Technology

Presented is a comprehensive analysis of Infrared Cameras (IRCs), documenting the principles behind its operation. Primary parts inclusive of the optical and electronic modules are also analyzed. The paper further describes typologies of infrared cameras giving the two broad categorizations those with cooled and with uncooled infrared image detectors. Practical applications of variant types of infrared cameras inclusive of surveillance, night vision, and firefighting among other functions are further researched. IR cameras special measuring capability within different conditions such as temperature, with examples are also analyzed. Limitations to the effectiveness of infrared cameras such as diffraction are also critically documented. The paper further gives manufacturing features such as spectral filtering, interchangeable optics, color display concluding with recommendations for future development and improvement.

Background
Infrared Cameras (IRCs) are not an entirely new concept, as is evidenced in the sizable amount of literature available revolving around the subject. As presented by a number of authors with some giving technical while others non technical descriptions, infrared cameras are basically devices that form images using infrared radiation, (Caniou, 1999). The infrared radiation energy is one part of the electromagnetic radiation, hence based on the black body principle, the higher an objects temperature, the more infrared radiation as black body it emits. IRCs are therefore cameras that have the ability to detect the emitted radiation furthermore, the detection can work even in total darkness since the device is not entirely reliant upon the ambient light, (Kaplan, 1999).

Structure and operation
Principles behind infrared technology reflect the findings of studies which individually have recognized the uniqueness of their design. Both Kaplan (1999) and Richter (2006) documents that similar to optical cameras, infrared cameras have an optical module responsible for image formation, a detection module that converts optical images into an electrical signal and the electronic module for conditioning and data processing. Extra modules for optomechanical scanning and cooling can be added depending on the type of detector, (Richter, 2006). The optic module is responsible for concentrating the radiation energy thereby enabling the camera to form an image in the focal plane. The optical system is constituted of lenses or mirrors, while the lens optic are more compact and requires classical glasses that transmit wavelengths no greater than 2.5 m, (Caniou,1999). The materials used in the manufacture of infrared cameras are often selenium sulphide or zinc sulphide, silicon or germanium, noticeably, materials with high refractive indices, (Caniou, 1999).

Block Diagram of an Infrared Camera, Courtesy of  HYPERLINK httpwww.freepatentsonline.com6894280.html httpwww.freepatentsonline.com6894280.html
The main limitation to the effectiveness of an infrared camera is diffraction. Capper  Elliot (2000) justifies this noting that the blur circle is dependent upon the ration D where D refers to the diameter of the optics. Hence in an atmospheric window III measuring 10 m, the wavelength is about 20 times longer than what is visible. The part before the detection module is the objective, which comprises of fixed focal length objectives and variable length objectives. Capper  Elliot (2000), Lovrek, Howlett  Jain (2008), and Williams (2009) all concur that it is the detector module or the detector which governs the performance of the camera. It plays the primary role in defining the overall structure and remains a determining factor of the total equipment price.

Circuitry of an infrared diagram courtesy of  HYPERLINK httpwww.freecircuitdiagram.com www.freecircuitdiagram.com
Focal plane analysis is enabled via the deflection of line of sight and by the use of an optomechanical device, (Caniou, 1999). The scan patter in a line frame type and the image rates are made directly compatible with standard video. This allows for the interconnection with peripheral equipment inclusive of the video, monitor and the tape recorder, (Caniou, 1999). Scanning devices may in some cases solely consist of mirrors (catoptric) or refractive (Dioptric) elements furthermore while the former forms system, the latter is in dioptric systems. The combination of both mirrors and refractive elements leads to the formation of the more effective catadiotropic system

An exemplification of a modern catadiotropic system presented by Lovrek, Howlett  Jain (2008) can have either of the following combinations 2 resonant mirrors for line and frame scanners 1 mirror for line scanning and 1 refractive prism for frame scan and in the third case 1 mirror for line scanning and 1 reflective prism for the frame scan. As concerns thermal mapping, a number of researchers have noted that that while IR cameras individual thermal detector elements may be slow in response (in the order of milliseconds), each of the thousands of elements in the array is however exposed to any incoming target radiation and is therefore fast enough for a full response to a typical 30 HZ video scanning rate.

Kaplan (1999) documents that presently high resolution thermal imagers that feature IRFPA detectors have entirely replaced the opto-mechanically scanned imagers hence, a larger proportion of commercially produced Infrared Cameras feature IRFPA detectors. In cases in which problems in temperature control and monitoring cannot be solved by measuring a single or several discrete points within a target surface, then thermal mapping of the entire area of the target surface is necessary, a feat achieved by the infrared technology.

