Thermal sensors: understanding and assessing the selection criteria for a cooled detector

To understand and assess the selection criteria for a thermal detector, simply take a look at the product's technical literature. In this article, we are going to focus our attention on cooled thermal detectors.

A cooled thermal camera features an imaging sensor combined with a cryocooler, which lowers the sensor temperature to cryogenic levels. Driving down the sensor temperature is a prerequisite for reducing thermally-induced noise to a lower level than the thermal noise carried in the signal from the scene in front of the camera.

This article provides an overview of the typical structure of a datasheet. You can then use that knowledge to effectively pick out the relevant information on a datasheet and gain a clearer insight into the different criteria for comparing thermal detectors.

But avoid falling into the trap of thinking that the datasheet is the be-all and end-all. Although it will give you the lowdown on the different performance levels, specifications and options, it will tend to gloss over the component's inner workings and how it can actually be used for a given application. In other words, when it comes to making the final choice, you will need to drop the vendor a line for guidance on which model offers the best fit for your specific requirements.

The structure

Most of the time, datasheets follow a fairly generic template. The header contains the part number and a summary of the main characteristics. Then you will see a brief description of the product, which can give you an idea as to whether the product is likely to tick your boxes. The description also touches on the different applications where the component can be used, which represents another invaluable source of information. The basic description is often followed by a list of the product's strengths and main features.

Subsequently, the datasheet moves onto the product's technical performance. The stated figures are typical values and cannot necessarily all be achieved at the same time, so they should be interpreted as reflecting just one specific context.  Remember that there will always be a trade-off between the intended application and the product's performance levels. However, the intel in this section can help you cast a critical eye over the infrared sensors on your shortlist.

The key criteria

1. Performance criteria:

• NETD

NETD stands for Noise Equivalent Thermal Difference. It is expressed in mK (or thousandths of a °C). It refers to the smallest temperature difference that a detector is capable of resolving. It corresponds to a signal-to-noise ratio (S/N) of 1. In other words, when the temperature difference is equal to the NETD, the variation in the detector's output signal corresponds to its noise. The lower the NETD, the more sensitive the detector.

Therefore, the NETD can be thought of as the infrared equivalent to contrast in visible light imaging. It will play a mission-critical role in scenes featuring a low level of thermal contrast, i.e. where all the objects are roughly at the same temperature, like a landscape.

Note that there are no standards or universally accepted methods for measuring the NETD. Make sure that the datasheet specifies the measurement method used.

For example, the NETD for LYNRED's cooled components tends to be measured in the mid-infrared region in front of a black body at 300 K.

• Storable charge 

The storable charge defines the gain of the readout circuit or the size of the integration capacitance in farads (i.e. the ability to convert photodiode current into voltage). Therefore, the lower the integration capacitance, the higher the readout circuit's gain, since:

 v = (i/c) x t where v = voltage    i = current    t = integration time

 i = c x dv/dt where i = current   C = capacitance   dv = change in voltage  dt = change in time

This has a direct influence on the detector's response in volts/watts for a given integration time and flow. Some detectors offer a choice between different storable charges that can be configured during operation. Standard practice is to achieve a charge of approximately 50% for a given scene flow by fine-tuning the storable charge and integration time.

• Frame rate 

The frame rate depends on the integration time and the circuit's master clock rate (meaning the read speed of the readout circuit: the higher the circuit's clock rate, the faster the pixel readout speed), both of which can be adjusted within certain limits, namely the maximum clock speed and the minimum integration time for accommodating the scene (refer to the storable charge). The type of application governs the required frame rate. The rate tends to be less than 50 Hz for visual surveillance applications and above 100 Hz for complex systems with embedded processing capabilities.

• Spectral band 

Cooled detectors allow for imaging in different spectral bands with low atmospheric absorption levels. A distinction is typically made between three IR bands:

• Short-wavelength (SW): 1 to 2 µm
• Mid-wavelength (MW): 3 to 5 µm
• Long-wavelength (LW): 8 to 12 µm

The technology and spectral filters incorporated into the detectors determine their spectral response band. The type of application and the project constraints will generally dictate the spectral band.

To learn more about cooled thermal detectors, download the free white paper here.

• Resolution

Once again, this criterion should be assessed against the intended application, since resolution determines the quality of the image produced. A higher sensor resolution equates to clearer and sharper points in the image. Higher resolutions can be used to measure small objects from further away.

2. Design size criteria:

• Consumption:

The amount of power drawn is mainly defined by the energy use of the detector's cooling system and the detector's operating temperature, which is the recommended temperature.

The operating temperature depends on the technology and the spectral band, but should not be confused with the ambient temperature. It refers to the cryogenic operating temperature of the focal plane. This temperature lies between 80 and 110 K. The further away the spectral band, the greater the need for cooling:

• SWIR band = ambient temperature
• MWIR band = between 80 and 150 K
• LWIR band = between 80 and 70/75 K (for superior performance)

 This is an important criterion for all battery-powered applications.

• Weight / size

This ratio can be a deal-breaker in terms of the detector's compatibility with the final system.

• Pixel pitch 

This parameter has a significant effect on the detector's optical performance and dimensions.

• Additional criterion: check whether the detector will be subject to a specific temperature range and prone to such mechanical constraints as impacts and vibrations. It is important to check that these environmental conditions are compatible with the intended application. Conditions can either be static (e.g. for surveillance systems or binoculars), aircraft-mounted or embedded in armored vehicles (where conditions are much harsher).

Datasheets represent an invaluable source of intel for analyzing the different solutions available and shortlisting your potential infrared detectors. You will find all the information needed to determine whether the component is capable of fulfilling your specific requirements.  Feel free to contact the vendor to take your search to the next level and hone your project brief. When choosing a thermal detector, always strive to achieve the best trade-off between the solution's performance and the requirements of the application.

To learn more about cooled thermal detectors, download the free white paper here.