Section No. 3

Speed and frame rate

1Hz 30Hz 1000Hz how fast do you need

Cooled cameras have much higher imaging speeds than uncooled ones. High-speed thermal imaging allows microsecond exposure times that stop the apparent motion of dynamic scenes and permit capturing frame rates exceeding 29,000 frames per second. Applications include thermal and dynamic analysis of jet engine turbine blades, automotive tire or airbag inspection, supersonic projectiles, and explosions to name a few. Cooled cameras have very fast response times and they make use of a global shutter.

This means that they will read out all pixels at the same time, as opposed to reading them out line by line, which is the case with uncooled cameras. This allows cooled cameras to capture images and take measurements on moving objects without image blurring.

The IR images in opposite compare the capture results of a car travelling at 70mph. The lower image was taken with a cooled thermal imaging camera. One would think the tire is not spinning, but this is the result of a very fast capture rate of the cooled camera that has stopped the motion of the tire. The uncooled camera capture rate is simply too slow to capture the rotating tire causing the wheel spokes to appear transparent and blurred (see upper image). You cannot accurately measure temperature on blurred images.

What we see

Spatial Resolution

Cooled cameras typically have greater magnification capabilities than uncooled cameras, because they sense shorter infrared wavelengths. Because cooled cameras have greater sensitivity characteristics, lenses with more optical elements or thicker elements can be used without degrading the signal to noise ratio, allowing for better magnification performance. The thermal images in Figure 2 compare the best close-up magnification that can be achieved with a cooled and uncooled camera system. The image on the left was taken with a 4× close-up lens and 15μm pitch cooled camera combination resulting in a 3.5μm spot size. The image on the right was taken with a 1× close-up lens and 25μm pitch uncooled sensor resulting in a 25μm spot size.

Sensitivity

how much detail do you need?

It is often difficult to fully appreciate the value offered by the improved sensitivity of cooled thermal cameras. How do you get a feeling of the benefit from a 50mK sensitivity uncooled thermal camera in comparison to a 20mK sensitivity cooled thermal camera?

To help illustrate this benefit we ran a quick sensitivity experiment (See Figure 3). For this comparison we put our hand on a wall for a brief few seconds to create a thermal handprint. The first two images show the handprint immediately after the hand was removed. And the second set of images shows the thermal handprint’s signature after two minutes.

The cooled camera can still see most of the thermal signature of the handprint, whereas the uncooled cameras only show the partial remains of the handprint. The cooled camera clearly can detect smaller temperature differences and for longer durations than the uncooled camera. This means the cooled camera will provide better detail on your target and help you detect even the faintest of thermal anomalies.

Spectral filtering

what would or wouldn't you like to see?

One of the great advantages of cooled thermal cameras is the ability to easily perform spectral filtering in order to uncover details and take measurements that otherwise would be unachievable with uncooled thermal cameras. In the first example shown in Figure 4, we are using a spectral filter, either placed in a filter holder behind the lens or built into the dewar detector assembly, in order to image through flame. The end user wanted to measure and characterize the combustion of coal particles within a flame. Using a “see through flame” spectral infrared filter, we filtered the cooled camera to a spectral waveband where the flame was transmissive and therefore we were able to image the coal particle combustion. The first image is without the flame filter and all we see is the flame itself. The second is with the flame filter and we can clearly see the combustion of coal particles.

Sync or Trigger

Start Stop how and when...

Precise camera synchronization and triggering makes the cameras ideal for high-speed, high sensitivity applications. Working in snapshot mode the FLIR A6750sc is able to register all pixels from a thermal event simultaneously. This is particularly important when monitoring fast moving objects where a standard uncooled thermal infrared camera would suffer from image blur. The images in Figure 5 are a good example. In this example, we dropped a coin and had a sensor trigger the camera to take an image. Two drops of the same coin triggered the camera at the same time, giving you the object in the same position each time as well. With an uncooled microbolometer camera, you would not catch the coin at all due to the inability to trigger that type of detector and if you did by luck it would be blurred.