Medical imaging is a science using a variety of medical imaging techniques to diagnose and treat human diseases. In these years the digitalizing process of medical images starts from collection, scanning, display to reconstruction, diagnosis, transmission, storage, etc is closely bounded up to the maturity of computer science. Thermogram means a photographic record made by thermography. In this paper a study is made about the Thermogram and the enhancement using the Thermography instrument
Keywords- Thermogram, pixels, images, infrared technology, gray image, temperature.
I. INTRODUCTION TO
THERMOGRAPHY:- INFRARED TECHNOLOGY
Infrared technology is predicated on the fact that all objects having a temperature above absolute zero emit energy or radiation. Infrared radiation is one form of this emitted energy. Infrared emissions, or below red, are the shortest wavelengths of all radiated energy and are invisible without special instrumentation. The intensity of infrared radiation from an object is a function of its surface temperature. However, temperature measurement using infrared methods is complicated because there are three sources of thermal energy that can be
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detected from any object: energy emitted from the object itself, energy reflected from the object, and energy transmitted by the object Figure 1.1.Only the emitted energy is important in a predictive maintenance program. Reflected and transmitted energies will distort raw infrared data. Therefore, the reflected and transmitted energies must be filtered out of acquired data before a meaningful analysis can be completed.
The surface of an object influences the amount of emitted or reflected energy. A perfect emitting surface,Fig. 1.2, is called ablackbodyand has an emissivity equal to 1.0. These surfaces do not reflect. Instead, they absorb all external energy and reemit as infrared energy.
Surfaces that reflect infrared energy are calledgray bodiesand have an emissivity less than 1.0 Fig. 1.3. Most plant equipment falls into this classification. Careful considerations of the actual emissivity of an object improve the accuracy of temperature measurements used for predictive maintenance. To help users determine emissivity, tables have been developed to serve as guidelines for most common materials. However, these guidelines are not absolute emissivity values for all machines or plant equipment.
Variations in surface condition, paint or other protective coatings and many other variables can affect the actual emissivity factor for plant equipment. In addition to reflected and transmitted energy, the user of thermographic techniques must also consider the atmosphere between the object and the measurement instrument. Water vapor and other gases absorb infrared radiation. Airborne dust, some lighting and other variables in the surrounding atmosphere can distort measured infraredradiation. Since the atmospheric environment is constantly changing, using thermographic techniques requires extreme care each time infrared data is acquired.
Figure1.1.Energy emissions. All bodies emit energy within the infrared band. This provides the basis for infrared imaging or thermography. A = absorbed energy. R = reflected energy. T = transmitted energy. E = emitted energy.
Figure1.2.Blackbody emissions. A perfect or blackbody absorbs all infrared energy. A = absorbed energy. R = reflected energy. T = transmitted energy. E = emitted energy.
Figure1.3.Graybody emissions. All bodies that are not blackbodies will emit some amount of infrared energy. The emissivity of each machine must be known before implementing a thermographic program. A = absorbed energy. R = reflected energy. T = transmitted energy. E = emitted energy.
Most infrared monitoring systems or instruments provide special filters that can be used to avoid the negative effects of atmospheric attenuation of infrared data. However the plant user must recognize the specific factors that will affect the accuracy of the infrared data and apply the correct filters or other signal conditioning required to negate that specific attenuating factor or factors.
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Collecting optics, radiation detectors, and some form of indicator are the basic elements of an industrial infrared instrument. The optical system collects radiant energy and focuses it upon a detector, which converts it into an electrical signal. The instrument’s electronics amplifies the output signal and process it into a form, which can be displayed. There are three general types of instruments that can be used for predictive maintenance: infrared thermometers or spot radiometers, line scanners, and imaging systems.
Infrared thermometers or spot radiometers are designed to provide the actual surface temperature at a single, relatively small point on a machine or surface. Within a predictive maintenance program, the point-of-use infrared thermometer can be used in conjunction with many of the microprocessor-based vibration instruments to monitor the temperature at critical points on plant machinery or equipment. This technique is typically used to monitor bearing cap temperatures, motor winding temperatures, spot checks of process piping temperatures, and similar applications. It is limited in that the temperature represents a single point on the machine or structure. However when used in conjunction with vibration data, point-of-use infrared data can be a valuable tool.
This type of infrared instrument provides a single-dimensional scan or line of comparative radiation. While this type of instrument provides a somewhat larger field of view, that is, area of machine surface, it is limited in predictive maintenance applications.
