Recent investigations into the suitability of infrared imaging for certain inspection problems were greatly slowed by a personal unfamiliarity with the associated terms and abbreviations. I had a need to make reference to texts, journals, and standards to find some meaningful descriptions and hence an understanding of the material being presented. The understanding of infrared terms and definitions is fundamental for a prospective user to consider the method’s suitability in-

dependently of others who may be closely associated with the equipment. This experience brought about a collation of definitions and descriptions which can be shared in the belief that there may be those in the NDT industry who will also need to equip themselves with similar knowledge. Hopefully, the reader will find it useful.

 

Background
The terminology which is used to define infrared systems and components originates from the fields of electronics, optics, and physics. Descriptions can appear complex and therefore be a distraction from an appreciation of the system capabilities and its suitability for an application. To complicate matters further, some of the longer terms become more obscure because they are abbreviated to a few letters. This article is not intended to be a superficial treatment of the subject matter but simply a brief and useful description of the terms most commonly encountered with infrared equipment and its performance.

 

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Here are brief and useful descriptions of the terms most commonly encountered with infrared equipment.

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Infrared wavelengths of the electromagnetic spectrum are between the visible and microwave wavelengths of about 10^-6 and 10^{-3 m}. For infrared applications that are made through the atmosphere (and most are), it is appropriate to understand that the main method of attenuation of infrared wavelengths is absorption by carbon dioxide gas molecules and water vapor. Atmospheric attenuation produces two main transmission wavebands of about 3-5 µm and 8-12 µm. Infrared systems are usually optimized for one or the other waveband. Other constituents of the atmosphere are much less significant in absorbing infrared radiation, but larger suspended particles do cause scatter. The larger the particle with respect to the wavelength of the radiation, the more significant the scattering effect. Smoke particles, for example, which are generally smaller than 2 µm, permit infrared radiation to pass relatively easily but absorb and scatter the shorter visible wavelengths.

Several ASTM documents and other texts deal comprehensively with the subject; notable among them is ASNT’s Nondestructive Testing Handbook, Volume 9. Other tutorial publications which would be most useful for anyone needing an introduction to an understanding of infrared systems are listed in the bibliography.

 

Definitions
Afocal Scanner. This is a miniature object space scanner used with an afocal telescope. This offers increased magnification and a corresponding reduction in field of view. The simplest example is a single flapping mirror and a long linear detector array.

Ambient Temperature Compensation. The temperature of an infrared camera and lenses can vary significantly and produce drift and hence erroneous readings from the instrument. A compensation system will correct for this variation.

Bandwidth. As noted above, there are two main bandwidths used in infrared testing. In general the 8-13 µm band is preferred for high performance thermal detectors because of the greater sensitivity to ambient temperature objects and good transmission through smoke. The 3-5 µm band may be more appropriate for hotter objects or if sensitivity is less important than contrast. For certain optical resolution it can use smaller optics, which may be useful in some circumstances.

Black Body. An ideal thermal radiator emitting and absorbing all possible thermal radiation at a given temperature, hence having an emissivity of 1.

Charge Coupled Device (CCD) Detector. The CCD detector passes the signal from each detector to the end of a row where it is read. Some of the signal (electric charge) is lost along the way.

Complementary Metal Oxide Semiconductor (CMOS). A CMOS device is created by a photochemical etching process which creates tiny circuits (semiconductors) for signal processing. A focal plane array is such a device. A CMOS detector has the signal from each detector element read individually, hence obtaining the exact value for processing.

Emissivity. The ratio of the radiance of a body at a given temperature to that of a black body at the same temperature. Instruments will require emissivity compensation to permit accurate temperature measurement across a range of materials and surface conditions.

Field of View (FOV). This is a function of the system optics and usually described in degrees of arc in the vertical and horizontal planes. Some systems include changeable lenses which will therefore change the FOV in the system.

Fill Factor. The ratio of the active area of each detector in an array to the inactive space which surrounds each detector.

Filters. Spectral filters are used to adapt the infrared system response to objects which have special spectral characteristics, such as measuring objects through flames, measuring energy in the CO2 absorption band, or suppressing certain wavelengths.

Focal Plane Array (FPA). An array is any grouping of detectors which has more than one vertical row and one horizontal line of detectors together. It could be as small as two detectors a line and two rows, four detector elements in all. More usually such arrays are 256 lines and 256 rows. The term focal plane refers to the location of the detector array in the optical path. The focal plane is that point where the image is focused.

Image Space Scanner. Radiation from the scene is imaged by an objective lens onto the infrared detector array.

