Choosing the Right Camera for Thermography Projects

Friday, September 11 2009

Infrared (IR) thermography is an indispensable tool for studying dynamic thermal phenomena. This type of imaging is accomplished with an IR camera (Figure 1) that converts infrared radiation into a visual image depicting temperature variations across an object or scene. In addition, a good IR camera makes accurate (±1°C to ±2°C) non-contact measurements of the object’s temperatures.

Purchasing one of these cameras for scientific research or product development is a significant investment. Several criteria should be considered prior to purchase:

1. What temperatures do you expect to measure?
2. How quickly will you need to capture data?
3. What is the size and distance to your target object?
4. What type of IR detector is best for your application?
5. What kinds of temperature analysis and report generation are required?
6. What camera accessories will you need?
7. What support and training are you likely to need?

Temperature Range and Resolution

Typically, an IR camera is used to characterize target object temperatures. Normally, this involves temperature differences among various locations on the object, or between the target object and its background or another reference temperature. Therefore, temperature range and resolution are important.

Temperature range is defined by the coldest and hottest temperatures on the target object or scene within the camera’s field of view (FoV). For example, a thermographic image of an aircraft idling on the runway might be used to compare the engine to the body of the aircraft. Between the body and engine, the temperature range might be 25°C to 500°C. You would need a camera that could cover at least that range within a single thermographic image.

Temperature resolution is the smallest temperature difference you need to measure and is commonly referred to as temperature sensitivity. IR camera sensitivities can range from about 0.025°C up to 0.1°C, depending on the camera’s detector design.

Data Capture Speed

To determine if an IR camera will meet your speed requirements, consider:

• The motion of your target object
• How quickly your target object heats up or cools down
• IR camera motion

An IR camera’s data acquisition time involves exposure time, frame rate, and total record time. Exposure time is how quickly an IR camera can capture a single frame of data (image), which is analogous to shutter speed on a traditional camera. Exposure time is determined by the integration time of the camera’s A/D converter or the thermal time constant of the camera’s detector.

Data capture speed is also related to a camera’s thermal resolution (sensitivity). For a given target or scene, a camera with higher sensitivity requires less exposure time than one with lower sensitivity. With a shorter exposure time and highly sensitive detector, a camera can provide superior images of cooler objects, and also capture fast movement or temperature changes without image blurring.

The camera frame rate is defined by the number of frames (images) the camera can acquire over a one-second time period. Frame rate should not be confused with the camera’s video display rate, which is the rate at which the camera displays frames to a video monitor. IR camera frame rates can vary from a few frames per second to thousands of frames per second.