Spectral Instruments Series 800, Руководство пользователя

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Appendix C
Sensitivity And Attenuation
SICCD cameras are designed to use CCDs that have a very large intrinsic signal
capacity – or full well. The full well capacity is specified in electrons and ranges
from 40,000 to more than 500,000 electrons in each pixel depending upon CCD
type. A SICCD camera that employs a 16-bit ADC converts the signal from the
CCD into numbers that range from 0 to 65535. A 12-bit ADC converts signal of
the CCD into numbers that range from 0 to 4095. Clearly it is not possible to
accurately digitize both full well level signals and very low signals with one
camera sensitivity setting. The sensitivity is the relationship between the analog
to digital converter output (ADUs) and the number of electrons in a pixel and is
expressed as e
-
/ADU. Sensitivity values are required that range from less than 1
e
-
/ADU, in order to digitize very low level signals, to greater than 50 e
-
/ADU to
digitize full well level images.
SI cameras that run 15- or 16-bit digitizers are typically set up so that very low
level images can be accurately measured. Generally this means setting the
system gain so that one digital unit corresponds to about 1/2 electrons of signal
or, 0.5 e
-
/ADU. In order to also measure high light level images with the same
camera, SI cameras offer two software selectable attenuation levels which can be
invoked in order to decrease the sensitivity of the camera and thus digitize higher
signal level images. The range of sensitivity settings for your camera is provided
in the test report that is supplied with the camera.
The highest level of attenuation is state 3, the lowest, the default state, is 0.
Changing the attenuation from 3 to 0 decreases the conversion factor which
increases the sensitivity. This permits more accurate measurement of low light
areas in an image while giving up the ability to measure bright areas in that same
image because they will have saturated the measuring circuit.
Operating a SICCD camera at the lowest attenuation number provides the most
accurate measurement of background signals in low light level images. This is
the attenuation setting used to measure the intrinsic system noise.
Dual Slope Integrator Sensitivity
The sensitivity of a SICCD camera using a dual slope integrator as the analog
signal processing system is determined by two factors: a) the attenuation, which
switches among discrete levels, and b) the dual slope integrator setting. The dual
slope integrator (DSI) setting determines how long each pixel is sampled
(integrated) before it is digitized. The longer the output node voltage (the pixel
or super pixel signal) is integrated, the better the readout because the noise is
decreased.