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.

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.

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°.

Contact

Michael Gadermayr, Tel.: +49 (241) 80-27906

Publications

	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
	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
	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
	Julie Klein Correction of longitudinal aberrations in goniometric measurement with a multispectral camera In: 19. Workshop Farbbildverarbeitung, p. 31-41 Ilmenau, Germany, October 2013
	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
	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
	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
	Julie Klein, Johannes Brauers and Til Aach Spektrale Charakterisierung einer Multispektralkamera In: 16. Workshop Farbbildverarbeitung, p. 18-27 Ilmenau, Germany, October 2010 Presentation
	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
	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