We developed a new measurement system for bidirectional reflectance distribution functions (BRDF). The system can obtain simultaneously isotropic BRDF of all scattering angles utilizing a semicircular ring and an image sensor. First, we predicted the performance of our measurement system using integrated ray tracing simulation. The light path is as follows: the light from the light source at 635 nm is reflected off the target material, and the light is reflected back at the semi-circular ring. The image sensor records the light reflected from the semicircular ring. The results show good agreement with original and simulation BRDF, but detailed analysis suggested. The system improves significantly measurement time and resolution of reflection angles. Furthermore, the system is not only more cost effective than other traditional measurement systems, but also eliminated the temporal fluctuation of the light source intensity.
We present a new ray tracing simulation of aero-optical effect through anisotropic inhomogeneous media as supersonic
flow field surrounds a projectile. The new method uses multiple gradient-index (GRIN) layers for construction of the
anisotropic inhomogeneous media and ray tracing simulation. The cone-shaped projectile studied has 19° semi-vertical
angle; a sapphire window is parallel to the cone angle; and an optical system of the projectile was assumed via paraxial
optics and infrared image detector. The condition for the steady-state solver conducted through computational fluid
dynamics (CFD) included Mach numbers 4 and 6 in speed, 25 km altitude, and 0° angle of attack (AoA). The grid
refractive index of the flow field via CFD analysis and Gladstone-Dale relation was discretized into equally spaced
layers which are parallel with the projectile’s window. Each layer was modeled as a form of 2D polynomial by fitting the
refractive index distribution. The light source of ray set generated 3,228 rays for varying line of sight (LOS) from 10° to
40°. Ray tracing simulation adopted the Snell’s law in 3D to compute the paths of skew rays in the GRIN layers. The
results show that optical path difference (OPD) and boresight error (BSE) decreases exponentially as LOS increases. The
variation of refractive index decreases, as the speed of flow field increases the OPD and its rate of decay at Mach number
6 in speed has somewhat larger value than at Mach number 4 in speed. Compared with the ray equation method, at Mach
number 4 and 10° LOS, the new method shows good agreement, generated 0.33% of relative root-mean-square (RMS)
OPD difference and 0.22% of relative BSE difference. Moreover, the simulation time of the new method was more than
20,000 times faster than the conventional ray equation method. The technical detail of the new method and simulation is
presented with results and implication.
The Space Optics Laboratory at Yonsei University, Korea, in cooperation with Breault Research Organization (BRO) in Tucson, Arizona, have invested significant research and development efforts into creating large scale ray tracing techniques for simulating “reflected” light from the earth with an artificial satellite. This presentation describes a complex model that combines the sun, the earth and an orbiting optical instrument combined into a real scale nonsequential ray tracing computation using BRO’s Advanced Systems Analysis Program, ASAP®. The Sun is simulated as a spherically emitting light source of 695,500 km in diameter. The earth also is simulated as a sphere with its characteristics defined as target objects to be observed and defined with appropriate optical properties. They include the atmosphere, land and ocean elements, each having distinctive optical properties expressed by single or combined characteristics of refraction, reflection and scattering. The current embodiment has an atmospheric model consisting of 33 optical layers, a land model with 6 different albedos and the ocean simulated with sun glint characteristics. A space-based optical instrument, with an actual opto-mechanical prescription, is defined in an orbit of several hundreds to thousands of miles in altitude above the earth’s surface. The model allows for almost simultaneous evaluations of the imaging and radiometric performances of the instrument. Several real-life application results are reported suggesting that this simulation approach not only provides valuable information that can greatly improve the space optical instrument performance but also provides a simulation tool for scientists to evaluate all phases of a space mission.