Biomedical Engineering Reference
In-Depth Information
ranges are possible). This way, color filters can be designed as well as spectral
imaging systems. The advantage of this novel technique is that the SP arrays are
relatively small in size (few micrometers per each color array), they do not apply
high loss of energy, and also the light that is dispersed is already polarized. There
is no current application based on this technique, yet this miniaturization of the
dispersing element and the freedom to design variable elements on tiny arrays can
become useful in future spectral imaging applications.
Another late development includes variable filters which are based on spectral
selectivity that depends on the filter rotation angle (VersaChroma, Semrock, USA).
These filters change the transmitted spectrum as a function of the angle of the
filter. The performance of the filters is high; by changing the angle of the filter, the
transmission spectral band changes, but the spectral band stays intact. The spectral
range of the filter is approximately 12 % of its central wavelength, for example,
490-555 nm with a bandwidth of 25 nm. Although limited to a specific spectral
range, these filters demonstrate the ever-growing number of techniques that can be
combined with spectral imaging.
4.4.2.3
Advantages and Limitations of Wavelength-Scan Systems
One obvious advantage of spectral imaging systems that are based on wavelength-
scan is the flexibility to select for each measurement the optimal spectral range. The
downside is the low transmission rate of these devices (typically reaches only 35-50
%, depending on the type of device and model). On the other hand, systems that are
based on fixed filters have very high transmission that can reach almost 100 % yet
the spectral range can be tuned only by change of filter set.
The trade-off between the abovementioned spectral imaging methods can be
viewed through a basic application used to determine the percentage of oxygen in
the blood. Its prerequisites limit detection to the spectral range of 520-600 nm so
that in principle three well-selected spectral ranges should suffice.
By using fixed filters, the spectral bandwidth of each filter is hardware dependent
and cannot be changed, which means that for any other application a different set
of filters is required. In contrast, both LCTF and AOTF provide variable filtering
schemes with no moving parts and a higher speed with respect to the switching times
of fixed filters. In addition, advanced AOTF systems provide a variable spectral
resolution as well. See, for example, the review by Gat [ 28 ].
Another advantage of filter-based systems is the ability to select a different
exposure time for each spectral band (filter). This can increase the overall quality
of the spectral image when the measured intensity from the image is not uniform
(in general) across the spectral range. One can also think about increasing the
dynamic range by measuring the image with each filter at two different exposure
times (or more). This can be important especially when there are varying intensity
ranges along the image; normally, high intensities enforce short exposure times,
thereby reducing the quality of the dimmer regions. Acquiring the same image at
varying exposure times can unveil specimen varied by intensity at equal quality.
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