Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 118

These parameters also depend on the shape of the measured spectrum. A sharp
laser line has a better detectable signal because the energy is concentrated at a
single wavelength when compared to a broad spectrum with equal energy which
is distributed over a large spectral range.
4. Spectral response
The spectral response of a system describes the efficiency of the detector in
translating photons at a given wavelength from the sample to intensity counts by
the system. It does not refer to the efficiency of the fore-optics, but to the spectral
transmission and response of the detector, which usually have a large effect on
the response. In addition, some of the spectral dispersion methods also affect the
response, sometimes in a significant manner. Some of the methods are limited
to a range where, min 2 max , for example, the spectral range can only be
between 400 and 800 nm. In that case, the spectral response will fall dramatically
around the edges or it may lead to artifacts in the measurement.

4.3 Spectral Imaging System Configuration

Spectral imaging combines the two methodologies, spectroscopy and imaging.
Whereas imaging provides the intensity at every pixel of the image, I.x; y/,and
a typical spectrometer provides a single spectrum, I./ (intensity at every given
narrow wavelength), a spectral image provides a spectrum at each pixel, I.x; y; /.
This is a three-dimensional (3D) data set and can be viewed as a cube of information.
One can consider I.x; y; / as either a collection of many images where each one
is measured at a different wavelength or as a collection of many spectral values at
each pixel (Fig. 4.2 ).

4.3.1 The Principles of Spectral Imaging Systems

Spectral imaging requires the combination of a dispersive element (or method) to
sequentially obtain the spectral information with an imaging system that measures
those spectra in an ordered manner.
The data collected at each pixel is the intensity at each wavelength, I.x; y; /,see
Fig. 4.2 . To date, there is no such three-dimensional (3D) detector that can collect
the whole spectral imaging simultaneously, at least not for a spectrum with a typical
resolution which is made of 10-100 separated wavelengths. Therefore, it is clear
that the spectral cube of information must be collected in few measurements after
dividing it somehow in a way that depends on the optical method that is being used.
The highest-rank detector available to date (except for a three-color method, as
will be explained later on) is a two-dimensional (2D) array detector, either a CCD
or CMOS, and with such detector, the spectral image cannot be acquired at once. If
a lower-dimension detector such as a line detector or a single-point detector is used,
the spectral image acquisition time will take even longer. This problem resulted