Glossary of Infrared Terms

Sierra Pacific Infrared offers this reference as a guide only and not a definitive source.

Choose a letter to find a particular term or simply browse the complete glossary below.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Absolute Zero

Temperature at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be -
273.15°C or -459.67°F.

Albedo

Earth reflected solar radiation. Global annual averages are: Perihelion 0.30+/-0.01, Aphelion 0.30+/-0.01, Mean 0.30+/-0.01.

Ambient Temperature

The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test.

Ambient Temperature Compensation

For years, it has been well understood that thermal imaging systems drift with variations in environmental temperature. This results from energy falling on the detector from components inside the camera such as the lenses and other internal objects. Each manufacturer has their own approach for dealing with this problem. Approaches range from sophisticated algorithms processing data that is collected from multiple temperature sensors throughout the camera and lenses, to systems that employ no compensation mechanisms at all.

Why should the P/PM user care about this feature? Due to the fluctuating nature of the environments that IR cameras are used in, the temperature of the camera and lenses vary significantly. This can cause rather severe drift if not properly compensated for. The drift manifests
itself in the form of erroneous readings from the instrument. The most comprehensive approach to solving this problem is by instrumenting each contributing component in the system with a temperature sensor, then the system can be calibrated through a variety of ambient temperature conditions during the manufacturing process. This capability is particularly important if you intend to make decisions on repair criterion based on absolute temperature measurements or trended data.

Analogous Systems

Two systems are said to be analogous when they both have similar equations and boundary conditions and the equations can be transformed into the equations for the other system by simply changing symbols of the variables. Thermal and electrical systems are two such analogous systems.

Ohm's Law:

q = GT

I = E/R

Quantity

Thermal System

Electrical System

Potential T E
Flow q I
Resistance R R
Conductance G 1/R
Capacitance C C

The analogy between thermal and electrical systems allows the engineer to utilize the widely known basic laws such as Ohm's Law and Kirchhoff's Laws used for balancing networks. Numerical techniques such as finite differencing, are used to solve the partial differential equations describing such systems.

Arithmetic Nodes

An arithmetic node can be used to represent the surface of a material. It could also represent the interface between two dissimilar materials, (for example a bondline). Arithmetic nodes have no thermal capacitance. They are sometimes called steady state nodes. Their temperatures are calculated by being brought into a steady state heat balance with the neighboring nodes. It can be used to represent nodes with very small capacitance relative to the rest of the model. In a transient analysis, this could result in a significant reduction in computer run time with only minor changes in overall accuracy.

ASIC (Application Specific Integrated Circuit)

In an effort to reduce the size, power consumption and cost of FPA cameras, the processing electronics need to be highly efficient and powerful. One means of achieving this without needing to support the software overhead and power consumption of off the shelf processors designed for PC applications is to utilize custom processor technology packaged in an Application Specific Integrated Circuit (ASIC).

ASICs are very common today and are used in everything from photocopiers to cellular phones. The concept behind these devices is to design an electronics processor which has been optimized in all aspects of performance for the particular application. The resulting electronics design is then packaged into an IC which becomes an ASIC. ASICs typically use a fraction of the power associated with standard PC processors and do not require the high software overhead associated with the DOS operating environment. Most ASIC processors offer advanced capabilities such as field upgradeability and very fast processing speeds.

The use of ASIC technology has benefited P/PM users by making FPA instruments smaller, lighter and less power consuming. Typically devices based on ASIC technology have relatively long battery life and support easy to use controls. The bottom line is that the FPA instrument should not be compromised by the choice of processing technology within the instrument. Low power, high speed, upgradeable processors are most desirable for hand held FPA systems.

Anomaly

An area or object within a thermal image that is different than what is expected and cannot be explained by normal visual, environmental, or historical information.

Aspect Ratio

The ratio of image frame width to height.

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B

Background

The area within a scene that does not include the target of interest.

Blackbody

A theoretical object that radiates the maximum amount of energy at a given temperature, and absorbs all the energy incident upon it. A blackbody is not necessarily black. (The name blackbody was chosen because the color black is defined as the total absorption of light energy.) For example freshly fallen snow and white paint have an IR absorptivity approaching 0.95.

Bandwidth

The range of frequencies over which a device is capable of operating within a specified performance limit.

Blooming

Image spreading and masking adjacent areas.

BTU

British thermal units. The quantity of thermal energy required to raise one pound of water at its maximum density, 1 degree F. One BTU is equivalent to .293 watt hours, or 252 calories. One kilowatt hour is equivalent to 3412 BTU.

