Limits...
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

Spatial resolution in dots per inch (DPI) as a function of the sensor pixel size for the distance l of the object from the sensor (a) 150 mm; (b) 250 mm; (c) 1000 mm. Black line in all three charts indicates the condition DoF , for the sample size  50 mm; (d) Spatial resolution as a function of measured sample size computed for maximum zenith angle .
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5375714&req=5

sensors-17-00423-f006: Spatial resolution in dots per inch (DPI) as a function of the sensor pixel size for the distance l of the object from the sensor (a) 150 mm; (b) 250 mm; (c) 1000 mm. Black line in all three charts indicates the condition DoF , for the sample size 50 mm; (d) Spatial resolution as a function of measured sample size computed for maximum zenith angle .

Mentions: Our design strategy is that we start with the distance l between the sample and the sensor given by the assumed size of the gantry (i.e., radius of the hemisphere), the sample size y and the objective focal length f and we search for the right F-number and pixel size Δ to achieve the highest spatial resolution calculated for the lowest camera. The spatial resolution is imposed by the required DoF because the camera views the sample at a high zenith angle. As can be shown by the formulae in Appendix A and in charts in Figure 6d, the maximum spatial resolution is determined by the sample size only, assuming DoF ≈ sample size y. No matter what the camera focal length or the sensor pixel size are, the maximum spatial resolution (in DPI) does not noticeably change (see Figure 6) and we can find the appropriate pixel size level for the target spatial resolution value. If we change the focal length f, only the pixel size changes and the DPI is kept the same. A change of the sample to camera distance l has almost no effect either (see Figure 6a–c). The relation between the sample size and the maximum spatial resolution is presented in Figure 6d.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
Spatial resolution in dots per inch (DPI) as a function of the sensor pixel size for the distance l of the object from the sensor (a) 150 mm; (b) 250 mm; (c) 1000 mm. Black line in all three charts indicates the condition DoF , for the sample size  50 mm; (d) Spatial resolution as a function of measured sample size computed for maximum zenith angle .
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f006: Spatial resolution in dots per inch (DPI) as a function of the sensor pixel size for the distance l of the object from the sensor (a) 150 mm; (b) 250 mm; (c) 1000 mm. Black line in all three charts indicates the condition DoF , for the sample size 50 mm; (d) Spatial resolution as a function of measured sample size computed for maximum zenith angle .
Mentions: Our design strategy is that we start with the distance l between the sample and the sensor given by the assumed size of the gantry (i.e., radius of the hemisphere), the sample size y and the objective focal length f and we search for the right F-number and pixel size Δ to achieve the highest spatial resolution calculated for the lowest camera. The spatial resolution is imposed by the required DoF because the camera views the sample at a high zenith angle. As can be shown by the formulae in Appendix A and in charts in Figure 6d, the maximum spatial resolution is determined by the sample size only, assuming DoF ≈ sample size y. No matter what the camera focal length or the sensor pixel size are, the maximum spatial resolution (in DPI) does not noticeably change (see Figure 6) and we can find the appropriate pixel size level for the target spatial resolution value. If we change the focal length f, only the pixel size changes and the DPI is kept the same. A change of the sample to camera distance l has almost no effect either (see Figure 6a–c). The relation between the sample size and the maximum spatial resolution is presented in Figure 6d.

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