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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.


LED module: (a) PCB with lens; (b) tube mounted on the holder; (c) the assembled unit with three adjusting screws to get required direction; (d) photograph of the assembled PMMA dome showing the baffler created by the structure of tubes that effectively diminishes stray light inside the dome; (e) the illumination intensity distribution at 250 mm distance from the LED module with the tube and without it.
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sensors-17-00423-f011: LED module: (a) PCB with lens; (b) tube mounted on the holder; (c) the assembled unit with three adjusting screws to get required direction; (d) photograph of the assembled PMMA dome showing the baffler created by the structure of tubes that effectively diminishes stray light inside the dome; (e) the illumination intensity distribution at 250 mm distance from the LED module with the tube and without it.

Mentions: The PCB allows for mounting lenses up to a maximum diameter of 31 mm. We carried out thorough research on off-the-shelf lenses for the Cree XP-G LED and based on their directional emitting diagrams we preselected 5 lenses from different manufactures for evaluation. For these 5 lenses we measured their directional emission distribution, their illumination uniformity and the size of the area lit at the distance used in our device. As the most appropriate model we selected the lens LEDIL FA11905_TINA3-S made of PMMA (Polymethyl methacrylate), with diameter 16.1 mm, height 11.4 mm, and the angle at FWHM 15. It achieves the most uniform illumination at the sample area at the distance used. The LED module with the lens mounted is shown in Figure 11a.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
LED module: (a) PCB with lens; (b) tube mounted on the holder; (c) the assembled unit with three adjusting screws to get required direction; (d) photograph of the assembled PMMA dome showing the baffler created by the structure of tubes that effectively diminishes stray light inside the dome; (e) the illumination intensity distribution at 250 mm distance from the LED module with the tube and without it.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f011: LED module: (a) PCB with lens; (b) tube mounted on the holder; (c) the assembled unit with three adjusting screws to get required direction; (d) photograph of the assembled PMMA dome showing the baffler created by the structure of tubes that effectively diminishes stray light inside the dome; (e) the illumination intensity distribution at 250 mm distance from the LED module with the tube and without it.
Mentions: The PCB allows for mounting lenses up to a maximum diameter of 31 mm. We carried out thorough research on off-the-shelf lenses for the Cree XP-G LED and based on their directional emitting diagrams we preselected 5 lenses from different manufactures for evaluation. For these 5 lenses we measured their directional emission distribution, their illumination uniformity and the size of the area lit at the distance used in our device. As the most appropriate model we selected the lens LEDIL FA11905_TINA3-S made of PMMA (Polymethyl methacrylate), with diameter 16.1 mm, height 11.4 mm, and the angle at FWHM 15. It achieves the most uniform illumination at the sample area at the distance used. The LED module with the lens mounted is shown in Figure 11a.

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.