4. Other detector technologies?
CMOS
Complementary Metal Oxide Semi-conductor (CMOS) detectors have come under considerable
scrutiny over recent years. In a CMOS detector, each individual photosensor or more
typically each column of photosensors has an amplifier associated with it. A row
of pixels can be readout in parallel with the row selected by an addressing register
or an individual pixel can be selected by column multiplexer. A CMOS device is essentially
a parallel readout device and therefore can achieve higher readout speeds particularly
required by imaging applications. CMOS detector technology however still requires
considerable development to compete against CCD for performance in scientific applications.
To achieve the parallel readout the CMOS amplifier uses multiple amplifiers, each
with its own gain, linearity and noise performance variation. Compensating for the
variations in the current state of the art CMOS devices is difficult over a wide
range of illumination levels and to the accuracy required by scientific applications.
High speed readout with high sensitivity can be achieved by EMCCD cameras.
Hybrid
Hybrid sensors, which combine CCD and CMOS technologies, can potentially deliver
performance superior to either CCD or CMOS bulk detectors. They look to be an interesting
longer term option, but there is still a considerable amount of development required
before they can be commercially viable. In particular, to overcome the issues associated
with compensating for the variation of the multiple amplifiers. Furthermore, they
will not be able to show read noise minimization to nearly the same degree that
either EMCCDs or ICCDs are able to, especially in dynamic imaging applications,
so have application limitations (e.g. they will not be capable of single photon
counting).