CHAPTER 11 Digital Microscope Cameras 63
are moved by electrical signals out to an electron voltage conversion
station. This architecture is the reason for the name: the charges
are moved from one bucked to the next, and they are coupled to each
other - thus, a Charge Coupled Device. The conversion station sits in
one or multiple corners of the sensor area. The collected raindrops
(the size of electron packets) get measured per pixel in this converter
and translated by an electronic analog-to-digital converter into digital
numbers that are sent to your computer for storage in an image file,
for display by your graphics adapter on your computer screen or for
further processing. This exposure / readout / transmission / storage
procedure happens for each single image and needs to be executed
really
quickly to be ready for the next image. If your sensor holds a lot
of pixels, it needs to be very fast to be ready with no noticeable delay
for the next exposure. Modern digital cameras are no longer restricted
to the speed values of the old video standards, which produced
25 frames per second (PAL-Standard) or 30 frames per second (NTSC
Standard). Modern cameras can go much faster, or go slower at
higher pixel counts.
So if you require high frame rates, all these readout steps must be
very short, and the same goes for the exposure time. Therefore, at
high frame rates the time to collect photons must be short, and that
means the signal will get dimmer since fewer photons can be collected.
To counter this, increase the light intensity of your lamp – but be
aware – there are limits. For example, if you are looking at living cells,
it might get so hot in the focus point, that the cells get cooked and die.
Or you might have a camera with large pixel buckets, that can collect
more photons. Or a highly noise-free camera, which can see the least
number of photons over the noise. Or you could just go slower and
give the camera more time to collect the photons.
The digital numbers represent the intensity signal of the photons: the
more photons, the higher the digital number. Dark parts of the image
show a signal close to zero, whereas bright intensities show a number
up to 256, which fits to 8 bit. More sophisticated cameras can
resolve even more signal intensities and represent these in 14 bit or
more. The dynamic
range refers to the darkest part and the brightest
part in a single image which can be handled by a digital camera device.
This is an important quality parameter since it enables you to see
very dim structures in your samples, while bright parts are still visible.
The sensitivity of a camera is limited visually by the signal intensity,
which is still detectable in the background noise. This is unavoidable
in technical systems. Minimizing these noise sources requires intense