Thermal detectors: How to read an infrared detector data sheet

The first step in your search for an infrared detector is usually to take a look at the product’s technical documentation. The product data sheet is a good place to start, as it provides a summary of a component’s main features and characteristics. If you don’t know the technical vocabulary and other conventions around infrared detectors, you risk missing important details.

Read on to find out how product data sheets are usually organized so that you can go right to the information you need and get a full understanding of what criteria to compare and how.

First, you should be aware that a data sheet cannot be all or do all. The role of the product data sheet is simply to provide an overview of a component’s characteristics, performance, and available optional features. However, the data sheet generally does not explain how the component works or how it is used in specific situations or applications. Therefore, once you have read the data sheet, you will need to reach out to the manufacturer to request further guidance on the best product for your unique needs so that you can make the best choice.

What to look for

Most product data sheets are organized in pretty much the same way: The product name and number are at the top of the page, followed by a summary of the main characteristics. Next you will often find a brief description of the product. Some product data sheets will also provide a recap of the main applications for the product. You will then often see a list of the product’s main features and benefits.

Finally, the product’s technical performance will be outlined. This is the “meat” of the data sheet. You will find the optimal values for the each of the different criteria, but not all may be possible at once. Therefore, you have to be able to interpret the data for a specific situation to know how the product will actually perform.  In other words, performance is always a compromise in terms of which criteria are most important for a given application. Nevertheless, the data sheet is still a good place to start and will allow you to begin to compare a shortlist of infrared detectors.

The main criteria

1. Performance:

• NETD

NETD stands for Noise Equivalent Thermal Difference. It is expressed in mK (millikelvin). It is the smallest temperature difference a detector can perceive. It corresponds to a signal-to-noise ratio of 1. In other words, the variation in the detector’s output signal for a temperature difference equal to the NETD corresponds to the detector’s noise level. The lower the NETD, the more sensitive the detector is.

NETD is analogous to contrast for visible imaging techniques. NETD is particularly important for scenes with low thermal contrast (scenes where all of the objects are pretty much at the same temperature, such as landscapes, for example).

However, it is important to note that there is currently no standard for measuring NETD.  So, you should check to ensure that the data sheet explains how the NETD measurement was taken. For example, LYNRED product NETDs are measured as follows: f/1 (lens aperture), 300 K (ambient temperature), 30 Hz (frame rate).

• Thermal time constant  

The thermal time constant indicates the pixel’s capacity to respond to the scene. This can be important depending on the target application. For machine vision or missiles, for instance, the objects in the scene will be moving rapidly, so responsiveness is crucial. The thermal time constant is also related to the pixel shape and design, which also influences NETD and sensitivity (the detector’s capacity to receive thermal energy). It is not generally included in the data sheet. Ask the manufacturer.

• Scene dynamics

This is another characteristic that must be considered in the context of the target environment (temperature, humidity, etc.). It is also closely related to NETD.

These three criteria are interrelated. Therefore, your choice will always be a compromise between the three.

FOM (figure of merit) is one way you can compare detectors. It is presented as a diagram comparing the NETD and thermal time constant. Remember: NETD alone is not enough to make your choice.

• Operating temperature

Your desired operating temperature will also depend on your target application. Defense applications will usually require an operating temperature range of –40 °C  to 85 °C. For hunting, however, a range of –20°C  to 60°C is sufficient. The wider the range, the more constraints it puts on the system.

• Resolution

Resolution should also be considered in light of the target application, because resolution is what ultimately determines the quality of the image obtained. The higher the resolution, the sharper the image. Higher resolutions can measure smaller objects at longer ranges.

Thermal imaging technology has the capacity to drive new product innovations. Download our free case study to see how. 

2. Form factor:

• Energy consumption

This is an indicator of a system’s battery life. It is important for portable devices (binoculars and thermal cameras for firefighting, military, and thermography applications).

• Weight/size

This is important in terms of integration into the target system.

• Pixel pitch

Pixel pitch has a significant impact on optics and form factor.

Data sheets are vital sources of information to help you effectively analyze and pre-select a shortlist of potential infrared detectors for your project. They include all of the information you need to determine whether or not a component is right for your needs.  Get in touch with the manufacturer for more detailed information than what is on the data sheet. Your thermal detector choice will be based on the best compromise between performance and the requirements of your target application.

Thermal imaging technology has the capacity to drive new product innovations. Download our free case study to see how. We’ll show you how LYNRED Customer Support helped one customer integrate infrared technology into a new product!