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esolution multispectral images, and outline the methodology employed to operate
it.
2. ‘Narrow-band’ multispectral imaging
Compared to RGB imaging, which is based on the theoretical framework of
colorimetry [7] and therefore ‘synthetizes’ color stimuli from the contributions of
objects, environment, and observer, multispectral imaging attempts to estimate
objects’ reflectances. It is therefore unaffected by the typical problems of RGB
imaging, including device-dependency [1], metamerism [7,1], and accuracy
limitations of the device sensors [8,9]. In fact, despite the availability of
colorimetric device-independent color spaces and international standards such as
ICC profiles [10] and sRGB [11], multispectral imaging remains the only way to
achieve complete independence from both the environment and observer.
Motivations for the use of multispectral imaging can be found in everyday
experiences like the phenomenon of metamerism, which shows that there exist
different ‘physical’colors (spectra) that sometimes get the same colorimetric
representation. At the same time, physics outlines that while color representations
in RGB imaging and colorimetry use parameters whose ultimate physical
significance is that of measuring the amount of light energy that is ‘registered’ by
the sensors considered (both human and electronic), such parameters have only an
indirect relationship with the fact that the objects observed are actually able to
reflect light towards those sensors.
The aim of multispectral imaging is then that of describing this ‘ability’ of color
surfaces as modelled by their reflectance function; as this function depends on the
physical properties of the surfaces considered, it is also much more invariant than
environmental conditions and observers sensitivity, and therefore more
‘fundamental’.
In general, the acquisition performed using a given sensor will return a value a in
the form
(1) = E(
) R(
λ)
S(
λ)
26
λ
2
∫
a λ dλ
.
λ
1
This value integrates contributions from the energy E that reaches the physical
sample observed, the color reflectance R of the sample, and the ‘sensitivity’ S of
sensor. The integration with respect to the wavelength λ is performed in the range
λ1 to λ2 of the sensor's sensitivity; if this range exceeds that of the visible light
spectrum, then appropriate steps must be taken to cut unwanted radiation off.
To obtain an estimation of the reflectance R, two different approaches are currently
used in multispectral imaging [12]. On one hand, direct measures of these values
can be attempted if the device’s sensors are sensitive to a very narrow wavelength