Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 324

Fig. 9.10 Basic diagram of a
polarization imaging system

The design of an optical system for fluorescence analysis must consider the
entire optical path because fluorescence signals are usually very weak. The low
fluorescence signal level requires a highly efficient optical system to improve light-
capturing abilities for higher throughput and to provide a higher dynamic range
to accommodate vast differences in fluorophore concentrations across the tissue.
To optimize fluorescence signal detection, excitation filters should be designed to
maximize blocking in the transmission passband of the emission filter in the
illumination path and to maximize blocking in the corresponding transmission
passband of the excitation filter in the detection path. In general, it is preferable
to block out-of-band light with an excitation filter instead of an emission filter so
that the sample will be exposed to less radiation. For systems whose excitation
and emission paths share common optical elements, such as a microscope objective
lens, and for systems whose excitation and emission paths do not share the same
elements but it is not appropriate to place the emission filter in front of the detection
path, special attention should be paid to the autofluorescence properties of optical
materials.
As a general guideline, the detection path should have as few optical elements
as possible to increase light transmission and minimize the autofluorescence of the
optical components. The same requirement is also desired for the excitation path.

9.3.3 Polarization Imaging

Figure 9.10 is a basic diagram of a conventional polarization imaging system.
A linear polarizer is placed in the illumination path to ensure a linear polarization
of illumination, and a second polarizer, with an orthogonal transmission axis, is
used as an analyzer, positioned between the sensor and the tissue. The specularly