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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.

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Related in: MedlinePlus

The spectral characteristics of evaluated light emitting diode (LEDs), (a) CREE X-lamp XP-G and (b) its newer version XP-G2, all performance groups into which producer classifies the LED one by one.
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sensors-17-00423-f008: The spectral characteristics of evaluated light emitting diode (LEDs), (a) CREE X-lamp XP-G and (b) its newer version XP-G2, all performance groups into which producer classifies the LED one by one.

Mentions: We need appropriate illumination in order to achieve a high fidelity of the colours in the measured data with the RGB trichromatic colour model used by the cameras in our proposed design. After general research of available products on the market and estimating the required illumination level we decided to use a high intensity white LED. First we studied the properties of various available LED models. After this initial study we selected 2 different high intensity white LEDs (CREE XP-G and CREE XP-G2) for further investigation. These LED models are classified to performance groups exhibiting different spectral characteristics and efficiency. As the information about their relative spectral power distribution was not available, we measured 14 different LEDs (7 models from the CREE XP-G group and the 7 from the CREE XP-G2 group). We measured spectra for 3 LEDs of each LED model to check for consistency of their spectra (in total 42 measurements). Within each of the two groups, there is a significant difference in spectra between the seven models, as shown in Figure 8. Fortunately, for any single LED model in either performance group, the variance of the relative spectral power distribution is insignificant.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
The spectral characteristics of evaluated light emitting diode (LEDs), (a) CREE X-lamp XP-G and (b) its newer version XP-G2, all performance groups into which producer classifies the LED one by one.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f008: The spectral characteristics of evaluated light emitting diode (LEDs), (a) CREE X-lamp XP-G and (b) its newer version XP-G2, all performance groups into which producer classifies the LED one by one.
Mentions: We need appropriate illumination in order to achieve a high fidelity of the colours in the measured data with the RGB trichromatic colour model used by the cameras in our proposed design. After general research of available products on the market and estimating the required illumination level we decided to use a high intensity white LED. First we studied the properties of various available LED models. After this initial study we selected 2 different high intensity white LEDs (CREE XP-G and CREE XP-G2) for further investigation. These LED models are classified to performance groups exhibiting different spectral characteristics and efficiency. As the information about their relative spectral power distribution was not available, we measured 14 different LEDs (7 models from the CREE XP-G group and the 7 from the CREE XP-G2 group). We measured spectra for 3 LEDs of each LED model to check for consistency of their spectra (in total 42 measurements). Within each of the two groups, there is a significant difference in spectra between the seven models, as shown in Figure 8. Fortunately, for any single LED model in either performance group, the variance of the relative spectral power distribution is insignificant.

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