Unlike other infrared techniques, thermal or infrared imaging provides the means to scan the infrared emissions of complete machines, process, or equipment in a very short time. Most of the imaging systems function much like a video camera. The user can view the thermal emission profile of a wide area by simply looking through the instrument’s optics. There are a variety of thermal imaging instruments on the market ranging from relatively inexpensive, black and white scanners to full color, microprocessor-based systems. Many of the less expensive units are designed strictly as scanners and do not provide the capability of store and recall thermal images. The inability of store and recall previous thermal data will limit a long-term predictive maintenance program.
Point-of-use infrared thermometers are commercially available and relatively inexpensive. The typical cost for this type of infrared instrument is less than $1000. Infrared imaging systems will have a price range between $8000 for a black and white scanner without storage capability to over $60,000 for a microprocessor-based, color imaging system.
Training is critical with any of the imaging systems. The variables that can destroy the accuracy and repeatability of thermal data must be compensated for each time infrared data is acquired. In addition, interpretation of infrared data requires extensive training and experience.
Inclusion of thermography into a predictive maintenance program will enable you to monitor the thermal efficiency of critical process systems that rely on heat transfer or retention; electrical equipment; and other parameters that will improve both the reliability and efficiency of plant systems. Infrared techniques can be used to detect problems in a variety of plant systems and equipment, including electrical switchgear, gearboxes, electrical substations, transmissions, circuit-breaker panels, motors, building envelopes, bearings, steam lines, and process systems that rely on heat retention or transfer.
Basic Infrared Theory
Infrared energy is light that functions outside the dynamic range of the human eye. Infrared imagers were developed to see and measure this heat. This data is transformed into digital data and processed into video images that are calledthermograms. Each pixel of athermogramhas a temperature value and the image’s contrast is derived from the differences in surface temperature. An infrared inspection is a nondestructive technique for detecting thermal differences that indicate problems with equipment. Infrared surveys are conducted with the plant equipment in operation, so production need not be interrupted. The comprehensive information can then be used to prepare repair time/cost estimates; evaluate the scope of the problem; plan to have repair materials available, and perform repairs effectively.
All objects when heated emit electromagnetic energy. The amount of energy is related to the temperature. The higher the temperature, the more electromagnetic energy it emits. The electromagnetic spectrum Fig. 1.4contains various forms of radiated energy including x-ray, ultraviolet, infrared, and radio. Infrared energy covers the spectrum of 0.7 to 100 μm.
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The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. A wave has several characteristics Fig. 1.5. The highest point in the wave is called thecrest. The lowest point in the wave is referred to as thetrough.The distance from wavecrest to wavecrest is called awavelength. Frequencyis the number of wavecrests passing agiven point per second. As the wave frequency increases, the wavelength decreases. The shorter the wavelength the more energy contained; the longer the wavelength, the less energy. For example: a steel slab exiting the furnace at the hot strip will have short wavelengths. You can feel the heat and see the red glow of the slab. The wavelengths have become shorter crest to crest and the energy being emitted has increased, entering the visible band on the spectrum. By contrast, (infrared energy) when the coil comes off of the coilers it has been cooled. There is a loss of energy. The wavelength have increased crest to crest and decreased in frequency.
Heat Transfer Concepts
Heat is a form of thermal energy. The first law of thermodynamics is that heat given up by one object must equal that taken up by another. The second law is that the transfer of heat takes place from the hotter system to the colder system. If the object is cold, it absorbs rather than emits energy. All objects emit thermal energy or infrared energy through three different types or modes. The three modes are conduction, convection, and radiation. It is important to understand the difference of these three forms.
Conduction.Conduction is the transfer of energy through or between solid objects. A metal bar heated at one end will, in time, become hot at the other end. When a motor bearing is defective, the heat generated by the bearing is transferred to the motor casing. This is a form of conduction.
Convection.Convection is the transfer of energy through or between fluids or gases. If you took the same motor mentioned above and placed a fan blowing directly on the hot bearing, the surface temperature would be different. This is convection cooling. It occurs on the surface of an object. An operator must be careful to identify the true cause and effect. In this case, the difference between good and bad source heating and the surface cooling due to convection.
Radiation.Radiation is the transfer heat by wavelengths of electromagnetic energy. The most common cause of radiation is solar energy. Only radiated energy is detected by an infrared imager. If our motor were sitting outside in the slab storage yard with slabs stacked around it, the electromagnetic energy from the sun and from the slabs would increase the temperature.