Boundary Nodes

Boundary nodes are used to represent constant temperature sources or sinks. Effectively, they have infinite thermal capacitance. Boundary conditions such as ambient air, electronic base plates, or deep space can be simulated by using boundary nodes. Boundary node temperatures are not altered by the solution routines. However, time varying boundary conditions can be modeled with modern thermal analyzers.

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C

Calorie

The quantity of thermal energy required to raise one gram of water 1°C at 15°C.

Capacitance (Thermal)

A thermal modeling term. The capacitance C of a node is computed from the thermophysical properties of the subvolume evaluated at temperature T of the node.

C = M * Cp

where:

C Capacitance of the node
M Mass of the node
Cp Specific Heat of the node

Steady state thermal modeling solutions are not dependent upon thermal mass The transient solution routine does require the thermal mass of the nodes. Nodes with small thermal capacitance (when compared to the rest of the model) can be input as arithmetic nodes. The computational time step used in the transient solution is driven by small thermal capacitance diffusion nodes which are connected by large thermal conductors. Therefore, arithmetic nodes, when used with discretion, can save considerable computer time.

CCD Readout

Today's FPA detectors have two basic types of readouts for taking each detector's signal and getting it to the camera's signal processor. These are known as CCD (Charge Coupled Device) and CMOS. The CCD Detector operates in a mode where the signal from each detector is determined by transferring its electrons from one detector to the next down the row until it reaches the end column where it is read out. You can think of this by envisioning a bucket brigade where the contents of a bucket at the beginning of a line is transferred to the end of the line by passing it from bucket to bucket.


The CCD transfer process is not perfect, since some of the charge is lost along the way, much in the same way some water would be lost after passing it through 255 buckets. This is known as "Charge Couple Transfer Loss Phenomenon." Also, when one detector cell becomes overfilled with photons from a hot source, it can "overflow" into the adjacent detector cells. This is known as "blooming". CCD detectors require significantly more power than their CMOS counterparts and thus require higher powered cooling devices typically.


CCD detectors are widely used in imaging applications since the losses encountered by Charge Couple Transfer Loss Phenomenon and blooming are typically not relevant in non-measurement scenarios. When a CCD detector is utilized in a measurement IR FPA camera, compensations must be done to reduce errors caused by this issue.

Celsius (Centigrade)

A temperature scale defined by 0°C at the ice point and 100°C at boiling point of water at sea level.

Chromatic Aberration

Chromatic Aberration is a phenomenon where different wavelengths of light are not all focused at the same time. For example, 35 mm cameras have had lenses that have "color correction" for years. What they mean by color correction is that the lens is designed to focus all colors of light simultaneously. So when you focus on a scene of a bouquet of flowers, each flower, regardless of its color will be in focus. If the lens did not have color correction, you might see an image where the red and yellow flowers were in focus, but the blue flowers would seem a bit fuzzy. This is known as chromatic aberration.


Chromatic aberration can occur in IR systems, since these systems typically sense energy over a wide range of wavelengths at one time. Without correction, you could have a scene in which energy at 3.5μm is focused and energy at 5.0μm is fuzzy. The result would be an overall image that would not be crisp and could be subject to measurement errors. Manufacturers of IR systems can correct for this problem by developing color corrected IR lenses. Typically this is done by having several optical elements in the lens just like is done with 35 mm camera lenses.

CMOS (Complementary Metal Oxide Semiconductor)

Complementary Metal Oxide Semiconductor (CMOS) refers to a manufacturing technology which is used widely today in most electronic devices. To a large degree, CMOS technology is what made the production of IR FPAs possible.


In a CMOS device, a photochemical etching process is used to create tiny circuits known as semiconductors for signal processing. Typically, a silicon substrate is used in conjunction with various metal compounds to make up the raw material; this is known as a wafer. The etching
process leaves metal areas which are used for electrical conduction and oxide regions which are used for insulation.


CMOS technology is used throughout today's FPA cameras. Most importantly however is the fact that this technology has allowed the volume manufacture of various types of IR sensitive material in array formats.

Conductance

The measure of the ability to carry a heat flow.

Conduction (Thermal)

A thermal modeling term. Heat flows from a region of higher temperature to a region of lower temperature. Conduction is the process by which heat flows within a medium or between different mediums in direct contact. The energy is transmitted by molecular communication.


Conductors which represent conduction or convection paths are referred to as linear conductors because the heat flow is a function of the temperature difference between nodal temperatures to the first power.

Qdot = G * (T1 - T2)
Linear conductors representing solid conduction are computed from the equation:
G = k * A / L
where:
G thermal conductance (i.e. Btu/hr-F or W/C )
k thermal conductivity (i.e. Btu/hr-ft-F or W/cm-C )
A cross-sectional area through which heat flows (i.e. FT2 or cm2 )
L length between adjoining node centers ( i.e. ft or cm )

Conductivity (Thermal)

The property of a material to conduct heat in the form of thermal energy.

