Sensor Sense: CMOS image sensors
CMOS (complimentary metal-oxide semiconductor) imaging technologies arrived on the scene about the same time as CCD (charge-coupled device) imagers.
However, it has been only within the last decade that
CMOS imaging has started to rival that of CCD. Both are widely used in
machine vision, but users still debate which technology is best.
Each technology tends to satisfy basic needs for the majority of
machine-vision applications. CCDs create higher image quality, produce
better signal-to-noise ratios, and possess greater reproduction
repeatability. On the
other hand, CMOS imagers
usually cost less,
consume less power,
and make for smaller
systems.
While CCD imagers
work by photon capture
(see last month’s
Sensor Sense), CMOS
imagers typically use
a photosensitive diode
or transistor that
changes conductivity
when struck by light. Each lightsensing
element, or photosite,
needs a corresponding electronic
control circuit to read
the amount of light striking it.
The circuit usually contains a
charge-to-voltage converter,
a sample-hold system, noise-correction
circuits, and multiplexers to synchronize and sequence the
photosite output with all of the other photosites on the imager.
With all their control circuitry residing on the same silicon wafer,
CMOS imagers typically do not need the kind of external electronics
that supports CCD imagers. However, CMOS control circuitry is complex.
Compared to a comparable CCD chip, the CMOS device has less
area available for light capture. Often its light-sensitive area is less
than 25% of the imager’s total surface area. This is one reason CMOS
may not be the first choice where low light levels predominate. The
photon-capture ability of CCDs performs an integration role to deliver
a more-robust image with low light levels.
Some CMOS imagers have a collection of tiny lenses, called a microlenticular
array, that covers the entire surface. The array focuses
light gathered over larger areas onto the sensitive photosites to boost
capture area and improve light sensitivity.
Finally, CMOS may not be the best way to see supersmall features.
CCDs work better where the geometries of the feature under inspection
need subpixel (typically less than 7-μm) accuracy.