Multispectral technology

 

The multispectral technology offers higher color accuracy than conventional RGB cameras: more color channels are utilized to sample the visible wavelength range and the acquired spectrum can thus be estimated. Color images acquired with RGB cameras contains on the contrary a systematic color error, since no basic linear dependency can be found between the spectral sensitivity of RGB cameras (see Fig. 1) and the spectral sensitivity of human observers.

 

rgb_sensitivity mspec_sensitivity
Fig. 1: Spectral sensitivity of the 3 color channels of an RGB camera
Fig. 2: Spectral sensitivity of our multispectral camera with 7 color channels

 

Multispectral cameras are utilized in domains aiming at color accuracy like recording of fine art paintings, pre-press, industrial quality control, remote sensing, or medical science, where the spectra of cells can give information for cancer diagnosis for instance.

mspec_camera

Fig. 3: Multispectral camera at the institute, featuring a filter wheel between sensor and lens

 

A widely-used high-quality type of multispectral camera utilizes a monochrome sensor and optical bandpass filters that are generally placed in a filter wheel. Such cameras are used at the Institute of Imaging and Computer Vision (see also our facilities), with either 7 or 19 color filters. The sensitivity curves of the 7-channel camera are shown in Fig. 2. The filter wheel is placed between the sensor and the objective lens, see Fig. 3. Hence, the acquisition of a multispectral image consists of 7 or 19 images (or color channels) that are acquired successively. The spectrum is sampled by the color channels and can be reconstructed.

 

Geometric calibration and compensation of multispectral cameras

 

Because of the color filters positioned in the filter wheel, rays are refracted differently in each color channel and aberrations appear. They have been thoroughly analyzed in the project “Geometric calibration and compensation of multispectral cameras”. This project was funded by the German Research Foundation (DFG) from 2009 to 2012.

Moreover, stereo acquisitions with multispectral cameras made of RGB cameras and color filters have been studied. More information about this project is available here.

 

Multispectral goniometric imaging

 

In our new research topics, the algorithms and methods developed so far are further improved and applied to more complex materials and objects, whose spectral characteristics strongly depend on the angles. Such materials require a goniometric measurement, i.e., a measurement from different acquisition angles and with different illumination angles.

Our measuring setup enables acquisitions where the normal to the material’s surface, the measuring device and the light source are in a plane (in-plane measurement). A spectroradiometer as well as a multispectral camera can be utilized as measuring device. We use the 19-channel multispectral camera in order to achieve a better spectral accuracy. An example of goniometric multispectral acquisition is shown in Fig. 4. An effect paint with obvious angle dependence has been measured from different acquisition angles (“Aufnahmewinkel”) with an illumination at the angle +45°.

effektlack_goniomeas

Fig. 4: Multispectral images of effect paint for different acquisition angles

 

Contact

Tarek Stiebel

 

Publications

 

ei2014_ta Summary of our research concerning multispectral imaging, as part of the “Special Session in Memory of Til Aach”:
Julie Klein
Multispectral imaging and image processing
In: IS&T/SPIE Electronic Imaging: Image Processing: Algorithms and Systems XII
San Francisco, California, USA, February 2014
ca_exp_setup Julie Klein, Johannes Brauers and Til Aach
Spatio-spectral modeling and compensation of transversal chromatic aberrations in multispectral imaging
In: Journal of Imaging Science and Technology, vol. 55 (6), 060502, December 2011
ei2014 Julie Klein and Georg Schmücking
Analysis of aberrations and pixel information in goniometric multispectral imaging
In: IS&T/SPIE Electronic Imaging: Measuring, Modeling, and Reproducing Material Appearance
San Francisco, California, USA, February 2014
fws2013 Julie Klein
Correction of longitudinal aberrations in goniometric measurement with a multispectral camera
In: 19. Workshop Farbbildverarbeitung, p. 31-41
Ilmenau, Germany, October 2013
longab_unsupcorrection Julie Klein
Unsupervised correction of relative longitudinal aberrations for multispectral imaging using a multiresolution approach

In: IS&T/SPIE Electronic Imaging: Color Imaging XVIII
San Francisco, California, USA, February 2013
stereo_aufbau Julie Klein and Bernhard Hill
Multispectral stereo acquisition using two RGB cameras and color filters: color and disparity accuracy

In: 18. Workshop Farbbildverarbeitung, p. 89-96
Darmstadt, Germany, September 2012
stereo_colconstancy Julie Klein and Til Aach
Spectral and colorimetric constancy and accuracy of multispectral stereo systems

In: IS&T Sixth European Conference on Color in Graphics, Imaging and Vision (CGIV 2012), p. 239-246
Amsterdam, Netherlands, May 2012
monochromator Julie Klein, Johannes Brauers and Til Aach
Spektrale Charakterisierung einer Multispektralkamera

In: 16. Workshop Farbbildverarbeitung, p. 18-27
Ilmenau, Germany, October 2010
Presentation
CA_analysis Julie Klein, Johannes Brauers and Til Aach
Spatial and spectral analysis and modeling of transversal chromatic aberrations and their compensation

In: IS&T Fifth European Conference on Color in Graphics, Imaging and Vision (CGIV 2010) 12th International Symposium on Multispectral Colour science, p. 516-522
Joensuu, Finland, June 2010
Presentation
direct_psf_estimation Johannes Brauers and Til Aach
Direct PSF estimation using a random noise target

In: IS&T/SPIE Electronic Imaging: Digital Photography VI, p. 75370B-1 – 75370B-10
San Jose, USA, January 2010

 

Last update: 2nd January 2014