Convection

1. The circulatory motion that occurs in a fluid at a non-uniform temperature owing to the variation of its density and the action of gravity. 2. The transfer of heat by this automatic circulation of fluid.


For heat transfer by convection, the conductor is calculated by the following equation:

G = hc * A
where:
hc is the convection coefficient (energy/length2-time-deg)
A surface area in contact with the fluid (length2)

Cryogenics

Measurement of temperature at extremely low values, i.e., below -200°C.

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D

Density

Mass per unit of volume of a substance. I.E.: grams/cm or pounds/ft

Detection

The ability to distinguish that an artifact within the field of view is an anomaly.

Diffractive Lenses

The use of Diffractive Lenses is a relatively new technology associated with modern FPA systems. Diffractive lenses provide the color correction capability of a set of multiple lenses with a single diffractive element.

By doing the work of several lens elements with only a single element, the size, weight and transmission of a lens can be improved. Diffractive lenses can be distinguished from standard lenses by noting the "rings" which are etched in the surface of the lens. These diffractive grooves cause light waves to be bent in a unique manner, thus correcting for chromatic aberration.

The use of diffractive lenses, provide P/PM users with FPA cameras which produce crisp images while minimizing the size weight and cost of the optics.

Diffusion Nodes

A diffusion node is used to represent normal materials. Diffusion nodes have thermal mass (capacitance) and store and release energy with time. This process is characterized by a gain or loss of potential energy which depends on the capacitance value, the net heat flow, and the time over which the heat is flowing. In the transient solution routine, diffusion node temperatures are calculated by a finite difference representation of the partial differential heat transfer equation. Typically three items are stored for each diffusion node: temperature, thermal capacitance, and nodal heating (if any).


Thermal capacitance is the product of the mass of the node and the specific heat of the material that comprises the node. The mass can be calculated and the Specific Heat can be found in reference materials.

Dynamic Range

The range of input signal values that can be accepted by a device or system without unacceptable loss of information in its output due to saturation.

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E

Emissivity

A value between 0 and 1 that represents a ratio between an object and a black body.  The value represents the surface emittance of that object.  For a more detailed explanation of emissivity see Understanding Emissivity.

Endothermic

Absorbs heat. A process is said to be endothermic when it absorbs heat.

Enthalpy

The sum of the internal energy of a body and the product of its volume multiplied by the pressure.

Eutectic Temperature

The lowest possible melting point of a mixture of alloys.

Exothermic

Gives off heat. A process is said to be exothermic when it releases heat.

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F

Field of View

Maximum cone or fan of rays subtended by the entrance pupil that is transmitted by the instrument to form a usable image.

Field Rate

The product of the frame rate and the interlaced ratio.

Fill Factor

In a Focal Plane Array, not all of the surface of the detector is sensitive to IR energy. Since the array is made up of rows and columns of individual IR detectors, there is an inactive region surrounding each detector forming the rows and columns. You can think of this like a matrix of corn fields with roads running around them. Corn is grown in the fields, but not on the roads providing transportation from field to field. The inactive area between the rows and columns of an IR FPA are pathways for electronic signals. The ratio of active IR sensing material on an FPA to inactive row and column borders is called the Fill Factor. An ideal detector would have a very high fill factor, since it would have a large majority of its area dedicated to collecting IR photons and a very small area dedicated to detector segregation. Today's best IR FPA detectors offer fill factors as high as 90%.


Fill factor can be an important parameter to the average P/PM user. A camera with a high fill factor detector will typically provide better sensitivity and overall image quality than one with a lower fill factor. Also, high fill factor detectors typically offer better cooling efficiency, so less power is utilized cooling the detector down to operating temperature. This translates into longer battery life and greater cooler reliability.

Focal Plane Array

A state of the art sensor that detects energy in the infrared spectrum, these un-cooled ir detectors are used in the latest generation cameras.

The first, and most widely used term to come with this new technology is the term Focal Plane Array, which describes the technology itself. A Focal Plane Array (FPA) detector is considered to be any detector which has more than one row of detectors and one line of detectors together. For example, the smallest conceivable FPA detector would have a configuration of 2 X 2 detectors (two rows and two columns). This configuration is basically described by the term Array. The term Focal Plane refers actually to the location of the detector array in the optical path. The Focal Plane of an optical system is a point at which the image is focused. Thus, in a FPA system, you have an array of detectors at a point where the image is focused on them. Most typical IR FPA systems available today have an array of 256 X 256 detectors or more (256 columns and 256 rows).


