Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 17

and the signal is read out with the shutter being closed. The scan speed is limited by
the shutter opening and closing compensation time. For some applications, shutter
is not needed because no light falls onto the CCD during readout. But “image
smearing” may occur without a shutter for slow scientific CCD cameras, because if
light keeps falling onto the CCD during readout, charge will continue to accumulate.

1.2.5.3 Software Binning and Hardware Binning

Binning is a process of combining the data of adjacent pixels to form a single output
called superpixel. It is commonly used in spectroscopy, particularly for low signal
detection such as fluorescence or Raman. The commonly used is hardware binning,
which is performed before the signal is read out. For hardware binning, the signal-
to-noise ratio (S/N) can be improved linearly with the number of pixels binned if
the signal is readout noise limited. For shot noise-limited signal, the S/N ratio is
improved roughly proportional to the square root of the number of pixels grouped
together. However, hardware binning is limited to the rectangular groups of pixels,
and the total number of pixels to be binned is limited by the well capacity of the shift
register. If the total charge binned together exceeds the capacity of the shift register,
the data will be invalid due to signal saturation, such as a Raman measurement with
very strong fluorescence background.
An alternative way is software binning, which is performed after the signal
readout. Software binning has more flexibility but is not as fast as hardware binning.
It is not limited to the rectangular groups of pixels. If there is any image aberration,
the pixels can be shifted before software binning. Software binning is not limited
by the well capacity of the shift register either, so higher signal may be produced
than hardware binning. However, the maximum improvement in S/N ratio is only
the square root of the pixels binned no matter the signal is readout noise limited or
shot noise limited. For some applications, a combination of hardware binning and
software binning can be used for improving the S/N ratio.
Except the intensity issues, conventional spectrograph equipped with a straight
slit also results in a parabolic-shaped image [ 24 ]. The parabolic shape arises from
the fact that rays from different positions along the length of the slit are incident on
the grating at varying degrees of obliqueness. The displacement of the image along
the slit is given by [ 24 ]
x D f 2 tan sin 1 sin ˛ sin 1 sin ˛

;

d c

(1.6)

where c D d cosŒtan 1 .y=f 1 /, f 1 and f 2 are the focal lengths of the input and
output lenses, ˛ is the incident angle, is the wavelength of the incident light, and d
is the grating fringe spacing. x is the displacement of the image, and y is the position
away from the slit center along the slit. An example of the displacement is shown
in Fig. 1.9 . It can be seen that the distortion is more severe for longer wavelength.
This distortion is also dependent on the focal lengths of the input and output lenses.