Instantaneous Field of View (IFOV). This describes the optical resolution of the system and may be expressed in milliradians (mRAD) or minutes of arc. It is equivalent to the horizontal and vertical fields of view of an individual detector and as part of the overall resolution of the system and can be calculated using IFOV = A^1/2/F_ND, where A^1/2 is the square root of the linear dimension of the detector, F_N is the f number of the optics (focal length), and D is the diameter of the front objective.

Isotherm. A locus of points of equal heat.

Line Scanner. A device which scans along one line of a scene providing a one dimensional temperature profile. Optomechanical scanning systems such as line scanners used in infrared equipment rarely have an efficiency greater than 40 percent.

Microbolometer Detector. These are thermal detectors (very small bolometers) and not photon detectors. The detector (thermistor) actually heats up when exposed to infrared energy, changing its electrical resistance proportionally, which can then be measured. No cryogenic cooling devices are required, but images using this detector type are generally less sensitive.

Minimum Resolvable Temperature Difference (MRTD). This is a measure of the compound ability of an infrared imaging system and an observer to recognize periodic bar targets on a display. The MRTD is the minimum temperature difference between the standard test pattern and its black body background at which the observer can observe the pattern. This capability is governed by system thermal sensitivity (NETD) and spatial resolution (MTF) and is greatly enhanced by inbuilt temperature profile functions in some equipment. The MRTD increases with an increase in spatial frequency.

Modulation Transfer Function (MTF). MTF is a function of spatial frequency and in infrared systems MTF is the mathematical description of the spatial distribution of amplitude attenuation. The ability of an infrared system to transmit the spatial frequency of a scene is described in terms of the MTF. The overall system MTF is obtained from the product of the MTFs of its subsystems.

Noise Equivalent Temperature Difference (NETD). NETD quantifies thermal sensitivity. It is the target to background temperature difference between a black body target and black body background at which the signal to noise ratio of the scanner is equal to 1. Along with the spatial resolution (MTF), it governs the overall performance of an imager. It is usually defined in terms of the minimum resolvable temperature difference (MRTD). For high sensitivity the NETD must be low.

Object Space Scanner. The optical path between the focusing lens and the detector in these scanners is fixed. The lens operates effectively on axis and the scanning is performed on the object side of this lens. The result is good focus over the entire field of view, but it requires relatively large scanning elements and power to drive them.

Optics. The lens system which focuses the scene on the detector. Transmission through the optics can be as low as 60 percent and this has a direct affect on the NETD.

Photon Detectors. Semiconductors whose electrical properties are altered by photon-induced transitions which excite carriers from a bound state to a mobile state.

Photoconductive Detectors. In these materials, the carriers generated by incident photons produce a measurable increase in the conductivity of the device.

Photovoltaic Vidicon. An uncooled thermal imaging camera using a scanning electron beam to read the charge induced onto a pyroelectric target by incident thermal radiation. Less sensitive than cooled devices and less expensive, it is best suited for close range applications.

Quantum Efficiency. This term relates to infrared detectors and refers to the relative efficiency at which the infrared photons are collected and converted into electrical charge. Platinum silicide for example is a common detector material and has a low quantum efficiency, less than 1 percent.

Staring Arrays. The pyroelectric vidicon is an example of a staring array in that the thermally sensitive target is exposed to the scene for one TV field time, producing a low NETD value with no cooling required. The staring array can have limitations in spatial resolution and amplifier noise.

Spatial Frequency. A measure of detail in terms of equivalent, uniformly spaced cylindrical patterns, expressed as units of cycles per milliradian or line pairs per milliradian. Sometimes called spatial resolution.

Thermal Detectors. A detector material which uses a temperature dependent property which produces a measurable physical change.

Variable Integration Time (VIT). The time taken for the array of detectors to collect the infrared photons is called the integration time. A typical FPA will take 16 ms for one complete frame. Variable Integration time indicates that the photon capture device can capture the photons over a shorter time and hence captures less energy at a given temperature. This will permit high temperature measurements and imaging without the need for filters.

 

Bibliography
ASTM 1316: Standard Terminology for Nondestructive Examinations, Section J: Infrared Examination. ASTM, West Conshohocken, PA.

The Infrared Handbook, Wolfe and Zissis, 1978, 1985. ERIM, Ann Arbor, MI.

Introduction to Infrared Imaging System Design, William L. Wolfe, 1996. International Society for Optical Engineering (SPIE), Bellingham, WA.

Optical Design Fundamentals for Infrared Imaging Systems, Max J. Riedl, 1995. International Society for Optical Engineering (SPIE), Bellingham, WA.

Practical Applications of Infrared Thermal Sensing and Imaging Equipment, Herbert Kaplan, 1992. International Society for Optical Engineering (SPIE), Bellingham, WA.

 

* Qantas Airways, Non-Destructive Test Section, M271/G Mascot Jet Base, Sydney, Australia 2020; fax [61] (2) 9691 7268.

Copyright © 1997 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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