Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 123

showninFig. 4.4 b is the 3CCD; like its name, it uses three CCD sensors with a set
of dichroic filters that split the image on to them [ 17 ]. Fig. 4.4 c shows the method
probably most common for high-end photography and video imaging to date. It is an
improvement to the typical method of a single-color array sensor (Fig. 4.4 c) where
each pixel of the device includes four real pixels on the array, in which each of them
has a color filter that is optimized for green, red and blue. Two of the filters are
green, one blue, and one red with a spectral response that imitates the human color
vision. In order to construct the image, an optimized spatial interpolation filter is
used to build the color image at each point.
This method, which was limited to the basic RGB colors, was lately boosted by
significant improvements of patterned dichroic filters on the area of the sensor [ 18 ].
The device called dichroic filter array (DFA) can, in principle, have a much larger
number of filters. It was demonstrated by selecting many sets of 2 3 pixels. One
of the pixels is left without a filter, and the other five have, each, a dichroic filter
with a desired bandwidth that can be tailored respective to the application. Such
a system was demonstrated lately [ 19 ] presenting a combination of 2 2 filter to
measure oxy- and deoxyhemoglobin. The filters were chosen to be at wavelengths
of 750, 772, 802, and 834 nm with a full width half maximum (FWHM) of 20 nm.
A CCD with 2;300 3;500 original pixels of 10 10 m was used; despite the size
of the pixels, it still provided an excellent resolution and large field of view, so that
a successful analysis of oxygen level in a tissue was demonstrated.
This method certainly has a potential for further improvements, especially with
the new generation of high quality scientific grade CMOS cameras that have a
relatively small pixel size.

4.4.1.1 Design Principles

Figure 4.5 shows two common implementation methods for simultaneous measure-
ment of a limited number of selected spectral bands. Figure 4.5 a demonstrates
simultaneous two-color imaging, using a rather simple wedged dichroic mirror.
Such a method was demonstrated [ 20 ] with a 3 ı wedged dichroic (Chroma
Technology) and was used for high sensitivity fluorescence measurements. The
focusing lens is mounted before the dichroic mirror, but it can also be located in
between the wedged dichroic and the CCD. The wedge angle together with the lens
focal length determines the spatial separation of the two images on the CCD and
can be designed to provide the desired separation, based on the field of view and the
spatial resolution.
Figure 4.5 b shows another implementation for simultaneous imaging of up to
four spectral bands. This setup is based on a set of dichroic filters combined with
mirrors. Small tilting of the mirrors provides the separation of different spectral-
band images on to the CCD. Although the figure shows the images one next to
another, a change in the optic setup enables folding the four images to form a square.
Such system, separating two spectral bands, is available from various sources,
such as OptoSplit (Cairn, UK), DV 2 (Photometrics, USA), and U-SIP (Olympus).