The purpose of this exercise was to make the thermography aware that there could be other causes of the thermal energy found or not found. In this case, was the motor hot because of a bad bearing or because of solar radiation? Was the motor missed and failed later because of the fan blowing on it and causing convection cooling? Conduction is the only mode that transfers thermal energy from location to location within a solid, however, at the surface of a solid or liquid, and in a gas, it is normal for all three modes to be operating simultaneously.
Emissivity.Emissivity is the percentage of energy that is emitted by an object. Infrared energy hits an object; the energy is then transmitted, reflected, or absorbed. A common term used in infrared thermography is blackbody.
A blackbody is a perfect thermal emitter. Its emissivity is 100 percent. It has no reflection or transmittance. The objects you will be scanning will each have a different emissivity value. A percentage of the total energy will be due to reflection and transmittance. However, since most of your infrared inspection will be quantitative thermography, the emissivity value will not be as important now.
EVALUATION OF INFRARED EQUIPMENT
Listed below is the criteria used to evaluate infrared equipment. It is important to determine which model fits your needs best before a purchase is made. Some of these points will be important to you and others will not. You will know more about yours needs by the end of this course.
Portability.How much portability does your application require? Does weight and size of the instrument affect your data collection? What kind of equipment will you be scanning?
Ease of Use.How much training is required to use the imager? Can it be used easily in your environment?
Qualitative or Quantitative.Does it measure temperatures? If yes, what temperature range will be measured? Will you need more than one range?
Ambient or Quantitative Measurements.What are the maximum upper and minimum lower ambient temperatures in which you will be scanning?
Short or Long Wave Lengths.Long wavelength systems offer less solar reflection and operate in the 8 to 14 μm bandwidth. Short-wave systems offer smaller temperature errors when an incorrect emissivity value is entered. The operating bandwidth for a short-wave unit is 2 to 5.6 μm.
Batteries.What is the weight and size of the batteries? How long will they last? Will you need additional batteries? How long does it take to charge?
Interchangeable Lenses.Do the ones available fit your application? What are their costs?
Monitor, Eyepiece, or Both.Will you need to show a live image to others while performing an inspection?
Analog or Digital.How will you process the images? Does the imager have analog, digital, or both capabilities?
Software.Can the software package produce quality reports, store and retrieve images? Do you require colonization and temperature editing?
INFRARED THERMOGRAPHY SAFETY
Equipment included in an infrared thermography inspection is almost always energized. For this reason, a lot of attention must be given to safety. The following are basic rules for safety while performing an infrared inspection:
Plant safety rules must be followed at all time.
Notify area personnel before entering the area for scanning.
Qualified electrician from the area should be assigned to open and close all panels.
Where safe and possible, all equipment to be scanned will be on line and under normal load with a clear line of sight to the item.
Equipment whose covers are interlocked without an interlock defect mechanism should be shut down when allowable. If safe, their control covers opened and equipment restarted.
INFRARED SCANNING PROCEDURES
The purpose of an infrared inspection is to identify and document problems in an electrical or mechanical system. The information provided by an inspection is presented in an easily and understandable form. A high percentage of problems occur in termination and connections, and especially in copper to aluminum connection. A splice or a lug connector should not look warmer than itsconductors if it has been sized properly. All problem connection should be dismantled, cleaned, reassembled, or replaced as necessary.
Type of Infrared Problems
When viewing thermal problems there are three basic types of problems.
Mechanical looseness occurs most frequently. A loose connection will result in thermal stress fatigue from over use. Fuse clips are a good example, the constant heat up and cool down creates a poor connection. An accurate temperature measurement, or use of an isotherm will identify a loose condition. When the isotherm is brought down to a single pixel, or temperature, it will identify the source of the loose condition.
Understanding the nomenclature of the problem can identify component failure. Specifically, the actual component will be the heat source. For example, a heat-stressed fuse in a three-phase assembly will appear hotter than the other two fusses.
Broken Strands.These hot spots are found at the support and at the cables termination.
Spiral Heating.This is found on stranded wire, which is heavily oxidized. The problem will show up as a hot spiral from one connection to another connection. There is a load imbalance between the strands, which results in a poor connection.
Ground Conductor.Usually there are no hot spots on a ground conductor. They do show up, however, as hot spots where there is abnormal leakage current to the ground. Be suspicious about such spots. Always point them out in the inspection report.
Parallel Feeders.A cold cable indicates a problem when parallel conductors are feeding the same load.
By this study we can conclude that thermogram is Infrared Imaging is a totally non-invasive, non-contact, medical imaging procedure for detecting and monitoring various diseases and physical injuries.
ACKNOWLEDGEMENT I wish to thank my guide for helping in providing the moral support and encouragement in preparation of this paper
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