アイテム詳細

要旨

Noise due to dark current is a serious limitation for taking
long exposure time images with a CCD digital camera. Current
solutions have serious drawbacks: interpolation of pixels with high
dark current leads to smoothing effects or other artifacts --
especially if a large number of pixels are corrupted. Due to the
exponential temperature dependence of the dark current, dark frame
subtraction works best for temperature controlled high end CCD imaging
systems.

On the physical level, two independent signals (charge generated by
photons hitting the CCD and by the dark current) are added. Due to its
random distribution, adding (or subtracting) the dark current noise
signal increases the entropy of the resulting image. The entropy is
minimal if the dark current signal is not present at all.

A dark frame is a good representation of the dark current noise. As
the generated dark current depends on the temperature equally for all
pixels, a noisy image can be cleaned by the subtraction of a scaled
dark frame. The scaling factor can be determined in an optimization
step which tries to minimize the entropy of the cleaned image.

We implemented a software system that effectively removes dark current
noise even from highly corrupted images. The resulting images contain
almost no visible artifacts since only the noise signal is removed. This
extends the range of usable exposure times of digital cameras without
temperature control systems by about one to two orders of magnitude.