Electromagnetic radiation covers a whole family of natural phenomena with the common characteristics of traveling with the speed of light. Radio waves, light, X-rays, and cosmic radiation are all part of this family, characterized by wavelengths with a different spectrum range. Light waves occupy only a tiny part of the overall range, bordered on either side by bands of invisible light: infrared with lower wavelengths than visible light, and ultraviolet with higher wavelengths than visible light. The infrared, with a spectrum range from about 1 micrometer to 1000 micrometer wavelength (one-millionth of a meter to one thousandth of a meter) is the type of radiation produced by heat.
Specifically, for measurement purposes, this spectrum is directed into short-wavelength infrared, with wavelengths from 1 micrometer to 5 micrometer, and long-wavelength infrared, with wavelengths from 5 micrometer to 15 micrometer. The reason for choosing these two ranges is that with infrared wavelengths greater than about 15 micrometer the attenuation produced by passage through air becomes increasingly evident. The short wave areas and the long-wave areas, on the other hand, provide minimal attenuation of infrared and are known as atmospheric windows. At even longer wavelengths — around 1000 micrometer or 1 mm which represents the extreme of the infrared spectrum (bordering on microwaves)—atmospheric attenuation again decreases. In fact, infrared is not itself a transducer. it's electromagnetic radiation. Transducers associated with infrared techniques are infrared detectors, which provide thermal images for temperature measurement and heat detection within industrial and medical fields. In its simplest form, such a detector, or true transducer, is a device, such as a photo sensitive diode, that generates an electric current when exposed to infrared radiation. This current is dependent on the intensity and wavelengths of the radiation and can be used in many different ways to produce an infrared picture or thermal picture. The current produced by such a detector is very small and needs to be amplified and processed. Amplification increases the strength of the signal. Processing reduces the possibility of interference due to external sources and internal sources, such as electrical noise. This particular requirement is complicated by the fact that semiconductor materials (say diodes) used as transducers always generate a certain amount of electrical noise, which is dependent on the temperature of the material. This is known as thermal agitation noise. Mounting the detector in a flask of liquid nitrogen at a temperature of -196 deg. C. will reduce this noise to a minimum. At this temperature, thermal agitation is very low. A further ad vantage of cooling is that it increases the cutoff range of the detector; that is, the wavelength range outside which the detector no longer responds to the radiation. BLACKBODY CONCEPT A perfect blackbody is an object that absorbs all incident radiation striking it and emits all of this energy in the form of transmitted radiation. No objects are perfect blackbodies, but the behavior of most objects can be related to perfect blackbody performance in terms of strength of radiation and wavelengths transmitted. In the case of a body being treated, the radiation from a blackbody—or spectral radiation emittance, as it's called—takes the form of clearly defined curves, as in ill. 16-1. These curves define the broad wavelength range over which radiation is emitted, which is also characterized by a distinct maximum peak. The actual shape of a curve and the peak value are dependent on temperature. In creasing temperature rapidly increases the peak radiation, and at the same time progressively shifts the peak wavelength. Blackbodies radiating at a temperature of 800 to 900 degrees Kelvin (K) emit a very low level of radiation with wavelengths less than 1 m and a peak wavelength of around 4 micrometer. The wavelength range for visible light is about 0.4-0.8 micrometer, so only minimum blackbody radiation appears at a wavelength of 0.8 micrometer. The major part is in the infrared spectrum. In a fully darkened room there would be just enough 0.8 micrometer radiation present to make the blackbody faintly visible as a dull red. ill. 16-1: Blackbody radiation curves. In this example, curve A represents the coolest temperature; curve B represents a warmer temperature; curve C, a still warmer temperature. Note. Blackbody temperatures are always quoted in temperatures to the Kelvin scale and designated as K (not K°) 0 K corresponds to absolute zero temperature, which is -273° C or -459° F. Thus, 0° C. or 32° F. correspond to 273 K; and 100° C. or 212°F correspond to 373 K (from 0 ° C. upwards the K scale corresponds numerically to ° C. + 273). The wavelength for maximum radiation from a blackbody is given by Planck’s law: γ(max) = 3000/T where γ (wavelength) is in micrometers, and T is the temperature in degrees Kelvin. Some interesting quick calculations can be made on this basis. The temperature of the sun, e.g., is 6000 K. Considering the sun as a perfect blackbody gives max (gamma) = 9000/6000 = 0.5 micrometer In other words, this lies towards the lower end of the visible spectrum range (0.4-0.8 micrometer