64 PART IV Digital Documentation
engineering and, combined with speed performance and sensor size,
drives up the price of a scientific camera.
CMOS is the abbreviation for Complementary Metal Oxide Semiconductor.
Wow! Basically, this is the technical term for a manufacturing
processes for electronic circuitries on a silicon substrate and does not
describe any function of an image sensor itself. Originally it was used
to build analog and digital integrated circuits. But as this technology
is really capable of defining a whole lot of functionality on a small
area of silicon and is also using the silicon base material, which is sensitive
to visual light, engineers worked hard to combine the functionality
of a light detector and its support electronics in one device. This
was a complicated process as digital electronics cause a lot of noise
and a lot of unwanted side effects. Light detecting structures can pick
it all up as disturbing image artefacts. But finally, we have produced
fast, large and sensitive CMOS image sensors. The basic principle of
a CMOS image sensor is the same as with a CCD: it offers a pixel matrix
structure, where the pixels are responsible for converting the light
into an electron-based charge package. However, no charge packages
are moved around, because the conversion from photo-generated
charge into a voltage signal is done directly in each single pixel and
not in the corners of the sensor. And below each column of pixels,
the voltage signals are converted into digital numbers in parallel with
many on-chip A/D converters, which can now be implemented on
the same chip. So the conversion can be much faster and at short exposure
times, higher frame rates can be reached. In addition, even all
the support electronics can be implemented on the same chip. That
means the cameras can be built much cheaper and the whole camera
needs less power overall. As soon as the noise performance was
exceeding the levels of CCD cameras, CMOS took over and is now
the main technology used in digital camera systems.
Color or Monochrome Cameras?
Before talking about the relative advantages and compromises of color and monochrome
cameras, we must first ask how cameras based on these types of sensors
can detect color signals at all! To answer that question, remember that light can
be described as an electromagnetic wave with a given wavelength. And don’t forget
that our eyes can see specific ranges of wavelengths as colors. So the trick now
is to get a CCD or CMOS sensor (same principles apply to both) to filter out some
of the wavelength bands and assign them color information such as red, green
and blue, which can be used to describe the universe of colors.