FPA detectors bring high resolution IR imaging capabilities into the P/PM users' hands. By having an array of detectors "staring" at the scene rather than a single detector being scanned across the scene, IR cameras have become much smaller, lighter and more power efficient. Today's modern IR FPA systems have the portability of video palmcorders and the imaging quality of black and white TV cameras.

Frame Rate

The reciprocal of the time in seconds between the appearance of successive images of all points on a line filling the entire sensor field of view in the direction parallel to the scan pattern.

Freezing Point

The temperature at which the substance goes from the liquid phase to the solid phase.

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G

Gray Scale

Image is represented as varying shades of gray similar to a black and white television and is commonly quantified as 256 shades of gray.

Ground Truth Data

Information that allows an analyst the ability to correlate remotely collect and measure data and then present it in a quantifiable fashion.

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H

Heat Sink

1. Thermodynamic. A body which can absorb thermal energy. 2. Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it.

Heat Transfer

The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are:
Conduction
Convection
Radiation
Mass Flow

Heat

Thermal energy. Heat is expressed in units of calories or BTU's. In the real world, there are many reasons why thermal energy can enter a system.

Hybrid FPA

The other common type of FPA is a Hybrid Array. A Hybrid array is an array where the IR sensitive detector material is on one layer and the signal transmission and processing circuitry is on another layer. You can compare this to a city where the buildings are on one layer and the public transportation is on a subway underneath. In a Hybrid FPA, the two layers are bonded together by small Indium "bumps" which transmit the signal from each detector element to its respective signal path on the multiplexer below, much like a staircase joins the subway to the street level.


This process is known as "Indium Bump Bonding." Although this process requires more steps and can be more expensive, it results in FPAs with significantly higher fill factor (~75-90%). The higher fill factor resulting from this geometry provides much higher sensitivity than typically found in corresponding Monolithic FPAs.


The greatest benefit provided by Hybrid FPAs to the P/PM user comes in the form of high thermal sensitivity. This results from the Hybrid FPA's relatively high fill factor. Some FPA cameras employing this technology provide sensitivity down to 0.02°C. Very high sensitivity can be useful in
NDT applications, air in-leakage surveys and building diagnostic studies.

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I

Indium Antimonide (InSb) FPA

Indium Antimonide (InSb) is a detector material that was very common in single detector, mechanically scanned units from the past. The material typically offers higher sensitivity as a result of its very high quantum efficiency (80-90%). The high quantum efficiency does not buy you as much as it may seem however. Most IR manufacturers design their systems so that the detector wells are filled at about the 80°C on Range 1. With PtSi, this means allowing the detector to collect photons for most of the available 1/60th of a second frame time. With InSb, the wells fill in a few microseconds and after that you have to dump the rest of the photons. As a result, for most applications there is little benefit to the added quantum efficiency.


Another drawback to InSb FPAs for general applications is their relative instability over time. InSb IR FPAs have been found to drift in their non uniformity characteristics over time, and from cool down to cool down, thus requiring "Two Point Non Uniformity Corrections" in the field periodically. This can be done, but typically makes the system more complex by including mechanical shutters, thermoelectric coolers and additional electronics in the camera. For this reason, few manufacturers utilize InSb FPA detectors for measurement applications.

The added complexity of an InSb system is generally warranted in applications where extreme thermal sensitivity is required. Examples include such applications as long range military imaging.

Infrared

Portion of the electromagnetic spectrum immediately adjacent to visible light approximately from 0.7µm to microwave.

Infrared Camera

Electronics, lens, and detector combinations that give the user an image, which can be viewed or recorded, of energy in the infrared spectrum.

Infrared, Thermal

The portion of the infrared spectrum that the majority of heat energy is recorded from.  This portion of the electromagnetic spectrum is defined as 3µm to microwave; however, the majority of infrared imaging occurs between 3µm and 14µm.

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K

Kelvin

International temperature scale in which 0°Kelvin represents absolute zero.

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M

Microbolometer FPA

An emerging technology which will also be incorporated into P/PM IR FPA devices is that of Microbolometer Detectors. These detectors are different than the previous detectors that have been reviewed in that a Microbolometer detector is a Thermal detector rather than a Photon
counter. A microbolometer detector actually heats up as a result of being exposed to IR energy. As the microbolometer detector heats up, its electrical resistance changes proportionally. This resistance can be measured by applying a bias current to the detector.

Microbolometer detectors offer several promising benefits to the P/PM user. Of most significance, is that a Microbolometer will operate at near room temperature. This means that cryogenic cooling devices could be eliminated which should lower costs and increase reliability. Also, Microbolometer based cameras will operate in the long wave region, which will be useful in outdoor and low temperature applications.

