B. U. Goniometer (B.U.G.) Laboratory
Funded by N.A.S.A.
A photometric goniometer measures the reflected light properties of surfaces from a variety of illumination and viewing positions. The majority of goniometers being used for planetary applications are limited, either by design or use, to measurements in the principal (or scattering) plane.
This instrument was designed to measure the scattering properties over the entire upper hemisphere. Additionally, it is an automated system that utilizes stepper motors and motion control and measurement software. The light source is a 100W quartz-tungsten-halogen research grade lamp. The output light is filtered using 50nm FWHM interference filters within the visible and near-infrared (400-1000 nm range), then chopped and focused into a glass fiber-optic bundle. The fiber optic cable terminates at the top of one of the two arms (incidence arm) with a collimating lens assembly, where it is then directed onto the sample. The source arm is capable of incidence angles of 0-60 degrees (depending on sample size), and also swivels in azimuth from 0-170 deg. The reflected light is detected by a silicon-detection system which is "locked in" to the chopper, and provides very stable, low noise measurements, even in low-level ambient light.
The detector is mounted on the longer arm (emission arm) and is capable of emission angles of -80 to +80 deg. The degree of movement described here allows a complete half-hemisphere of scattered light to be measured (including the principal plane). For isotropic samples, the other half-hemisphere is theoretically the mirror image (across the principal plane) of that measured, and this is checked at selected places. The width of the collimating lens and it's distance from the sample limits the phase angle to a minimum of 3.0 deg (the detector is occulted by the optics at smaller phase angles). The maximum useful phase angle is 140 deg for most applications, but with special sample holders we have gone up to 160 deg or so for a few samples.
Goniometer in "parked" position (i=0, e=0, az=0). The detector is on the emission arm. The light source is directed along the incidence arm and terminates in a small collimating lens at the top.
Spectrograph. A single-grating spectrograph (Oriel Corp. FICS/Linespec system) is also available to document the characteristics of each sample. Spectral coverage is 400-1000nm with spectral resolutions of ~6nm.
CCD Camera System. In the past, a Panasonic CCD color video camera is used to capture images of the surface with resolutions as high as 60um (15um pixels) at a 1cm FOV. That system has passed on and we now use binocular microscopes equipped with digital cameras to document the samples.
Example: JSC Mars-1 Analog Soil
1. Lunar Regolith
2. Models of Bidirectional Reflectance
Past Work: MER03 Calibration Target