Securing Angle of Light
Wednesday, February 15, 2017 @ 04:02 PM gHale
Nearly all the light that enters our eyes has bounced off something first, bringing with it information about the nature of the objects it encountered on the way.
But that information is strongly affected by the angle at which the light strikes the object, and also the angle at which the reflected light reaches the eye of the beholder.
That’s why the properties of light reflected at different angles and intensities are enormously important to the way cars end up painted, fabrics are made, plastics and coatings are colored, printed materials are produced, optical systems are engineered, and remote-sensing images are created and interpreted.
Moreover, instruments deployed for public service and national security uses — such as satellite monitoring of weather and natural disasters, or detecting activity by potential adversaries — contain components that must end up precisely calibrated against a known standard to ensure accurate characterization of the effects of light reflectance and scattering.
The National Institute for Standards Technology (NIST) maintains the national scale for reflectance, and is about to launch an improved system for measuring the intensity and spectrum of light reflected and scattered off samples as large as 30 cm square in practically any direction.
Called the Robotic Optical Scattering Instrument (ROSI), it will offer new capabilities in increasing demand by industry and science.
“We began work to develop this system years ago,” said project leader Heather Patrick of NIST’s Physical Measurement Laboratory, “arriving at progressively more sophisticated designs before the current version of ROSI. By the middle of 2017 we expect to make the first set of functions available to customers. About a year later, all of the system’s features will be fully operational.”
Perhaps most importantly, ROSI will permit both in-plane and out-of-plane measurements. In the former, the light source, the sample, and the receiver are all in the same plane; in the latter, the receiver is in a different plane.
Out-of-plane measurements were not possible with ROSI’s predecessor, NIST’s workhorse Spectral Tri-function Automated Reference Reflectometer (STARR). “But they are important to anyone who does reflectance measurements in the field, such as those who study ocean color or infrared monitoring of heat signatures,” said NIST scientist Catherine Cooksey. “In addition, they are important to ‘gonioapparent’ coatings — that is, coatings that reflect different colors depending on the direction of viewing or illumination, like iridescent colors. For example, car paint that seems to change colors as you move around the car.”
In addition, ROSI extends the range of wavelengths from the ultraviolet to the near-infrared, and can provide 100 times more incident light than STARR, enabling detailed measurements of samples with low reflectance and opening new possibilities for research.
Three Automated Components
The ROSI system combines three integrated, fully automated components. One is a laser-based light source tuned to a specific desired color, intensity, and polarization before it is focused on the sample. The beam makes a spot 1 cm in diameter when shining exactly perpendicular to the surface of the sample, but broadens to an ever-wider and fainter ellipse as the angle between the source and sample becomes increasingly oblique.
The sample mounts on the end of ROSI’s second major component, a 6-axis robotic arm that can move the sample into almost any angle with respect to the beam. The third component is the receiver which detects the amount of light scattered from the sample at a specific viewing angle. The receiver can move around the axis of the robot arm, a design that facilitates out-of-plane measurements.
That capacity is critically important for characterizing the materials that satellite systems such as the long-running Landsat series — which maps changes in the global landscape — use to calibrate their on-board sensors. Those devices receive reflected and scattered light arriving from a wide array of angles, and the quality of observations depends on understanding the way that those angles affect the signal.
The impact is large: Since Landsat 8’s launch in 2013, more than 30 million images have been downloaded from the program site.
NASA provides many of its other satellite projects with reflectance measurements, and NASA’s reflectance scale is traceable to the national reflectance scale through yearly NIST calibrations.
ROSI is able to make three kinds of measurements. The least complex involves mirror samples, in which nearly all the incident light is reflected at one angle. That is the first capability that will come online later this year. The second kind goes by the name of bidirectional reflectance distribution function (BRDF), which basically means both the angle at which the incident light strikes the sample and the angle between the sample and the receiver can be separately adjusted to measure how the changes modify the properties of the reflected/scattered light.
ROSI will be able to produce “hemispherical” measurements in which light reflected from the sample is recorded at numerous points constituting a complete hemisphere and producing a comprehensive data set.
“This new facility offers NIST’s customers an expanded capability, and NIST itself with extended research potential,” Cooksey said. “Previously, we could only measure within a single plane. Now we can measure full hemispherical space above a sample point by point with significantly increased intensity of incident light. This increases the types of materials we can now measure, such as coatings with interesting BRDFs or very dark, black samples.”
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