Microbolometer detectors do have some drawbacks. At this time, no one is manufacturing these detectors in production quantities due to the lack of experience in the process, which makes them not practical for use in commercial cameras today. Also, Microbolometer detectors will be less sensitive and produce poorer quality images than their cooled counterparts. Lastly, microbolometer detectors are less likely to produce the accuracy and stability that P/PM users have become accustomed to with cooled sensors. This is due to the fact that a very small change in detector temperature will result in a fairly large change in output reading with these detectors.

In any case, this technology is likely to lower the costs of IR cameras somewhat and will provide users with cameras that are truly "solid state" with no moving parts.

Monolithic FPA

Today, there are basically two types of IR FPAs: Monolithic and Hybrid. Monolithic FPAs have both the IR sensitive material and the signal transmission paths on the same layer. You can think of this like a city that has both buildings and transportation all on the surface of the land. Monolithic FPAs have the benefit of typically being easier and less expensive to manufacture, since fewer steps are required in the process. On the other hand, Monolithic FPAs are typically considered to have lower performance than their Hybrid counterparts because they have a significantly lower fill factor (~55%). Monolithic FPAs have a lower fill factor because both the IR sensitive detector material and signal pathways are on the same level.


Most P/PM users will see the difference between a system with a Monolithic FPA array and a Hybrid array manifested in image quality. Systems with Monolithic arrays typically have less sensitivity than those utilizing a Hybrid array and as a result may have a poorer quality image. This is particularly noticeable when viewing low temperatures or scenes with small temperature differences. Also, until recently, advanced features such as variable integration time have not been found in Monolithic array designs due to the lack of flexibility with this design approach. This would mean that optical filters would be required to achieve high temperature imaging versus utilizing electronic signal attenuation methods which can be done with arrays having variable integration timing.

Multiplexer

A Multiplexer is the device that organizes and formats the signals from each detector in a repeatable fashion. Typically, a multiplexer takes the output from the 65,000 or more detectors and feeds them to one or more outputs. The way that the signal is taken from each detector and sent to the signal processor is determined by the detector Readout type.

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N

Natural (Free) Convection

The convection coefficient hc is a complicated function of fluid flow, thermal properties of the fluid, and the geometry of the system. Good ENGINEERING JUDGMENT is required to cool effectively with natural convection. Without going into any of the underlying equations, the natural convection coefficient can be approximated by:
hc = .52 * C * ( (Tw - Tair) / L ) **.25
where:
L is characteristic length
C is the configuration factor
Tw is the wall temperature
Tair is the air temperature
For a vertical plate, L is the length of the plate and C is .56. For a horizontal plate, L is given by:
L = 2 * length * width / (length + width)
For a horizontal plate facing up, C is .52; a plate facing down is .26. Units for these constants are BTUs, hrs, feet, F.

Nodes

In order to develop a thermal network and solve it using numerical techniques, it is necessary to subdivide the thermal system into a number of finite subvolumes called nodes. The thermal properties of each node are concentrated at the central nodal point of each subvolume. Each node represents a capacitance and has a temperature.


The temperature assigned to a node represents the average mass temperature of the subvolume. The thermal capacitance assigned to a node is computed from the specific heat of the material evaluated at the temperature of the node. Because a node represents a lumping of
parameters to a single point in space, the temperature distribution through the subvolume is linear. In a homogeneous material, the temperature at a point other than the nodal point may be approximated by interpolation between adjacent nodal points where the temperatures are known.

The error introduced by dividing the system into finite sized nodes rather than an infinite number of nodes depends on numerous considerations: material properties, boundary conditions, node size, node center placement, and time increment in transient calculations.

Typically a node is associated with a finite thermal mass (thermal capacitance) and a temperature which varies as a result of changes in the environment. These nodes are called diffusion nodes (mass nodes). The temperatures predicted by these nodes represent a "lump" of finite mass. In thermal modeling it is usually expedient to hold some aspect of the system constant. This portion of the thermal system is called a boundary condition. The boundary conditions drive the thermal system. The system does not drive the boundary. Examples are: Deep space for spacecraft modeling; A base plate of an electronic box for board modeling. The nodes used to model these conditions are called boundary nodes. Constant temperatures are usually assigned to these nodes however they can also be made to vary. Very often it is important to know the surface temperature of a material. Remember, diffusion nodes predict a "lump average" temperature. A two dimensional node can be made to represent the surface. It has no thickness and therefore has no thermal mass. These nodes can also represent interfaces between nodes. These nodes are called arithmetic nodes (massless nodes).

