Part 2: How fast do you need to capture the data?
When answering this question there are three things to consider:
- Exposure time
- Frame rate
- and total record time.
Exposure time is how quickly an IR camera can capture a single frame of data, which is similar to shutter speed on a traditional visible-light camera. The exposure time for IR cameras is referred to as integration time, or the detector’s thermal time constant. These two terms simply refer to the amount of time it takes to capture a single thermal image.
Let’s explore the analogue of the IR camera’s exposure time, i.e., a traditional camera’s exposure time with respect to the advantages of longer and shorter exposures. For both cameras, the shorter the exposure time, the less likely there will be blurring for high-speed events. However, since the exposure is shorter there is less time for the cameras to image the target; so you may be under-exposed. On the other hand, longer exposure times allow you to collect more light (for the traditional camera) or heat energy (for the IR camera) from your object of interest. The drawback, of course, is that if your target is moving fast you may see blurring.
So there is a trade-off between short and long exposure times. But if you recall from seeing Table 1, some cameras have better thermal resolution and are therefore more sensitive. We can deduce that it requires less exposure time for the higher-sensitivity cameras to get the same image as the lower-sensitivity cameras when looking at the same thermal target. For cameras that have these better thermal-resolution detectors, you get the best of both worlds—good imagery of cooler objects and no motion blur.
To determine if a particular IR camera will meet the speed requirement of your application you will need to consider:
- The motion of your target object.
- How quickly/slowly your target object will heat up or cool down.
- The motion of the IR camera.
Frame Rate (Frames/Second)
A camera system’s frame rate describes how many thermal images per second you can collect from the IR camera. IR camera systems with fast frame rates allow you to capture the thermal signatures of fastmoving targets, such as ballistic projectiles or explosion scenes. If data acquisition is fast enough, it’s even possible to capture a sequence and play it back in slow motion. So, the higher the camera’s frame rate, the better the results for dynamically changing targets.
As you would imagine, shorter exposure times allow for faster frame rates. Thermal cameras have frame rates that vary from a few frames per second to thousands of frames per second. Here is Table 1 again with an added column showing frame rates and exposure times:
Table 2: Frame rates and exposure times of common IR cameras
Total Record Time
Do you plan on capturing data at high speed for long periods, high speed for short bursts of data, or data log at slow rates for hours? There are as many data recording options as there are cameras, so all data-collection scenarios should be explored to determine the type of IR recording system you will need.
It is important to know the frame rate and total record time you require to select a camera and data system that will work best for your application. Certain IR cameras, such as FLIR’s handheld T-series, have built-in storage capabilities where they can record to internal flash memory or a removable compact SD card. Other cameras, such as the FLIR X6900sc, stream highspeed thermal data over Gigabit Ethernet, CameraLink, or CoaxPress to a PC or laptop for recording. FLIR’s high-speed X-Series cameras give you the capability to perform burst recording to on-camera RAM with direct playback and data storage to a removable solid-state drive (SSD). For high-speed, extended-length recording there are solutions where data is streamed to a RAID array of disks that can handle fast frame rates and feature large amounts of disk space. If data security is a concern, the storage media is removable on both the camera and high-speed data recorder. Simply take it out and store it in a safe place.