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Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site

View Article: PubMed Central - PubMed

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∘ 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.

No MeSH data available.


Related in: MedlinePlus

The photograph of the auto-collimator output at the display during operation: (a) the misaligned laser beams giving two spots; (b) aligned (i.e., collimated) laser beams giving one brighter spot.
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sensors-17-00423-f017: The photograph of the auto-collimator output at the display during operation: (a) the misaligned laser beams giving two spots; (b) aligned (i.e., collimated) laser beams giving one brighter spot.

Mentions: In the fourth step the auto-collimator is adjusted. This is a slightly more difficult procedure, as we have to find the orientation of the lightdrum at the normal of a surface and adjust the laser module direction at the same time. At the beginning, we have neither the correct position of the device with respect to the surface normal nor the right adjustment of the laser module. The lightdrum is put onto the holding frame for measurement on the desk. The mirror is placed in the sample position on the desk. By repeatedly rotating the device to the positions 0, 90, 180, and 270, adjusting the device against the mirror in the boundary position and the laser orientation in the second direction, we get a converging procedure so finally the device is perpendicular to the mirror and the laser direction is properly adjusted too. At the rotation angle 0 we tilt the device from the current point on the holding frame to the middle to direct the retroreflected beam from the mirror onto the matte screen observed by the camera. By adjusting the laser we move the beam spot to the centre. We then rotate the gantry to 180 and repeat the procedure, first by tilting the gantry, second by tilting the laser. Then we rotate back to 0 and repeat the procedure until we get a sufficient accuracy of adjustment. After adjustment with sufficient accuracy in this one direction we repeat the procedure for perpendicular directions 90 and 270. We then repeat both procedures in both directions () and () until we are satisfied with the convergence. The LED laser module direction at the auto-collimator is finely adjusted in two axes by the three M3 screws. The photography of the display with the image taken by the camera auto-collimator is shown in Figure 17 for two cases, misaligned and aligned beams.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
The photograph of the auto-collimator output at the display during operation: (a) the misaligned laser beams giving two spots; (b) aligned (i.e., collimated) laser beams giving one brighter spot.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5375714&req=5

sensors-17-00423-f017: The photograph of the auto-collimator output at the display during operation: (a) the misaligned laser beams giving two spots; (b) aligned (i.e., collimated) laser beams giving one brighter spot.
Mentions: In the fourth step the auto-collimator is adjusted. This is a slightly more difficult procedure, as we have to find the orientation of the lightdrum at the normal of a surface and adjust the laser module direction at the same time. At the beginning, we have neither the correct position of the device with respect to the surface normal nor the right adjustment of the laser module. The lightdrum is put onto the holding frame for measurement on the desk. The mirror is placed in the sample position on the desk. By repeatedly rotating the device to the positions 0, 90, 180, and 270, adjusting the device against the mirror in the boundary position and the laser orientation in the second direction, we get a converging procedure so finally the device is perpendicular to the mirror and the laser direction is properly adjusted too. At the rotation angle 0 we tilt the device from the current point on the holding frame to the middle to direct the retroreflected beam from the mirror onto the matte screen observed by the camera. By adjusting the laser we move the beam spot to the centre. We then rotate the gantry to 180 and repeat the procedure, first by tilting the gantry, second by tilting the laser. Then we rotate back to 0 and repeat the procedure until we get a sufficient accuracy of adjustment. After adjustment with sufficient accuracy in this one direction we repeat the procedure for perpendicular directions 90 and 270. We then repeat both procedures in both directions () and () until we are satisfied with the convergence. The LED laser module direction at the auto-collimator is finely adjusted in two axes by the three M3 screws. The photography of the display with the image taken by the camera auto-collimator is shown in Figure 17 for two cases, misaligned and aligned beams.

View Article: PubMed Central - PubMed

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∘ 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.

No MeSH data available.


Related in: MedlinePlus