Non-Reimaging Lens Design

A Non-Reimaging Lens Design is a lens that has the IR image focused at only one point in the optical path. This single point of focus is on the FPA detector itself. This type of lens design does not have any elements designed for absorbing off axis stray radiation. These lenses are used widely in imaging only FPA products since the effects of stray radiation are of little concern in nonmeasurement devices. A benefit to this type of design is a reduction in lens size and weight. Typically non-reimaging lenses have fewer elements and are less expensive to manufacture than their reimaging counterparts.


P/PM users can use systems with non reimaging lenses in non measurement applications. When using this type of system in measurement scenarios, the user should be aware of external sources of IR energy in the survey environment and how they can change the resulting image and measurement data obtained with the camera.

Non-uniformity Correction

One of the less desirable characteristics of modern FPA detectors is their relative non-uniformity from detector to detector. This results from variations in the manufacturing process and the detector material itself. The fact remains that all FPA detectors are fairly non-uniform in their
response to temperature when they are built.


To correct for this, virtually all FPA cameras have some type of non-uniformity correction built into the camera. Methods for correcting this problem vary greatly from manufacturer to manufacturer. The most simple approach is when a lens cap is placed on the camera and a "NUC" button is depressed and the camera corrects for uniformity based on the temperature of the lens cap. Other systems have a uniform temperature "paddle" within the camera which is inserted in the optical path periodically to correct the detector. Some systems have permanent multi-point non-uniformity correction, where the detector is corrected at a variety of scene temperatures for each range and
then the data is stored within the unit, so the user never has to perform a non-uniformity correction in the field. This appears to be the best approach since it requires no user intervention and also provides for non-uniformity correction at several temperatures and not just at the lens cap temperature as with other approaches.


Non-uniformity correction is an important parameter for the P/PM user to consider given that it needs to be done each time you change ranges, lenses, or when the camera operating temperature varies. Systems that do this automatically will prove to be the easiest to use in the field. The best non-uniformity correction will be accomplished at a temperature as close to the object temperature as possible. For example, when looking inside a furnace at 1300 degrees F, a nonuniformity correction on the lens cap at 75 degrees F is of little value. The best approach in this case, is to have a non-uniformity correction point that would "equalize" the array at a temperature around 1300 degrees F. Today, this can only be accomplished with systems that feature permanent multi-point nonconformity correction.

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O

Oscillating Coolant Heat Exchange

The Oscillating Coolant Heat Exchanger has recently been awarded 2 patents. This device behaves like a mechanically driven heat pipe, however it is not limited by the operational constraints that often limit the usefulness of heat pipes. The device has not been applied commercially to date.

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P

Parallel Conductors Thermal

A modeling term. Many times a conductor representing a complicated geometry can be evaluated on a piece-wise basis, then recombined into one conductor value. One or more parallel conduction paths between nodes may be summed to create one conductor value by the following equation:

G(tot) = G1 + G2 + G3 +...+Gn

Peltier Effect

When a current flows through a thermocouple junction, heat will either be absorbed or evolved depending on the direction of current flow. This effect is independent of joule IR heating.

Phase Change Thermal Control

Method by which a material's heat of transition is used to advantage. This could include, but is not limited to, boiling water or melting wax.

Planetary (Earth) IR

Perihelion 234+/-7 W/m2
Aphelion 234+/-7 W/m2
Mean 234+/-7 W/m2
72 to 76 Btu/ft2-hr

Platinum Silicide (PtSi) FPA

Platinum Silicide (PtSi) is today's most common FPA detector material. The reason for this is that PtSi operates in the short-wave region (1-5 micro m), has good sensitivity (as low as 0.05 degrees C) and has excellent stability. PtSi is also used because it is manufacturable using semiconductor production techniques with fairly high detector yields resulting in reasonable costs.


PtSi detectors have been desirable for measurement cameras since it is a highly stable material that resists drift over time in its responsively to temperature. PtSi FPA detectors have been fielded for more than 10 years now, and have an extremely well proven reliability and long term stability record One drawback to PtSi as a detector material is its low quantum efficiency of <1%. However, modern signal processing techniques coupled with Hybrid construction and CMOS readouts have made PtSi into a leading material for use in P/PM and scientific IR imaging environments.


PtSi is a good detector material choice for general purpose P/PM applications. The detector offers a good mix of sensitivity, accuracy and stability to meet most IR imaging needs.

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Q

Quantum Efficiency

Quantum Efficiency can be thought of as "Collection Efficiency." Most IR detectors are photon counters, they count IR photons over very short periods of time. Quantum Efficiency refers to the relative efficiency at which IR photons are collected and converted into electrical charges. A high quantum efficiency is a good thing to have since it makes signal processing easier. Surprisingly, the most popular IR FPA detector material today, Platinum Silicide (PtSi) has a very low quantum efficiency (less than 1%).

