A thermal imager measures infrared (IR) radiation for determining temperature. Every object emits electromagnetic waves in the infrared region. The amount of energy emitted depends on the temperature and the material properties of the object. The ability of any material to emit infrared waves is called the emissivity. Perfect emissivity is "unity" (1.0) and is called a "blackbody." Most materials have an emissivity of less than 1.0.

To make an accurate temperature measurement, the emissivity of the object has to be known. It is a variable that has to be entered into most measuring instruments.

The most common infrared radiation is heat which the skin can feel but the eye cannot see. A sensor of infrared radiation, called a microbolometer, came into commercial use about 20 years ago. It had been developed by the military and is standard in today's thermal imagers for temperature detection. The microbolometer is an array of IR sensitive pixels similar to a digital camera. The difference is size. The pixels in the microbolometer chip are much larger. The most common sensors are 320 x 240 pixels and 160 x 120 pixels. Sensors with fewer pixels are common in low cost imagers.

The image is displayed through a viewfinder or an LCD screen at a frame rate of 30 - 60 Hz similar to a TV signal for most models. Also, all models have image memory for PC downloading. The memory may be JPEG format and transferred on a flash memory card or by USB or Ethernet cable.

Infrared is the region beyond red light, approx 0.75u (micron) to more than 15u wave length. This is the spectral range for most instruments. The thermal sensitivity for microbolometer arrays ranges from 0.05 to 0.08 C at 30C and 30Hz. This is a reference given for comparison of different models. The temperature range typically ranges from -40 C to 600 C. A reading range usually is about 150 to 200 C. Many models come with 2 or 3 temperature ranges standard. Some models have optional temperature ranges up to 2000 C. The reading accuracy for most microbolometer imagers is +/- 2 C or +/- 2% of reading.

The field of view (FOV) with standard optics is in the 23 - 27 degrees x 17 - 20 degrees range. The focus ranges from 12" to infinity and can be manual or automatic. A laser pointer is standard on many models.

All models are battery powered with rechargeable batteries. Battery life varies from 2-1/2 to 5 hours. Some models also operate with an AC adapter.