Zeiss LSM 880, Operating Manual

Page: 128 / 690

CHAPTER 1 - SYSTEM OPERATION
ZEISS Left Tool Area and Hardware Control Tools LSM 880
122 000000-2071-464 10/2014 V_01
(2) Analog (Grey Value) Display
In this display, photons are transformed into grey values, which are displayed in discrete intervals in the
histograms. Intervals can be calculated by:
Interval = (pixel period x frequency) / (2n-1) = (pixel period x CRmax/digital gain) / (2n-1),
where n = 8 for an 8 bit image and n = 12 for a 12 bit image (see Fig. 161 for an example).
Fig. 161 Analog display in photon counting image. Settings were digital gain 1, scanning speed 8
corresponding to a pixel period or pixel dwell time 1.52 µs, 8 bit image
Example:
With the GaAsP detector (Big.2), software limits the maximum count rate to 4 Mhz = 4 photons / µs.
At scanning speed 8 with a pixel dwelling time of 1,52 µs one can obtain a maximum of 4 x 1,52 = 6,08
photons. Since photons are integer, this value must be rounded down to 6 photons maximum / pixel.
The spacing interval is calculated for the 8 bit image as 255/6=42.5. This value is rounded down to the
next fifth value, which is 40.
So 1 photon will correspond to a grey value 40, n photons to a grey value n x 40, with a maximum
n of 6).
Grey value 0 is reserved for 0 photons, 255 for any overflow. In this way, the histogram is stretched to
cover as much as possible of the available grey scale. Therefore, even at lesser or greater photon counts,
the image will maintain its brightness.