upwards), so the sun appears yellowish in color. Emission in the infrared range (0.8 micrometer upwards) has a much lower intensity but is still readily detectable. Stars cooler than the sun have peak radiation at a longer wavelength. We can, in fact, easily tell the temperature at which they would become largely invisible (that is, with a peak radiation wavelength of, say 1 micrometer). Using the same formula as above, we get 1= 3000/T or T = 3000 K or 2727 deg. C. Such a star would not be seen through an ordinary telescope but would readily be detectable by its infrared radiation. EMISSIVITY The total emitted radiation energy of a blackbody is given by W = sigma T^4 W/m^2 where sigma is the Stefan-Boltzmann constant, 5.67 x 10^-8 No object is a perfect blackbody, but its relative efficiency in this respect can be related to blackbody radiation by its emission factor or emissivity. The law for nonblackbodies then becomes W=5.67 x 10x-8 x ET^4 where E is the emissivity. Emissivity can vary from 0 (a perfectly reflective surface) to 1 (for a perfect blackbody). It will also vary somewhat with temperature. Table 16-1 gives some typical emissivity values for familiar surfaces. Digressing a little, it's interesting to note that the unclothed human body is a near-perfect blackbody as far as heat loss is concerned. At a temperature of 27 deg C. or 30 deg K, e.g., heat loss is W = 5.67 x 10^-8 x 0.98 x 300^4 = 450 W/m^2 Because the surface area of a man’s body is about 2 m^2 this represents a radiated heat loss of nearly 1 kW. GRAY BODIES and COLORED BODIES A body with a constant emission factor (emissivity) regardless of wavelength is called a gray body. A body with an emission factor that varies with the wavelength of the radiation is called a colored body. An example of a colored-body curve, compared with that of a blackbody, is shown in ill. 16-2. Apart from having an emissivity of less than 1 at any wavelength, the principal difference between the two is that a colored body tends to behave like a blackbody at certain wavelengths, but a gray body does not. THERMOGRAPHIC IMAGES A thermographic image or thermogram is an image showing the heat radiation from a particular object at a particular temperature. In a complex object not all parts will be at the same temperature, so a thermogram also shows the heat distribution of the object. This is where a transducer/scanner unit is necessary to compile the thermogram. Table 16-1. Typical Emissivity Values for Surface.
The first requirement is an optical system to focus the radiation on the detector. The detector will only detect the strength of such focused radiation received. Thus, to produce a complete thermal picture the optical system must be flexible in the sense that it must allow the detector to sense the object point by point. In this way a complete thermal picture of the object can be presented, somewhat like a still TV picture. ill. 16-2:. A colored-body radiation curve. There are several methods of making the detector scan the object. The most suitable is usually an optical system using both horizontal and vertical rotating prisms. One prism then deflects vertically and the other horizontally, scanning the object with a pattern like that shown in ill. 16-3. In the AGA scanner a horizontally deflecting prism rotates in the scanner at a speed of more than 18,000 rpm, a vertically deflecting prism at 180 rpm. Therefore, the detector constantly monitors different points, and the electrical current from the detector varies in strength with the radiation of the object at every point. This variable current is used to produce a picture in a gray scale where the most intensely radiating points are the brightest (hot test areas). The pictures built up comprise 70-line fields. The scanning velocity is 25 fields per second. In order to prevent the lines in the screen from being too apparent, four fields in succession are displaced in such a way that they will form an interlaced frame. The interlacing of four fields to one frame gives a frame frequency of 6 1/4 complete frames per second. ill. 16-3. Scanning pattern for infrared imaging. (The vertical scale is greatly exaggerated.) Resolution The resolution, or, more correctly, the geometrical resolution, indicates how distinctly small details can be reproduced. The geometrical resolution is measured in milliradians (mrad), which is an angle measurement. The resolution is dependent on the size of the detector and the optical arrangement in the scanner. A lens with long focal length (telephoto lens) will give better resolution, e.g., than a lens with a wide angle. This follows the principle that a smaller part of an object can be viewed using a telephoto lens than with a normal relatively wide-angle lens. Aperture To enable as much radiation as possible to be focused on to the detector, use a lens with a wide aperture. When viewing a very intensively radiating object (e.g., a very hot oven), the radiation intensity will be too strong, and the system will be overloaded; in this case it treats all radiation as maximum, and the entire picture becomes white even if the picture controls are adjusted to their end positions. The system will not break down because of this, but it's overmodulated. This in itself can be overcome by