Although Quantum Efficiency is only one measure of a system's design, it is a good way to evaluate the overall sensitivity of an IR detector. IR FPAs with high quantum efficiency typically offer better sensitivity and performance at low temperatures. Quantum Efficiency defines a detector's "Collection Efficiency"

Quantum Well (QWIP) FPA

A relatively new FPA detector available is Quantum Well Infrared Photodetector (QWIP). Due to the unique bandgap of this material, these detectors operate in the long wavelength region (9- 10 micro m). QWIP detectors have a quantum efficiency of 5-10% at 9.5 micro m and offer very high thermal sensitivity (0.015 degrees C).


At this point, this technology is relatively unproven and immature. One question yet to be answered is the long term stability and uniformity of this material. Another drawback to these detectors is the requirement for cooling the detector to ~ 65 degrees K (-208 degrees C), which puts an added load on the cooling device inside the camera.


Assuming that the technical concerns can be addressed, QWIP detectors could benefit the P/PM user by providing a FPA camera with very good imaging and measurement performance while operating in the long wave region. These units could be useful in outdoor applications where solar reflections are a problem or in applications where very low ambient temperatures are a factor.

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R

Radial Conductors

Thermal modeling term. For conductors between nodes which are circular sections, the radiation conductors equation should be used.

Radiation Conductors

A thermal modeling term. The value of a radiation conductor is input in units of energy per unit time per degree**4. It is be computed as:

G = A * e(eff) * F(i-j) * s
or
G = A * F(i-j) * s
where:
G value of the conductor
A area of the surface i
e(eff) emittance (dimensionless)
s Stefan-Boltzmann Constant (energy/length2-time-deg4)
F(i-j) black body view factor from surface i to j (dimensionless)
F(i-j) gray body view factor from surface i to j (area)

The emittance ε, is a measure of how well a body can radiate energy as compared with a black body. Emittance is the ratio of total emissive power of a real surface at temperature T to the total emissive power of a black surface at the same temperature. The emittance of surfaces is a function of several things including the material, surface condition, and temperature. The emittance may be altered by polishing, roughing, painting, etc.


The view factor F(i-j) is a function of the geometry of the system only. Many computer programs have been developed to compute the view factors between complex geometry's; however view factors between some surfaces with simple geometry's can be hand calculated. The methods and equations are found in several heat transfer texts.


The gray body view factor F(i-j) is the product of the geometric shape factor F(i-j) and a factor which allows for departures from black body conditions (i.e. reflections). For example, for two parallel flat plates:


F(1-2) = F(2-1) = 1
F(1-2) = [ 1 / ( 1/e1 + 1/e2 -1) ] x F(1-2)

The effective emittance e* between two surfaces may be used to compute the gray body view factor with the following equation:

F(i-j) = e* x F(i-j)

The error induced by the use of e* is the result of neglecting secondary reflections from surfaces other than the two for which the effective emittance was determined.

Reimaging Lens Design

There are two types of lens designs currently in use with modern FPA systems: Reimaging and Non-Reimaging. A Reimaging lens is one that has the image in focus at two points within the optical path. One point is on the detector (as with all lenses) and the second point is in the middle of the lens at a point called a intermediate focal plane. This point in the middle of the lens, where the image is refocused, is used for placing a device in the optical path which will capture energy from objects outside of the normal field of view (referred to as off-axis stray radiation).


The device that is placed at the intermediate focal plane is called a Field Stop. The field stop has an opening in it which corresponds to the field of view of the lens. This is an important feature, since without this capability imaging and measurement data can be corrupted by hot or cold objects that reside outside the field of view of a camera's lens.


P/PM users who are using IR FPA cameras for measurement purposes in industrial environments should be aware of this design factor. Systems with Reimaging lenses can be used in environments where there are a variety of hot and cold objects around the object that is being
measured. Systems that do not have this type of lens design can be subject to measurement errors as a result of off-axis stray energy falling on the FPA detector.

Resistance Temperature Characteristic

A relationship between a thermistor's resistance and the temperature.

Resistance (Thermal)

The resistance to the flow of heat.

Resistance = 1 / Conductance

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S

Series Conductors

Thermal modeling term. Series conduction paths between nodes may be combined to create one conductor value by the following equation:
G(tot) = 1 / (1/G1 + 1/G2 +...+1/Gn) = R1 + R2 + ... + Rn

Second Surface Mirror

The metal deposit provides the absorptance property. Silver and aluminum are the most popular metals and are often used on 2.0 and 5.0 (50 and 125 microns) FEP, also referred to as FOSR, (Flexible Optical Surface Reflector). This combination of materials obtains low absorptance over emittance ratios for low operating temperatures.


