Dark Charge - Teledyne Princeton Instruments Sophia-XO System Manual

Chapter 7
7.3

Dark Charge

Dark charge (or dark current) is the thermally induced buildup of charge in the CCD
over time. The statistical noise associated with this charge is known as dark charge
noise. Dark charge values vary widely from one CCD array to another and are
exponentially temperature dependent. In the case of cameras with MPP type arrays, the
average dark charge is extremely small. However, the dark-charge distribution is such
that a significant number of pixels may exhibit a much higher dark charge, limiting the
maximum practical exposure. Dark charge effect is more pronounced in the case of
cameras having a non-MPP array (such as deep-depletion devices.)
With the light into the camera completely blocked, the CCD will collect a dark charge
pattern, dependent on the exposure time and CCD temperature. The longer the
exposure time and the warmer the camera, the larger and less uniform this background
will appear. Thus, to minimize dark-charge effects, you should operate with the lowest
CCD temperature possible.
Do not be concerned about the DC level of this background. This is not noise but rather
a subtractable bias pattern. By acquiring/saving a dark charge background image under
conditions identical to those that will be used to acquire live data, this background
image can then be subtracted from the acquired image, thus reducing dark-charge
effects.
Although the dark charge pattern will be subtracted from the acquired image, both the
dark charge pattern and the acquired image include system readout noise, N
additive. Therefore, when acquiring a dark charge pattern, it is strongly recommended
that multiple frames of dark data be acquired and averaged since the cumulative noise
within the dark background is reduced by the square root of the number of frames that
have been averaged. The cumulative readout noise, which is the sum of the acquired
image readout noise plus the dark data readout noise, is calculated as follows:
where:
•
•
•
Exposure
!
CAUTION!
If a sudden change in the baseline signal is observed, there
may be excessive humidity in the camera vacuum enclosure.
Turn off the camera and contact Teledyne Princeton
Instruments Customer Support. Refer to
Information on page 136 for complete contact information.
N = Total Readout Noise;
RT
N = Readout Noise;
R
F = Number of Dark Pattern Frames Acquired.
Contact
2
N
2
R
= +
-------- -
N N
RT R
F
43
, which is
R