Glossary I through Q
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.
Volume 1
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Volume 2
IJKLM
NOPQ
Volume 3
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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.

- J -
- K -
- L -
- 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.
- 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 non-measurement 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.


Nonuniformity Correction
One of the less desirable characteristics of modern FPA detectors is their relative nonuniformity 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 nonuniform in their response to temperature when they are built. 

To correct for this, virtually all FPA cameras have some type of nonuniformity 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 nonuniformity 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 nonuniformity correction in the field. This appears to be the best approach since it requires no user intervention and also provides for nonuniformity correction at several temperatures and not just at the lens cap temperature as with other approaches.

Nonuniformity 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 nonuniformity correction will be accomplished at a temperature as close to the object temperature as possible. For example, when looking inside a furnace at 1300°F, a nonuniformity correction on the lens cap at 75°F is of little value. The best approach in this case, is to have a nonuniformity correction point that would "equalize" the array at a temperature around 1300°F. Today, this can only be accomplished with systems that feature permanent multi-point nonuniformity correction.

- 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.
- P -
Parallel Conductors Thermal 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 Aphelion Mean
234+/-7 W/m2 234+/-7 W/m2 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 (1-5µm), has good sensitivity (as low as 0.05°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.

- 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 (9-10µm). QWIP detectors have a quantum efficiency of 5-10% at 9.5µm and offer very high thermal sensitivity (0.015°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°K (-208°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.