An interesting combination of materials such as 5 mil (125 microns) FEP and chromium can produce a "black" mirror.

Metal Deposits Solar Absorptance
Silver .06 - .09
Aluminum .10 - .14
Copper .20 - .30
Germanium .50 - .70
Inconel .60 - .70
Chromium .70 - .80
FEP Thickness
Inches
Emittance
0.0005 0.4
0.001 0.5
0.002 0.6
0.005 0.77
0.010 0.85

Set Point

The temperature at which a controller is set to control a system.

SI

System Internationale. The name given to the standard metric system of units.

Solar Constant

In space, normal to the sun line.

Perihelion 1414 W/m2
Aphelion 1323 W/m2
Mean 1367 W/m2
433 Btu/ft2-hr

Specific Gravity

The ratio of mass of any material to the mass of the same volume of pure water at 4ºC.

Specific Heat

The ratio of thermal energy required to raise the temperature of a body 1º to the thermal energy required to raise an equal mass of water 1º.

Stefan-Boltzman Constant

5.6697E-08 W/m2-K4
5.6697E-12 W/cm2-K4
1.355E-12 cal/cm2-K4-sec
1.714E-09 Btu/ft2-hr-R4

Super Cooling

The cooling of a liquid below its freezing temperature without the formation of the solid phase.

Super Heating

1. The heating of a liquid above its boiling temperature e without the formation of the gaseous phase.
2. The heating of the gaseous phase considerably above the boiling point temperature to improve the thermodynamic efficiency of a system.

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T

Thermal Coefficient of Resistance

The change in resistance of a semiconductor per unit change in temperature over a specific range of temperature.

Thermoelectric Cooler

A solid state refrigerator based on the Peltier effect. Typically very small; on the order of 1x1 inch. A single stage devices can create a -37C temperature differential between hot and cold sides. A three stage devices can create a -60C temperature differential. Use ranges from beer coolers to spacecraft.

Thermal Conductivity

The ability of a substance to conduct heat. Mathematically, the ratio of heat flow to the rate of temperature change in the particular substance.

Thermal Control

Thermal control is the engineered approach to control the thermophysical aspects of a system. Typically, temperature is controlled but sometimes heat flow is the controlled parameter. Reasons for thermal control include:
• Preclude catastrophic thermal failure
• Increase some performance characteristic
• Increase reliability
Reliability concerns have taken on new importance as a result of various studies that show a strong correlation between electronic equipment failures and:
• High temperatures
• Thermal cycling

Thermal Gradient

The distribution of a differential temperature through a body or across a surface.

Thermal Model

First and fore most a thermal model is tool. It is used to build the knowledge base of the thermal engineer. This knowledge enable the engineer to create a design that meet the requirements.

This could be accomplished (and sometimes is) with physical models and prototypes. The time and expense of this approach is often prohibitive.


Another approach is to construct a computer based, mathematical model of a thermal system. Such a model can be run through multiple conditions or configurations in seconds or minutes. Parametric (sensitivity) studies can also be quickly performed. Expense is incurred chiefly during the initial construction of the model not the running of the model.

Thermistors

Negative temperature coefficient thermistors are used to measure temperatures below 150C. They have sensitivities of several hundred ohm per 1C. Their cost range from $1 to $20. Their various configurations range from glass beads to stainless steel probes. Drawback is the non-linear response.

Transpiration Cooling

Transpiration cooling requires a liquid or gas coolant that flows through the surface of a severely heated component and exits the component from the heated surface through small pores in the surface. The coolant will both reduce the connective part of any heating and also removes heat from the surface in a very efficient way. Transpiration cooling is presently used in local regions of commercial turbine and rocket engines.

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U

Ultraviolet

That portion of the electromagnetic spectrum below blue light (380 nanometers).

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V

Variable Integration Time

Variable Integration Time (VIT) refers to a characteristic of the acquisition control of a FPA. The Integration Time is the period of time that the FPA is allowed to collect IR photons. Typically, an FPA runs at a maximum integration time of 16 milliseconds, which is one complete frame.


Arrays that have Variable Integration Time have the capability of capturing photons over shorter periods of time. This reduces the amount of energy that the detector captures at any given temperature. A common use for FPAs with VIT is to have high temperature imaging and measurement capabilities without needing filters. Some modern FPAs will operate up to 450ºC simply by using VIT.


For the P/PM user, having a FPA with Variable Integration Timing is a time saver since one can view higher temperatures by changing the electrical characteristics of the detector rather than installing an optical filter. Systems without adequate VIT typically require several filters to cover a span of -10ºC to 1500ºC.

Video Field

Any one of the two or more parts into which a frame is divided in interlaced.