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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.


(a) Auto-collimator principle; (b) solid drawing of the auto-collimator assembly.
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sensors-17-00423-f013: (a) Auto-collimator principle; (b) solid drawing of the auto-collimator assembly.

Mentions: We have incorporated an auto-collimator used in optical devices to allow for the proper adjustment of the BTF measurement device against the measured sample. A mirror affixed tightly onto the sample must be used. If the gantry is put exactly perpendicularly to the mirror, the collimated light beam emitted by the auto-collimator is retroreflected to the same position. The principle is shown in Figure 13a and its construction variants were discussed in [29]. The auto-collimator design consists of a 25 mm right angle prism, a mm cube beam splitter, a 25 mm square glass diffuser, a laser module as the source of collimated light and a miniature camera pointed at the glass diffuser. The three parts made of glass are cemented together. The beam from the laser module propagates through the beam splitter to the right angle prism and then goes to the mirror placed temporarily on the sample surface. The right angle prism is used only to minimise the height of the auto-collimator as the camera is then oriented horizontally. The use of an auto-collimator then increases the height of the gantry by only 25 mm. A solid drawing of the assembled auto-collimator unit with the camera is shown in Figure 13b.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
(a) Auto-collimator principle; (b) solid drawing of the auto-collimator assembly.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f013: (a) Auto-collimator principle; (b) solid drawing of the auto-collimator assembly.
Mentions: We have incorporated an auto-collimator used in optical devices to allow for the proper adjustment of the BTF measurement device against the measured sample. A mirror affixed tightly onto the sample must be used. If the gantry is put exactly perpendicularly to the mirror, the collimated light beam emitted by the auto-collimator is retroreflected to the same position. The principle is shown in Figure 13a and its construction variants were discussed in [29]. The auto-collimator design consists of a 25 mm right angle prism, a mm cube beam splitter, a 25 mm square glass diffuser, a laser module as the source of collimated light and a miniature camera pointed at the glass diffuser. The three parts made of glass are cemented together. The beam from the laser module propagates through the beam splitter to the right angle prism and then goes to the mirror placed temporarily on the sample surface. The right angle prism is used only to minimise the height of the auto-collimator as the camera is then oriented horizontally. The use of an auto-collimator then increases the height of the gantry by only 25 mm. A solid drawing of the assembled auto-collimator unit with the camera is shown in Figure 13b.

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.