Thermal or Infrared Camera

Infrared and thermal cameras are often mentioned together, but they have distinct roles and capabilities. In the broadest sense, thermal cameras are a specific subset of infrared cameras. All thermal cameras are infrared cameras, but not all infrared cameras are capable of measuring temperatures, as thermal cameras do.

An infrared camera can refer to any camera that detects infrared radiation, including devices for night vision or specialized machine vision. A thermal camera, also called an infrared thermographic camera, is an infrared device focused on sensing surface temperatures and providing compensated and calibrated temperature information.

At Optris, both the terms infrared camera and thermal camera are often used interchangeably, as the distinction between them is not rigid in everyday usage. Since all Optris cameras are radiometric and operate in the infrared range, the practical separation between the two terms is less critical, and both are commonly applied to the same products.

Thermal cameras are engineered to directly measure emitted infrared energy and translate it into a temperature-map image. A thermal camera senses the heat signatures of everything in its field of view and produces a thermogram, which is an image where the pixel intensity or color indicates the object’s temperature. These thermal cameras are working radiometrically.

A radiometric thermal camera has undergone calibration against known temperature standards, taking into account the physics of thermal emission, sensor response, and optics transmission. This allows the output image to carry absolute temperature information for every pixel.

Unlike general infrared cameras, a thermal camera provides temperature values instead of just brightness levels. Each pixel in the thermal image corresponds to a real, measurable surface temperature. The data can be used for quantitative analysis, process control, and automated decision-making, in addition to visual inspection. Optris thermal cameras are described as radiometric because they are calibrated instruments for temperature measurement, not just for infrared visualization.

Because of their differing purposes, infrared and thermal cameras use different sensor technologies. A typical infrared camera for near-IR might use a silicon-based sensor or an image intensifier tube to capture IR photons just beyond visible light. These sensors treat IR much like visible light, and the output is an image where brightness corresponds to the amount of IR light hitting each pixel.

A thermal camera, on the other hand, typically employs a microbolometer or other thermal detector array instead of a standard photo-sensor. Microbolometers are tiny thermal sensors, often made of materials like vanadium oxide or amorphous silicon, that change their electrical resistance when heated by incoming infrared radiation. Each pixel in the sensor array absorbs the IR energy from the scene and warms up by an amount proportional to the incident infrared flux. The thermal camera reads out these changes as electrical signals for each pixel. As the thermal detectors produce a signal even without incoming radiation, each pixel has its dark value, forming an offset that must be subtracted from measurements. Therefore, A thermal camera performs periodic self-calibrations, known as a non-uniformity correction, to maintain accuracy across its detector. To do so, a shutter flag, blackened to act like a black body, periodically covers the detector to record the current dark frame. These values are then subtracted from subsequent images. During warm-up, chip temperature drift requires frequent flag cycles to maintain accuracy.

Thermal cameras require radiometric calibration to accurately translate sensor signals into temperature readings. They keep track of the intensity of infrared radiation captured in each pixel and use internal models and calibration data to infer the true temperature of the object at that pixel. A non-radiometric infrared camera may produce an image that looks similar, but its pixel intensities have no fixed relationship to true temperature. They represent only relative brightness levels in the infrared spectrum.

Thermal cameras visualize and measure heat, whereas other IR cameras only visualize light in the IR spectrum.

Practical Application Example to differentiate Thermal Cameras from pure Vision Infrared Cameras

In practice, all thermal cameras are infrared cameras, but they form a special category designed for radiometric thermography. One could call many devices “infrared cameras,” from night-vision goggles to IR-sensitive machine vision cameras, but those might not be thermographic.

For instance, a security IR camera at night might flood the area with near-IR light and capture the reflected IR to produce a clear image in darkness. Non-thermal infrared cameras can produce images with fine detail of objects, but those images don’t inherently tell you the objects’ temperatures, as the cameras don’t undergo radiometric calibration and might not have a built-in shutter for offset corrections.

To illustrate, consider an industrial example: suppose we have a conveyor oven in a factory and we need to ensure products are heated to 200°C. A thermal camera installed at the oven exit can directly measure the temperature of each item coming out, showing a thermal image with the items glowing brightly if they’re at the target temperature, and even alarm if any item is underheated. A regular IR camera, even if it could see the items in the dark, would only give a grayscale or false-color view based on IR contrast. It might show the items glowing if they’re red-hot visible, but at 200°C they might not visibly glow. While the thermal camera provides measured data enabling process control, a generic IR camera would provide only a visual check.

Choosing Between Thermal and Infrared Cameras

The choice between an infrared camera and a thermal camera in an engineering context boils down to the information required.

Use an infrared (non-thermal) camera when visual imaging in IR is sufficient or desired. If the goal is to see an object in darkness or through certain optical filters, or to perform a visual inspection under IR illumination, then a standard IR camera is appropriate. These cameras are generally cheaper than thermal cameras and can provide higher image resolution and clarity for identifying objects. In industrial automation, IR cameras are sometimes used alongside visible cameras to provide additional contrast for enhanced visual analysis.

Use a thermal camera when actual temperature data is needed. If the task involves monitoring or controlling temperature, such as keeping a process within a thermal tolerance, protecting equipment from overheating, or detecting thermal anomalies, a calibrated thermal imaging camera is indispensable.