Glossary A through H

Select the first letter of the word from the list to jump to appropriate section of the glossary. If the term you are looking for starts with a digit or symbol, choose the '#' link.
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- 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.
Perihelion Aphelion Mean
0.30+/-0.01 0.30+/-0.01 0.30+/-0.01
global annual average

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.

Quantity Thermal System Electrical System
Potential T E
Flow q I
Resistance R R
Conductance G 1/R
Capacitance C C
Ohm's Law q=GT I=E/R
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.

- B -
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

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.
- 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. 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.


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

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.

- E -
Emissivity:
The ratio of energy emitted by an object to the energy emitted by a blackbody at the same temperature. The emissivity of an object depends upon its material and surface texture.

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.

- F -
FPA (Focal Plane Array)
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.


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.


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

- G -
- 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. For example;
  • Electrical components produce Joule heating.
  • Two parts rubbing together can generate frictional heat
  • A clothes dryer with a gas burner
  • Electric elements of a toaster
  • A laser beam striking a mirror will leave a part of its energy with the mirror.
  • Gamma radiation can interact with the atomic structure of a material to cause internal heating.
  • A satellite, orbiting the earth, is exposed to three major types of environmental heating.
    • Direct sunlight incident on its surfaces
    • Albedo, or reflected sunlight, from the surface of the planet
    • Infrared (IR) radiation from the planet.

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.


Diagram of typical Hybrid FPA

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.