using the aperture control. Then the radiation intensity can be reduced, and the detector will not be overloaded. With the scanner set to the highest aperture number and with out the use of external filters, objects of a temperature up to 800 deg C. can be viewed. FILTERS There are filters for a number of different purposes and applications, all aiming at increasing the scope of the thermal scanner. e.g., there is a high-temperature filter that both decreases the intensity of the radiation received from very hot objects and cuts off short wavelengths, in which the radiation from very hot objects is concentrated. This is used when registering the long-wave radiation from hot objects. For special applications to measure the temperature on liquid (fluid) glass, having a temperature of 700 – 800 deg C., a problem arises. Glass is transparent to short infrared wavelengths, and therefore the temperature of the background tends to be measured instead of that of the glass. By using the filter it's possible to remove the very shortwave radiation. Therefore, the long-wavelength infrared radiation will be measured, and the glass will be completely opaque, so that we measure the temperature of the glass and not of the surroundings. PRESENTING THE THERMAL PICTURE The final thermal picture is presented on a picture tube similar to that used in television. The detector output signal is in analog form. After being amplified, it needs processing into a video signal. All values of intensity between the signal’s minimum and maximum values, respectively, form the dynamics of the tube. In a gray scale the dynamics range from dark black to brilliant white. The thermal resolution (the smallest variations in temperature that can be registered on AGA’s equipment), is approximately 0.1 deg. C. at room temperature. It is of great importance that the thermal resolution of the system be dictated by the characteristics of the thermal scanner. What is missed by the thermal scanner can't be replaced by refined processing of the signal. The above procedure implies that objects of the same temperature, or at least what the scanner recognizes as the same temperature, should be reproduced in the same gray tone. In the processing of the video signal from the detector, it's possible, by marking the temperature chosen with saturated white pulses in the pictures, to add a function that will emphasize an area with the same temperature. COLOR PRESENTATION The human eye has a very high sensitivity when it comes to separating various color tones from each other. This is the explanation for using a color monitor instead of the usual black and white monitor when illustrating thermal pictures. The number of lines, and therefore the resolution, remains unchanged but the various levels of temperature are illustrated by different colors instead of gray shades. The colors are usually ranged such that blue illustrates the coldest parts, and yellow and white the warmest. This order can be changed if it for some reason should be better to increase the contrast between close parts in the object. Simultaneously, a range of colors is displayed on one side of the picture, indicating the various isotherms for reference purposes. PICTURE ANALYSIS AGA’s thermal picture is built of a number of lines, 4 x 70, where 70 lines constitute a field and four fields fill a frame. The horizontal and vertical resolution is set by the size of the detector and is 100 points per line. However, lines and points are overlap ping each other, so the true resolution results in 280 x 100 = 28,000 points. When a digital rendition of the picture is made, the information is first quantized. The value of the signal is measured frequently. This is called sampling. A sample can be quantized to one out of, say, 24 values. The analog signal can take an unlimited amount of values; the quantizing causes the analog value of the sample to be replaced by the closest possible digital value (ill. 16-4). ill. 16-4: M example of digitizing, or quantizing, an analog signal. The values of all points, which together build up a picture, is stored in a memory, from which it can later on be fetched for further processing. This is normally done by a computer. A line is divided into 100 points, where each point forms a picture element (pixel). Each point is quantized into one of 256 possible values. With 256 levels the quantizing differences are minimal, and the digital rendition is essentially equivalent to the analog rendition. One of the advantages of digital processing is that, with the help of a digital computer, it's possible to “subtract” pictures from each other in order to procure variations that otherwise would be very hard to obtain. Furthermore, in this way, complex picture analyses can be carried out. e.g., quick events can be analyzed, and the gradual change from picture to picture can be shown very clearly. An example of very advanced picture processing is the evaluation of pictures taken from American space shuttles. The very violently heated front and wing leading edges on the space shuttle have been investigated when reentering the atmosphere. This was thermographed with the assistance of a modified AGA Thermovision camera. PREV: Bimetal Strips |
Updated: Tuesday, February 10, 2009 18:55 PST
