Glossary
GUPPY Technical Manual
V4.0.1
218
Fixed pattern noise
abbr.
FPN
If the output of an image sensor under no illumination is viewed at high
gain a distinct non-uniform pattern, or fixed pattern noise, can be seen.
This fixed pattern can be removed from the video by subtracting the dark
value of each pixel from the pixel values read out in all subsequent frames.
Dark fixed pattern noise is usually caused by variations in dark current
across an imager, but can also be caused by input clocking signals abruptly
starting or stopping or if the CCD clocks do not closely match one another.
Mismatched CCD clocks can result in high instantaneous substrate currents,
which, when combined with the fact that the silicon substrate has some
non-zero resistance, can cause in the substrate potential bouncing.
The pattern noise can also be seen when the imager is under uniform illu-
mination. An imager which exhibits a fixed pattern noise under uniform
illumination and shows no pattern in the dark is said to have
light pattern
noise
or
photosensitivity pattern noise
. In addition to the reasons men-
tioned above, light pattern noise can be caused by the imager becoming
saturated, the non-uniform clipping effect of the anti-blooming circuit,
and by non-uniform, photosensitive pixel areas often caused by debris cov-
ering portions of some pixels.
FOV
FOV =
f
ield
o
f
v
iew
see field of view
FPN
FPN =
f
ixed
p
attern
n
oise
Related with the dark current is its electrical behavior to be regionally dif-
ferent on the sensor. This introduces a structural spatial noise component,
called fixed pattern noise, although it’s not meant temporal, visible with
low illumination conditions.
FPN is typically more dominant with CMOS sensors than with CCD, where it
can be ignored mostly.
This noise nfpn [%] is usually quantified in % of the mean dark level.
Frame
An individual picture image taken by a digital camera. Using an interlaced
camera, a frame consists of 2 interlaces fields.
Frame grabber
A component of a computer system designed for digitizing analog video
signals.
Frame rate
Frame rate is the measure of camera speed. The unit of this measurement
is
frames per second
(fps) and is the number of images a camera can cap-
ture in a second of time. Using area of interest (AOI) readout, the frame
rate can be increased.