Gonio-photometer
RGL relies on a customized PAB pgII gonio-photometer for most material measurements. This device simultaneously illuminates and observes a vertically mounted sample from arbitrary directions using a set of four motorized axes (double arrows). Illumination arrives from a static light source installed in an adjacent room partition (arrow from the left); the illumination direction is thus controlled by rotating the sample mount. The walls of the room are covered with a black fabric to absorb light reflected from the sample.
The pgII is fast: in the default measurement mode, the sensor head revolves around the sample at about 3 meters/second, while acquiring measurements at a frequency of 1Khz with an angular precision of 1 mrad (≈ 0,06°). The resulting data fully characterizes the optical behavior of the measured material and can be used either as a ground truth for existing models, or to reproduce the material in a physically-based renderer.
Sensors
The device provides the following imaging modalities:
- 4x large-area (1cm2) silicon photodiodes with a broadband response that can be customized using filters (e.g. RGB + Near-infrared). These are ideal for high-resolution measurements of specular materials due to their high dynamic range and extremely fast response.
- Zeiss MMS1 compact grating spectrometer covering the range from 310 to 1100 nm using 256 samples (one sample every 3-4nm).
- Two high-resolution cameras: Prosilica GT5120NIR (IR-enhanced CMOS, monochromatic, 5120×5120 resolution) and Prosilica GT6600 (RGB CCD with Bayer grid, 6576×4384 resolution). When using these cameras, our acquisition pipeline generates high-quality HDR output in OpenEXR format by fusing exposure series. They are mounted using a different arm that can support their increased weight.
The first two sensors would normally be used for BRDF/BSDF measurements, while the cameras enable measurement of spatial variation in addition to directional behavior. Note that the camera and spectrometer have an intrinsic exposure time and cannot measure while the machine is moving, hence a slower procedure is used where the device drives to specific configurations and halts until the exposure is done.
Light sources
Various light sources can be mounted on two light rails in a separate room partition. They illuminate the sample through two small circular holes visible above. The following options are currently provided:
- Xenon arc light source (very bright, broadband illumination from 300-1000nm)
- Light-emitting plasma source (stabilized, broadband illumination from 360-750nm)
- Tungsten halogen source (black-body spectrum)
- Stabilized HeNe laser (633 nm)
- Laser diodes of various wavelengths (~400-1000nm)
The lamp rails can also accomodate beam conditioning optics and other optical devices, including filter wheels (e.g. to measure fluorescent materials) and motorized rotation stages (e.g. for polarized light measurements). The image below shows a setup where the left rail is used to measure retroreflection using a coherent laser source that passes through a beamsplitter.
Sample mounts
Two different sample mounts can be installed:
- phirot2 (left) holds two samples in ISO-A4 format; the goniophotometer can automatically switch between them. Alternatively, smaller (10×10cm) samples can be held using inserts. A hole in the middle allows the unscattered beam to pass through so that it can be measured, e.g. to compute the normalization constant of a BRDF.
- phirot (right) holds large samples up to about 70×70cm and has a flexible mounting system that can accomodate a variety of shapes.
Samples
Ideally, the sample surface to be measured is perfectly flat and covers at least 10×10cm - 20×20cm. Larger samples are helpful because the beam used to illuminate the sample stretches out into a long ellipse at grazing angles (> 85°)—if the sample is too small, parts of the sample holder would be measured in such configurations. The image below shows a measurement of an iridescent butterfly (Morpho melenaus) whose surface is not perfectly flat. In such cases, the resulting measurements will specify an “effective BRDF” of the illuminated region.
Example data
Click the screenshot on the bottom right to see an example of the typical output produced by the device—the file will open an interactive viewing tool. This is a high-resolution measurement of a holographic paper measured using a silicon photodiode. Each point corresponds to a direction projected from the hemisphere onto the disk, whose height is proportional to the logarithm of the intensity. The purple arrow denotes the direction (θᵢ=30, ϕᵢ=0) from which the material was illuminated. The bump on the opposite side is the specular reflection, and the numerous ridges are caused by diffraction from the paper's grating-like microstructure.
