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An
Overview of Infrared Technology
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This glossary is presented here courtesy of Inframetrics,
manufacturers of infrared cameras. Select from the following terms
for a description of Infrared terminology.
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| 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
mechanism at all. |
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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
sever drift if not properly compensated for. The drift manifests
itself 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. |
| ASIC
(Application Specific Integrated Circuit) |
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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). |
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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. |
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The use of ASIC technology has benefited P/PM users by making FPA instruments
small, 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 with the instrument.
Low power, high speed, upgradeable processors are most desirable for hand
held FPA systems. |
| 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. |
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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. |
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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 IF FPA camera, compensations must be done
to reduce errors caused by this issue. |
| Chromatic
Aberration |
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Chromatic Aberration is a phenomenon where different wavelengths of light
are not all focused at the same time. For example, 35mm 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. |
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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 3.5um is focused and energy at 5.0um 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 35mm camera
lenses. |
| CMOS
(Complementary Metal Oxide Semiconductor) |
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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. |
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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. |
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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. |
| CMOS
Detector |
A CMOS detector has a readout that is made up of a series of MOSFET (Metal
Oxide Silicon Field Effect Transistors) that provide Direct Access to the
signal from each detector. In a CMOS detector, the signal from each
detector is read out column by column and row by row, until each detector
has been addressed individually. The benefit to this technique is
that the exact value for each detector is transmitted to the signal processor
for measurement.
CMOS circuits are ideal for low power applications. Power dissipation
in a detector readout circuit is critical because it must be cooled with
the detector to approximately -200 degrees Celsius. Even with a highly
efficient cooler, each milliwatt of power dissipated by the readout requires
about 25 milliWatts of battery power for cooling. Optimum battery
life is achieved by the use of a CMOS multiplexer detector readout and
high efficiency rotary Stirling cooler. |
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CMOS detectors are generally thought to provide better accuracy for measurements
as a result of their direct access readout capability. P/PM users
who require high measurement accuracy and long battery life can benefit
from this technology. |
| 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 lightwaves to be bent in a unique manner, thus correcting for chromatic
aberration. |
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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. |
| 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 |
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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%. |
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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. |
| Hybrid
FPA |
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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. |
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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 degrees Celsius (32
degrees Fahrenheit). Very high sensitivity
can be useful in NDT applications, air in-leakage surveys and building
diagnostic studies. |
| Indium
Antimonide (InSb) FPA |
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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 degrees Celsius
on Range 1. With PtSi, this means allowing the detector to collect
photons for most of the available 1/60th of a second time frame.
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. |
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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 cooldown to cooldown,
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. |
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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. |
| 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. |
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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 longwave region, which will be useful
in outdoor and low temperature applications. |
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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 output reading with
these detectors. |
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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 |
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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. |
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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 |
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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. |
| Non-Reimaging
Lens Design |
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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. |
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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 |
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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
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. |
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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 degrees Fahrenheit (704 degrees Celsius),
a nonuniformity correction on the lens cap at 75 degrees Fahrenheit (24
degrees Celsius) 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 degrees Fahrenheit. Today, this can
only be accomplished with systems that feature permanent multi-point nonuniformity
correction. |
| Platinum
Silicide (PtSi) FPA |
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Platinum Silicide (PtSi) is today's most common FPA detector material.
The reason for this is that PtSi operates in the shortwave region (1-5um),
has good sensitivity (as low as 0.05 degrees Celsius) 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. |
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PtSi detectors have been desirable for measurement cameras since it is
a highly stable material that resists drift over time in its responsivity
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. |
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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 |
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%). |
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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
Well (QWIP) FPA |
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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 (9-10um). QWIP detectors have a quantum
efficiency of 5-10% at 9.5 um and offer very high thermal sensitivity (0.015
degrees Celsius)(32 degrees Fahrenheit). |
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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 Kelvin (-208 degrees Celsius)(-342
degrees Fahrenheit), which puts an added load on the cooling device inside
the camera. |
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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 longwave 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. |
| Reimaging
Lens Design |
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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 an 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). |
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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. |
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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. |
| Variable
Integration Time |
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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. |
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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 450 degrees Celsius (842 degrees Fahrenheit) simply by using VIT. |
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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 -10 degrees Celsius(14 degrees Fahrenheit) to 1500 degrees Celsius(2732
degrees Fahrenheit). |
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