, enjoy a hot tea or warming fireplace in winter. We feel temperature, but we cannot ‘see it’ with our eyes.

Thermal imagers make visible this part of the electromagnetic spectrum that is hidden from our eyes. Since virtually every object in our environment emits thermal radiation, thermal imagers can produce an image of the surroundings even in absolute darkness. When used for security & surveillance, little remains hidden from a thermal imager.

With full radiometric calibration, these imagers can not only make temperature differences visible, but provide an absolute temperature measurement. A thermal imager with a VGA sensor and 60 Hz frame rate, can give more than 18.4 million individual temperature measurements per second.

Miniaturization and Performance Optimization

Producing a straightforward thermal imager that visualizes heat (i.e. detects and translates the relative heat distribution of a scene under consideration into a displayed image) is no longer state-of-the-art. You can accessorize your iPhone with a thermal imager add-on for less than $200. The real technical challenge begins with increasing demands on sensitivity and measurement accuracy – displaying the smallest temperature differences or measuring temperatures as precisely as possible – while simultaneously miniaturizing the thermal imager and still meeting the performance needs of the broader market.

Figure 1. Infrastructure security application thermal image.

Today’s leading manufacturers of thermographic imagers combine both the lowest NETD (Noise Equivalent Temperature Difference) and the best temperature measurement accuracy in a single device. Historically, these high-performance cameras were sensitive and accurate - but also large and heavy. This is where the trends of miniaturization and system integration are opening new applications for thermal imagers.

More and more applications are using thermal imagers, linking them with other sensors, such as color video cameras or LiDAR sensors. The applications are diverse and range from intelligent surveillance cameras for public and private security to building automation; from manufacturing process control to aids for firefighting and rescue services; from night vision solutions for autonomous vehicles to predictive maintenance.

Key Performance Parameters

Figure 2. Radiometric temperature measurement application thermal image.

As diverse as these applications are, so too are the requirements for thermal imager technology miniaturization and system integration – measurement characteristics, detectors, optics, data interfaces, housings and other design and manufacturing parameters must be optimized for the specific application. However, easy integration into application-specific system solutions is a particular challenge for many OEMs and system integrators, as powerful thermal imagers are often only offered as standard products with limited configurability.

The most effective thermal imagers are tailored to overcome these challenges and fill exactly this gap in the market. Three areas where thermal imager performance and ease of integration are impacted are in the detector, imager radiometric capabilities, and the optical & electrical features that are offered.

Advanced detector technology is constantly reducing the pixel to pixel spacing. This trend has been largely driven by the cellphone industry and has been most evident in visible cameras but has helped improve thermal imagers as well. The smaller the detector pixel pitch – while maintaining or improving sensitivity (NETD) and noise performance – the more effective a thermal imager is. Smaller pitch generally supports lower power, smaller mechanical design constraints, and can usually accommodate slower f/# optics (further system size and weight savings). Additionally, smaller pixel pitch FPAs also allow for higher resolution thermal imagers without a significant penalty in size, weight, and power.

Figure 3. Representative thermal Imager: The Jenoptik EVIDIR alpha 320 with 14 mm lens.

Radiometric performance properties include expansion from Region of Interest (ROI) only measurements to full radiometric measurements on every pixel of the FPA, enhanced temperature measurement accuracy, and extended target temperature ranges.

Extreme market competition in the gaming, virtual and augmented reality, smartphone, telecom, and autonomous vehicle industries has dramatically increased electronic processing capabilities while driving down component footprint and power consumption. As a result, thermal imager designers have a wider array of electronic component options to choose from that did not exist several years ago. The broader variety of component options also facilitates the design and integration of more compact, more efficient, and higher-performing systems. Meanwhile, optical and opto-mechanical fabrication techniques have also advanced, which allows designers to replace conventional optics with nontraditional optics to further reduce the size and weight of optical subassemblies. These broader industry trends, coupled with continued detector and radiometric advances within the thermal imaging segment, can offer OEMs and system integrators a wide range of flexibility when the right thermal imager supports these options.

Radiometric Thermal Imager Market Sample

Table 1. Representative samples of commercial radiometric thermal imagers

Table 1 summarizes a number of key performance parameters for five current radiometric thermal imagers with a QVGA pixel format.

To illustrate some of the key performance parameters identified in the previous section, we focus on the first entry in the table, the JENOPTIK EVIDIR. EVIDIR alpha is a family of compact, “plug and play”, infrared imagers that offer a variety of standard configuration options such as video format (VGA or QVGA), frame rate, mechanical shutter or long-term stable shutter-less operation, multiple standard communication interfaces (USB, GigE, and CMOS) and lens options to allow imager fields of view from 5 degrees to 60 degrees.

The imagers are fully functional as stand-alone devices, but they have been designed to facilitate integration into OEM (original equipment manufacturer) applications. EVIDIR’s “toolbox” approach of available configuration options is employed for easy customization. Options such as lenses, shutter, video format, and communication interface are modular, allowing the customer to mix and match to best suit their application.

The QVGA and VGA versions of both the imager and radiometric imagers share common mechanical, optical, electrical and command interfaces to allow for easy upgradeability and to minimize OEM development time. The imagers are built around state-of-the art, 12 μm pixel, uncooled micro-bolometer focal plane arrays and stable, uniform, thermal images with better than 30 mK NETD standard. The imager core is a two-board stack consisting of the focal plane array (integrated into a printed circuit assembly) and processor board.

The native output format of the imager is CMOS and OEM integrators can utilize this format to minimize size and power or select an interface board to convert the output to the desired standard image formats (e.g. USB or GigE). CMOS versions of the imager draw <950mW for a QVGA imager running at 60Hz. The imager aluminum housing assembly measures 20mm × 30mm × 30mm and weighs 27g without lens. There are various lens adapters to allow a multitude of off-the-shelf lenses to be utilized to best match the application. Radiometric cameras get an enhanced calibration but are otherwise identical to the imager versions.

Summary

Today’s state-of-the-art thermal imagers offer OEMs a higher performance, lower cost, alternative to prior QVGA and VGA thermal and radiometric cameras on the market. When designed appropriately, thermal imager customization allows OEM’s to tailor the camera to best suit their application and minimize cost.

Continued industry enhancements will provide customers with exactly the right product for their needs including SXGA (1280 × 1024) sensors with 12μm pitch, enhanced XGA (1024 × 768) sensor and VGA (640 × 480) technology, resolution enhancement, sub-frames, digital electronic zoom (e-zoom) and electronic image stabilization, shutter or shutterless operation, additional selectable frame rates, and optional electrical interfaces such as CameraLink.

This article was written by Dr. Daniel Brenner, MBA, Light & Optics Division, JENOPTIK Optical Systems GmbH (Jena, Germany). For more information, click here .



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Photonics & Imaging Technology Magazine

This article first appeared in the November, 2021 issue of Photonics & Imaging Technology Magazine.

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