Many Photometrics cameras operate in three distinct
detection modes that are specifically designed to
fully utilize the characteristics of the CCD. Before
describing each mode it is useful to define a few
System Gain: The number of photoelectrons
per analog-to-digital unit (e-/ADU).
Single-Pixel Full Well Capacity: The
number of photoelectrons an individual pixel can
contain before saturation.
Output Node: A summing well
just prior to the analog output of the CCD large
enough to accommodate the combination of charge
Output-Node Full Well Capacity:
The number of photoelectrons the output node can
contain before saturation.
"HIGH SENSITIVITY" MODE
system gain is set such
that the entire 0 to 4095 analog-to-digital converter
(ADC) range falls within the first 1/4 of the single-pixel
full well capacity. The camera is most sensitive
to light where only a slight change in light intensity
registers as a change in an ADU. This mode is best
suited for low-light applications.
"HIGH DYNAMIC RANGE" MODE
mode sets the system gain such that the entire single-pixel
full well capacity is realized. The ADC is adjusted
to a point where an ADU value of 4095 is reached
at the single-pixel full well capacity. This mode
measurements (comparing both bright and dim features
within the same image) at the full spatial resolution
of the camera.
"HIGH SIGNAL-TO-NOISE RATIO"
For a CCD camera, the maximum signal-to-noise
ratio (SNRmax) is a function of the full well
capacity of the sensor. Specifically, the SNRmax
is equal to the square root of the full well capacity.
Thus, the greater the full well capacity, the better
the maximum SNR. Typically, the output-node full
well capacity is much greater than that of a single
pixel. When this mode is selected and a minimum
of 2x2 binning is performed, the ADC is set to span
the output-node full well capacity. The high SNR
mode provides the highest precision when measuring
small intensity changes on a bright background.
(Gain) of a Kodak CCD
For a CCD with the
Single-Pixel Full Well Capacity: 80,000 electrons
Full Well Capacity: 160,000 electrons
The system gain would be as follows for each
12-bit detection mode:
High Sensitivity: 5 photoelectrons/ADU
Therefore, assuming a quantum efficiency of 40%,
only 12 photons are required per pixel to toggle
High Dynamic Range: 20 photoelectrons/ADU
The ADC spans the entire range of the single
pixel, enabling intensity differences of up to 4096:1
to be measured within the same image while taking
advantage of the CCDs full spatial resolution.
High Signal-to-Noise Ratio: 40 photoelectrons/ADU
A signal that varies as little as 0.5% on a bright
background can be detected with 96% confidence.
Each mode can be selected via software so that
changes can be made during an experiment or pre-programmed
for automated situations.