Glossary I through Q
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.
|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.
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.
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.
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.
|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 )
||is characteristic length
||is the configuration factor
||is the wall temperature
||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,
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.
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.
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.
|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
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:
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
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.
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.