Broad-band color filters with arbitrary spectral transmittance
using a Liquid Crystal Tunable Filter (LCTF)
Satoru Toyooka, Kazuhiko Kurashiki, Kanae Miyazawa, and Markku Hauta-Kasari *
Graduate School of Science and Engineering, Saitama University,
255 Shimo-okubo, Urawa-shi, Saitama-ken 338-8570, Japan
*
Department of Computer Science, University of Joensuu,
P.O.Box 111, FIN-80101 Joensuu, Finland
We propose a rewritable transparent broad-band color filter system whose transmitting wavelengths can be
changed arbitrarily using a Liquid Crystal Tunable Filter (LCTF).
Holding -time for each transmitting
wavelength of the LCTF is controlled corresponding to a computationally designed filter function, and a
time-integrated intensity image is taken with a CCD camera. The optically implemented filter functions and
the expected ones corresponded well. The filter system is easy to carry and can be controlled only electrically.
1. Introduction
The spectral based color image analysis has recently been progressed rapidly, and it is used, for example, for machine
vision, 1 telemedicine, 2 agrobiology, 3 and art painting reproduction. 4 Spectral images can be obtained, for example,
with a CCD camera through narrow-band interference filters, an acousto-optical tunable filter, a liquid crystal
tunable filter, a prism-grating-prism based line camera, or Fourier-transform-based methods. In these multispectral
methods, spectral images are measured precisely for each wavelength range. In any case, however, many images
have to be taken, depending on spectral resolution, and a large number of image data have to be processed and stored.
For spectral image compression, there are techniques for designing a low-dimensional color filter set, which can be
directly used in optical implementations. 5 The color filters are broad-band filters and they are adaptive to various
applications. The optical implementation of these color filters makes the acquirement of the compressed component
image set possible. 6,7 This low-dimensional component image set can be directly used in optical pattern recognition
tasks.
In this study we present the experimental setup for the imaging system to implement the broad -band color filters with
arbitrary spectral transmittances using the LCTF. The experimental results of the filter implementation are presented
and they are compared with the expected ones.
2. Experimental setup
An experimental setup for a proposed rewritable broad -band color filter system using the LCTF (CRI, VariSpec) is
shown in Figure 1. An intensity image of an object illuminated by a white light source is taken with a monochrome
CCD camera through the LCTF. The CCD camera, a lens and the LCTF are attached together. A standard white
board was placed on the object plane and it was illumina ted by a white light source. The purpose of this experiment
is to check the accuracy of the optically implemented broad -band color filters.
In order to implement broad-band filters with arbitrary spectral transmittance, we controlled the holding -time for
each narrow-band corresponding to a computationally designed filter function, and a time-integrated intensity image
was taken with the CCD camera. For one broad-band color filter used in our experiments, the LCTF was controlled
at the wavelength range from 450nm to 650nm as follows: First, the central wavelength of transmitting light of the
LCTF was set to 450nm, and held for 16 seconds. The shutter of the CCD camera was open during the holding time
and a time-integrated intensity image was taken with the camera. Then the LCTF was set to 460nm, and held for 11
seconds and the time-integrated intensity image was taken with the camera.
The holding times were calculated
computationally for each narrow-band a priori and then the similar procedure was repeated until 650nm. In this
experiment, total exposure time was 120 seconds.
ccd camera vision freezer
lctf
object
white light source
computer
Figure 1: Experimental setup for a broad-band color filter system using the LCTF.
Finally, the optically implemented 21-component color filter functions were constructed by averaging the 10 x 10
pixels areas in each of the 21 intensity images. The accuracy of this system was comparable to that of the systems
proposed in Refs. 8, 9.
3. Conclusions
We presented a new optical setup for implementing broad -band color filters with an arbitrary spectral transmittance
using the LCTF. The optically implemented color filters corresponded well with the expected ones. This system can
implement arbitrary spectral transmittances, and therefore, the computationally designed color filters for various
applications can be implemented optically. The system can be used to acquire the low -dimensional component image
set for representing the spectral data. The low-dimensional component image set can be also directly used in pattern
recognition tasks.
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