Scientific Cameras Push Perfection
Manufacturers continue to increase frame rates while improving sensitivity.
A scientific camera does much more than simply capture a picture of an object under study. If
an acquired image is to reliably reflect measurements of chemical concentration,
cell growth, etc., the digital values associated with each pixel must directly relate
to the amount of light detected. It is imperative, therefore, that cameras designed
for scientific imaging maintain this quantitative relationship between input and
Although any digital camera that preserves this
relationship might be considered scientific, these cameras distinguish themselves
based on their ability to do so at very low light levels, over a wide dynamic range
and/or at high frame rates. High-performance scientific cameras are engineered to
deliver photon-noise-limited imaging down to near-single-photon input levels. Ultimately,
the perfect scientific camera would be capable of detecting every incident photon,
with precision limited only by photon shot noise across the entire dynamic range
of the image.
Scientific cameras allow
researchers to image individual green fluorescent protein molecules in a polyacrylamide
gel. The data is from the lab of W.E. Moerner, department of chemistry, Stanford
High-performance scientific cameras
improve sensitivity by running the CCD slowly to minimize on-chip amplifier noise.
Over the past several years, improvements in amplifier design and signal processing
have boosted attainable frame rates. State-of-the-art on-chip amplifiers provide
camera noise floors of three to six electrons at digitization rates ranging from
1 to 20 MHz. Quantitative performance, meanwhile, is preserved over a full 16 bits
of dynamic range with very high linearity.
A new approach to reducing camera noise
even more is being pioneered by Marconi Applied Technologies in Elmsford, N.Y.,
and Texas Instruments Inc. in Dallas. These CCDs provide a gain structure prior
to the on-chip amplifier. When the amplifier noise is referenced to the light input,
it is effectively reduced by the gain factor. An on-chip gain of 100 times reduces
the effect of amplifier noise by 100 times, so that a noise floor of 10 electrons
would drop to an equivalent 0.1 electrons.
However, this strategy introduces excess
noise during the charge-multiplication process. Although the on-chip gain should
allow single-photon events to be detected (providing incredible sensitivity), the
excess noise means that the number of photons measured is more uncertain. Fortunately,
the gain mechanism can be turned off when additional precision is required. Improved
cameras based on the next generation of this technology are slated for release in
New detection technologies are also
being used to overcome the traditional compromise between keeping sensor noise very
low and running a CCD output amplifier as fast as possible to obtain high pixel
rates. Reading the CCD through multiple output ports is one solution to this performance
trade-off. The parallelism increases frame rates, but the implementation of associated
off-chip electronics becomes increasingly less efficient as the number of outputs
grows beyond even a few. CMOS sensors excel in this area. Amplifiers and analog-to-digital
converters can be placed on every column, increasing the parallelism during readout
by 1000 times or more for a large sensor. For low-light applications, however, CMOS
does not yet have the low-noise amplifier performance and high quantum efficiency
required for low-light scientific imaging.
System integration is also driving
innovation in scientific imaging. Many design trends are found in the consumer
camera market. For instance, numerous companies are adopting FireWire to simplify
the computer/camera interface. Also, 12- to 16-bit analog front ends are facilitating
the placement of much of the difficult low-level signal processing into a single
integrated circuit, allowing smaller, less expensive packages. Some popular CCDs
now come in hermetically sealed packages with integrated thermoelectric coolers.
As manufacturers continue to push the
limits of performance, they must be careful not to sacrifice the quantitative advantage
that underpins the cameras they seek to perfect.
Meet the author
Robert LaBelle is head of life science business
at Roper Scientific Inc. in Tucson, Ariz.
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