Lightdrum-Portable Light Stage for Accurate BTF Measurement on Site

Vlastimil Havran, Jan Hosek, Sarka Nemcova, Jiri Cap, and Jiri Bittner

Vlastimil Havran and Jiri Bittner are at the Faculty of Electrical Engineering, Czech Technical University.

Jan Hosek, Sarka Nemcova, and Jiri Cap are at the Faculty of Mechanical Engineering, Czech Technical University

Abstract

We propose a miniaturised light stage for measuring the bidirectional reflectance distribution function (BRDF) and the bidirectional texture function (BTF) of surfaces on site in real world application scenarios. The main principle of our lightweight BTF acquisition gantry is a compact hemispherical skeleton with cameras along the meridian and with light emitting diode (LED) modules shining light onto a sample surface. The proposed device is portable and achieves a high speed of measurement while maintaining high degree of accuracy. While the positions of the LEDs are fixed on the hemisphere, the cameras allow us to cover the range of the zenith angle from 0 to 75 degrees and by rotating the cameras along the axis of the hemisphere we can cover all possible camera directions. This allows us to take measurements with almost the same quality as existing stationary BTF gantries. Two degrees of freedom can be set arbitrarily for measurements and the other two degrees of freedom are fixed, which provides a tradeoff between accuracy of measurements and practical applicability. Assuming that a measured sample is locally flat and spatially accessible, we can set the correct perpendicular direction against the measured sample by means of an auto-collimator prior to measuring. Further, we have designed and used a marker sticker method to allow for the easy rectification and alignment of acquired images during data processing. We show the results of our approach by images rendered for 36 measured material samples.

Sensors, Volume 17, issue 3, article no. 423, doi:10.3390/s17030423 [BibTeX] [Paper web page at Sensors] with PDF to download

Representative image

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Important: the data on this webpage accompanying the article are distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0). [Video 1 WMV] - auto-collimator adjustment, measurement debugging, raw data example [Video 2 WMV] - image alignment for 120 view positions [Video 3 WMV] - image alignment for 120 view positions - selected steps visualization [Video 4 WMV] - image alignment for 120 view positions - compensating sticker thickness [Marker sticker PDF] as shown in the paper [Marker sticker PDF] with white interior [Marker sticker PDF] inverted, for silk-screen printing master [Marker sticker code in C++] requires The Libboard C++ library by Sebastien Fourey

Any questions, remarks and comments including commercial interest of the lightdrum are welcome to e-mail: havran@fel.cvut.cz