Thermography is an imaging method which renders visible the thermal radiation (mid-infrared to far infrared) of an object or body that is not visible to the human eye. In thermography, temperature distributions on surfaces and objects are acquired and displayed. Thermography is a non-contact measurement method.
Images that are generated by infrared cameras are initially gray scale. Current camera models are capable of displaying up to 256 (8 bit) gradients of gray. However, it is impossible for a human observer to resolve such fine gray gradients with certainty. Therefore, it is useful to generate images in a false-color mode, of which almost all thermographic cameras are capable. The eye is better able to distinguish color differences than brightness differences. The intensity of the thermal radiation in the image that is colored in this way is represented by the hue and not by different gray values. Different color palettes are commonly available for coloring the grayscale images. Often, the brightest—warmest—part of the image is shown in white, intermediate temperatures are shown in shades of yellow and red, and the dark, i.e., colder, parts of the image are shown in shades of blue.
Thermographic cameras may be divided into two types: systems with cooled infrared image detectors and those with uncooled detectors.
Cooled infrared detectors usually comprise an array of photoreceivers which operate on the principle of inner photoelectric effect. The detectors are commonly accommodated in a vacuum-sealed housing and are cooled cryogenically. Accordingly, as a rule, the detectors are much colder than the objects to be observed so that there is an important increase in the thermal sensitivity (temperature resolution) of the thermographic system compared to uncooled systems. One disadvantage of this method is that if the cooling of the detector fails, the thermographic system is “blind.” Other drawbacks of cooled systems include the higher acquisition costs and operating costs and the occasional long start-up periods before the system has cooled the detector down to operating temperature. On the other hand, the image quality is excellent compared to uncooled systems.
Uncooled thermographic cameras use detectors operating at ambient temperature. All modern uncooled systems operate on the principle of change in resistance, voltage, or current intensity when the detector is heated by infrared radiation. These changes are measured and compared with the values at operating temperature. The amount of received radiation is determined from this and a temperature is calculated with the aid of a preadjusted emission factor.
Uncooled infrared sensors are kept at constant temperature by thermoelectric Peltier coolers (TECs) to reduce signal drift of the receiver elements. There are also thermally unstabilized (TEC-less) detectors. Systems of this kind make do without expensive, cumbersome cooling devices. Therefore, these thermographic systems are appreciably smaller and less expensive than cooled systems. However, they deliver comparatively poorer results.
Uncooled detectors use pyroelectric or microbolometer arrays. The detector cell of a microbolometer array comprises a microbridge structure, a thermally insulated plate having a thickness of <1 μm which is suspended over the substrate by two electric contacts. The plate is made of a material with a highly temperature-dependent resistance (for example, vanadium oxide). The incident infrared radiation is absorbed and leads to an increase in temperature of the plate which in turn changes the resistance. The measured voltage drop is outputted as a measurement signal.
Pyroelectric sensors on the other hand deliver a voltage with a very high source impedance only when there is a change in temperature.
Both microbolometer arrays and pyrometric sensors require a mechanical chopper or at least an intermittent shading of the image sensor. In pyrometric sensors, the reason for this is that they can only respond to temperature changes. In bolometer arrays, the chopper or shutter is used for obtaining a dark image which is subtracted from the recorded image pixel for pixel as a sensor-specific reference (each pixel has a distinctly different resistance). This is usually done by means of a two-point correction (DE 698 30 731 T2), i.e., a correction by means of an offset and a slope, that is, a linear approximation. Also, the presentation of the image data is then carried out as a rule either directly or by means of a linear transfer characteristic (by means of an offset and a slope).
However, a linear signal display requires a compromise between resolution and dynamics. With a steep characteristic, cold objects are capable of good thermal resolution, but high object temperatures quickly lead to saturation. Conversely, with a flat characteristic that is optimized for high temperatures, details can no longer be perceived.