Thermography: Infrared, Liquid crystal, Microwave

THERMOGRAPHY

Thermograph, thermal imaging, or thermal video, is a type of infrared imaging. Thermo graphic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 900–14,000 nanometers or 0.9–14 µm ) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermograph makes it possible to see one's environment with or without visible illumination.

The amount of radiation emitted by an object increases with temperature, therefore thermograph allows one to see variations in temperature (hence the name). When viewed by thermo graphic camera, warm objects stand out well against cooler backgrounds; humans and

other warm-blooded animals become easily visible against the environment, day or night. As a result, thermograph’s extensive use can historically be ascribed to the military and security

services. Thermal imaging photography finds many other uses. For example, firefighters use it to see through smoke, find persons, and localize the base of a fire.

With thermal imaging, power lines maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air-conditioning.

Thermal imaging cameras are also installed in some luxury cars to aid the driver, the first being the 2000 Cadillac Deville. Some physiological activities, particularly responses, in human beings and other warm-blooded animals can also be monitored with thermo graphic imaging. The appearance and operation of a modern thermo graphic camera is often similar to a camcorder. Enabling the user to see in the infrared spectrum is a function so useful that ability to record their output is often optional. A recording module is therefore not always built-in.Instead of CCD sensors, most thermal imaging cameras use CMOS Focal Plane Array (FPA). The most common types are InSb, InGaAs, HgCdTe and QWIP FPA.

The newest technologies are using low cost and uncooled microbolometers FPA sensors. Their resolution is considerably lower than of optical cameras, mostly 160x120 or 320x240 pixels, up to 640x512 for the most expensive models. Thermo graphic cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often export-restricted. Older bolometer or more sensitive models as require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or liquid nitrogen.

Methods of Thermography

Infrared thermography

Liquid crystal thermography

Microwave thermography.

1. INFRARED THERMOGRAPHY

Infrared thermography is the science of acquisition and analysis of thermal information by using non contact thermal imaging devices.Human skin emits infrared radiation as an exponential function of its absolute temperature and the emissive properties of the skin temperature.

The maximum wavelength λ_max = 10 µm and range from 4 to 40µm.The thermal picture is usually displayed on a TV tube may be photographed to provide a permanent record.

Every thermo graphic equipment is provided with a special infrared camera that scales the object. The camera contains an optical system in the form of an oscillating plane mirror which scans the field of view at a very high speed horizontally and vertically and focuses the collected infrared radiations onto chopper.

The chopper disc interrupts the infrared beam so that a.c signals are produced. Then they are given to detector. The detector is infrared radiation detector. The detected output by detector is amplified and led to phase sensitive.

2. LIQUID CRYSTAL THERMOGRAPHY

Liquid crystals are a class of compounds which exhibit colour temperature sensitivity in the cholestric phase. Scattering effects with the material give rise to iridescent colours, the dominant wavelength being influenced by very small changes in temperature.

The high temperature sensitivity makes cholesteric liquid crystals useful for thermal mapping.In this technique, the temperature sensitive plate consists of a blackened thin flim support into which encapsulated liquid crystals cemented to a pseudo solid powder ( with particle sizes between 10 to 30 ) have been incorporated.

Thermal contact between the skin surface and plate produces a color change in the encapsulated liquid crystals; red for relatively low temperatures through the visual spectrum to violet for high temperatures. But in infrared thermograms, the violet colour is used to identify the low temperature regions and the bright colour or red is used to identify the temperature regions.

If we want to study a breast’s temperature distribution, several different plates are necessary to cover a breast temperature range from 28⁰C to36⁰C. Each plate covers a range of temperature 3⁰C. A record of the liquid crystal image may be obtained by colour photography. The response time varies according to the thickness of plate ( ranges from 0.06mm to 0.3 mm) and is 20 to 40 seconds.

3. MICROWAVE THERMOGRAPHY

Eventhough we get microwave emissions from the skin surface, that intensity is very small when we compare with Infra red radiation intensity . (10 wavelenght emission intensity is 10⁸ times greater than 10 cm wavelength emission intensity). But using modern microwave radiometers one can detect temperature change of 0.1K. since body tissues are partially transparent to microwave radiations which orginates from a tissue volume extending from the skin surface to a depth of several centimeters. Microwave radiometers consisting of matched antennae placed in contact with the skin surface for use at 1.3 G Hz and 3.3 G Hz have been used to sense subcutaneous temperature.

The present day thermographic systems, using Infrared radiation, only give a temperature map of the skin due to low penetration depth of the short wavelength of the infrared component of the emitted radiation. Using a microwave receiver with a frequency response from 1.7 GHz to 2.5 GHz a penetration depth of 1 cm in tissue and 8 cm in fat and bone can be obtained.

A severe problem is the unknown emissivity of the body surface for microwaves, as part of the radiation is reflected back into the body.In a conventional radiometer this gives rise toa measurement error proportional to the temperature difference between the body surface and the applied antenna. This error lies in the order of 1-2 K which is too high for medical applications.

The problem has been solved iv an elegant way by adding artificial microwave noise from the antenna,thus providing a radiation balace between the receiver and body surface. With this a temperature sensitivity of 0.1 K could be obtained. Based on the transducer attachment on the skin surface, we can classify the thermography into contact thermography and tele-thermography.

Advantages of Thermography

Get a visual picture so that you can compare temperatures over a large area It is real time capable of catching moving targets

Able to find deteriorating components prior to failure Measurement in areas inaccessible or hazardous for other methods It is a non-destructive test method

Limitations & disadvantages of thermography

Quality cameras are expensive and are easily damaged

Images can be hard to interpret accurately even with experience

Accurate temperature measurements are very hard to make because of emissivities Most cameras have ±2% or worse accuracy (not as accurate as contact)

Training and staying proficient in IR scanning is time consuming Ability to only measure surface areas

Applications

Healthy Cases

Tumors

Inflammation

Diseases of peripheral Vessels

Burns and Perniones

Skin Grafts and Organ Transplantation

Collagen diseases

Orthopedic Diseases

Brain and Nervous Diseases

Harmone Diseases

Examination of Placenta Attachment