Thermography

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Thermography can refer to a printing process and an imaging process. A thermogram is an image produced by thermography.

1 Thermographic imaging
2 Thermographic printing
3 Thermography, or thermal imaging
4 See also
5 External links

[edit] Thermographic imaging

Image of a small dog taken in mid-infrared ("thermal") light (false color)

Thermographic image of two ostriches

Thermographic image of a snake held by a human

Thermographic image of a lion

Thermographic image of traditional and 'passive house' buildings

Thermography, or thermal imaging, is a type of infrared imaging. Thermographic 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, thermography 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 thermography allows one to see variations in temperature (hence the name). When viewed by thermographic 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, thermography'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 thermographic imaging. [1]

The appearance and operation of a modern thermographic 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, 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. Thermographic cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often export-restricted. Older bolometers or more sensitive models as InSB require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or liquid nitrogen.

See also: infrared camera, infrared detector

[edit] Thermographic printing

Thermographic printing refers to two types of printing, both of which rely on heat to create the letters or images on a sheet of paper.

The simplest type is where the paper has been coated with a material that changes colour on heating. This is called thermal printing and was used in older model fax machines and is used in most shop till receipt printers. This is called direct thermal.

More complex is thermographic printing that melts print off a ribbon and onto the sheet of paper (thermal ink transfer printing). This is called thermal transfer.

Thermography printing is also a post print process done immediately after printing. It can be done inline or offline as long as the ink is wet. Thermography powder is applied to the sheet of paper after it leaves the printing press. The sheet is then vacuumed or shaken to remove excess powder. This leaves powder only where there is wet ink. The sheet then travels through a heat tunnel; there the heat causes the powder to melt which leaves a raised effect.

It is commonly used on letterheads, business cards, greetings cards, gift wrap, packaging and can also be used to print braille text.

[edit] Thermography, or thermal imaging

Thermography, or thermal imaging, is a type of infrared imaging. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum approximately 900–14,000 nanometers. Infrared Thermography is a picture of heat.

A thermographer takes and interprets IR images. They need to be familiar with thermal dynamics, heat transfer laws, and of why the temperature of objects may vary , and how similar objects of similar temperature may appear to have completely different temperatures to each other. Thermographic images can be difficult to measure and interpret.

Infrared is an portion of the light spectrum extending from 0.75 to 1,000 microns which is invisible to the human eye. All objects warmer than absolute zero (0 Kelvin or -275.15 °C) emit energy somewhere within that range.

Thermography deals with the infrared spectral band between optical red and microwave wavelengths.

Difference between IR film and thermography

IR film is sensitive to temperatures between 250 °C and 500 °C while thermography is sensitive to approximately -50 °C to over 2,000 °C. So for a IR film to show something it must be over 250 °C or be reflecting infrared radiation from something that is at least that hot. Night vision goggles normally just amplify the small amount of light that is available outside like starlight or moon light and can't see heat or work in complete darkness.

Advantages of Thermography

You get a visual picture so as 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

Limitations & disadvantages of thermography

Quality cameras are expensive easily damaged
Images can be hard to interperate 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

Applications

Condition monitoring
Medical imaging
Research
Process control
Non destructive testing

Thermal infrared imagers convert the energy in the infrared wavelength into a visible light video display. All objects above 0 degrees Kelvin emit thermal infrared energy so thermal imagers can passively see all objects regardless of ambient light. However most thermal imagers only see objects warmer than -50C

Radiation is proportional to a object surface temperature. This makes it possible for a thermal camera to display a object's temperature. The radiation seen by a camera does not only depend on the temperature of the object, but is also a function of the emissivity of the object. Radiation also originates from the surroundings and is reflected in the object. The radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.