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 Cameras
from Princeton Instruments are the only
deep-cooled, CCD-based systems in the world
that provide guaranteed high-vacuum performance
for the entire working lifetime of the detector.
Exclusive XP technology is available in
both the Princeton Instruments PIXIS® and
ProEM product line. Dark current rates of
0.001 to 0.002 e-/p/s, along with system
read noise as low as <1 e-rms for the
ProEM and 2.5 to 2.6 e- rms for the PIXIS
to give all users truly superb sensitivity.
The Need to Be Cool
The classic
struggle to minimize dark current in high-performance
CCDs is well known. Thermally generated
dark current, which grows linearly as pixel
area increases, is inversely proportional
to temperature. As a rule of thumb, it is
halved for every 5 to 7°C of additional
cooling. Keeping a detector’s dark current
— and associated dark noise —low is one
of the elements critical to the success
of scientific imaging and spectroscopy applications.
Cool Then, Cool
Now Historically,
several methods have been available to achieve
deep cooling, such as the use of thermoelectric
devices, liquid nitrogen, or external cryogenic
compressors. Of these techniques, thermoelectric
cooling offers the most convenience — but
only if designed and implemented properly.
Presently, there are several other manufacturers
offering cameras that claim -70 to -90°C
cooling. However, these systems must employ
forced circulation of water as cold as +10°C
to achieve it. The additional requirement
for external water circulation eliminates
the biggest benefit of thermoelectric cooling
— convenience. Moreover, a +10°C water supply
is not particularly common in a typical
lab setting. Even when it is available,
cold water can lead to unwanted problems
(e.g., condensation).
The Vacuum
To achieve deep
cooling, a world-class vacuum chamber needed
to be designed to house the CCD. The contents
of the vacuum chamber had to be minimized
to keep the thermal load and out-gassing
as small as possible. An innovative “getter”
material was used to absorb any residual
out-gassing, albeit minute, that might occur
over time. The XP design also called for
customized UHV (ultrahigh vacuum) components
made of stainless steel. Additionally, the
metalglass interface between the chamber
and the input optical window was brazed,
hermetically sealing the entire detector.
Next came the thermoelectric cooler itself.
Princeton Instruments engineers worked to
create a Peltier device that would keep
the CCD at a very low temperature while
also stabilizing a “control” point to minimize
the variation in dark current. Princeton
Instruments addressed these criteria by
designing a “feedback” circuit that can
provide ±0.05°C stability across a wide
temperature range.
Manufacturing Expertise
While
designing the world’s best CCD vacuum technology
involved overcoming a number of formidable
challenges, all the hard work would be for
naught if the XP detectors could not be
manufactured reliably. Fortunately, years
of manufacturing experience at Princeton
Instruments made this task a relatively
easy endeavor, even though the process is
actually quite complex. For starters, all
vacuum parts are stored in special, “dry”
containers until they are handled. The vacuum
processing is done using oil-less "molecular"
pumps. During vacuum processing, which is
one of the longest steps in the detector
assembly, the seals are continually checked
for leaks. Once assembled, every detector
undergoes a strictly defined, comprehensive
inspection to ensure that rigorous specification
limits are met. This process ensures the
best performance and reliability available
on the market today.
Single
window for maximum QE and sharper images
Princeton
Instruments cameras utilize only one anti-reflective
coated window; this reduces the chance of
multiple reflections, stray light and interference
patterns in the optical path degrading the
image. Princeton Instruments uses only windowless
CCDs and handles them in a strict clean
room environment. While this obviously involves
greater time and expense, it is the only
way to produce cameras which deliver the
ultimate in sensitivity, spatial resolution
and image MTF.
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