The mapping is done though the use of 2D staring FPA detectors with each detector in the array staring at one point at the target plane. Richter (2006) also affirms that the cameras are designed to enable the output to be intensity modulated relative to the total existent radiant energy at a specific target point on the surface. Therefore, the image produced is presented in color or monochrome in which the gray shades or color hues are intended to represent a specific thermal level at the target surface.


Circuitry diagram of an infrared camera courtesy of  HYPERLINK httpwww.freecircuitdiagram.com www.freecircuitdiagram.com

Typologies
A number of researchers have written on the types of infra red cameras with both Williams (2009) and Caniou (1999) affirming that they serve to emphasize quality rather than measurement capability. Literature surveys affirm two broad categorizations of thermographic cameras those with uncooled detectors and those with cooled infrared image detectors. Evidently, the former is the most common as documented by both Lovrek, Howlett  Jain (2008) and Richter (2006) who note that majority of general purpose thermal viewers use uncooled ferroelectric or bolometric detector arrays. The first thermal viewers to use uncooled FPA detectors were introduced in the late 1990s, following the modification of the IR night vision devices that had been developed by for the military. Caniou (1999) further subdivides the two broad categories into five types dependent upon their commercial production and applicability.

The most common form, the Uncooled IR cameras are applicable in predictive maintenance and condition monitoring used in buildings, roofs and infrastructure process monitoring and control and in medical and biological studies, (Caniou, 1999). Additionally, they are useful in material evaluation and nondestructive testing and in security, surveillance, night vision, firefighting and search and rescue. In cases of special application requiring improved sensitivity, spectral selectivity or high speed, cooled photo detector arrays are used. These Cameras have temperature sensitivity of between 0.08o and 80 MilliKelvins at 30oC, (Caniou, 1999).

They further have an estimated spectral range of between 7.5 and 13m, spatial resolution of between 1.3 milliradian (320240 elementbolometric FPA) and a frame repetition rate of 5060HZ, (Richter, 2006). Although multiple researches indicate the need for comprehensive training and certification prior to the handling of the project, Williams, (2009) documents that a complete system which includes image storage and battery can easily be handled by one person. Currently commercially produced, Lovrek, Howlett  Jain (2008) argues that as in the 90s, the imagers were initially offered to the security and law enforcement personnel.

Measuring Capability
IR Cameras have a special feature of providing quantitative temperature-measuring capability thereby enabling high image quality, (Capper  Elliot, 2000). Higher performance imagers are additionally endowed with fast scan capabilities besides having a spatial scan resolution of up to 512512 elements per frame. However, as noted by Richter (2006) to reach such performance output the Cameras would require cooled detector arrays most of which are electric powered and posses Stirling-circle and nitrogen gas coolers. Arguments by Williams (2009) indicates that majority of IR Cameras operate within the range of 0.9-1.9 m (NIR), the 3-5 m (NWIR) or the 8-14 m(LWIR) atmospheric windows.

The quantitative temperature measuring capability in an uncooled IR camera often offer excellent spatial resolution estimated at about 1 mrad and a minimum resolved temperature ranging between 0.02-0.2o C. Most manufacturers offer a number of features inclusive of spectral filtering, interchangeable optics for different FOV, isotherm graphic features, color or monochrome display on board image storage, computer capability and video recording capabilities, (Williams, 2009). Some researchers have focused on the importance of improving the current knowledge on IR photography indicating that performance parameters for the IR cameras continue to improve with variations and daily modifications being undertaken daily under controlled laboratory conditions. The introduction of multielement arrays, served to enable serial parallel scanning offering a two dimensional array combining. Prior to this there was either serial or parallel scanning, (Caniou, 1999).

Conclusion
Conclusively, Infrared cameras are capable of operating to the room temperature thermodynamic limit. The two types of thermographic cameras are those with cooled and uncooled infrared image detectors with both categories having a number of real life applications. According to Caniou (1999), material choice for variant parts inherently determines the quality and cost of an infrared camera although they have basic operation principles. Applications are wide and varied some of which include night vision, police and military target detection, astronomy, automotive application, process monitoring, roof inspection, aerial archaeology and technical surveillance counter-measures among others. Individual cameras specifications should be inclusive of sensor lifetime, Noise Equivalent Temperature Difference, dynamic range, field of view, input power, mass and volume and the number of pixels.