Reflectivity Sphere

Undergraduate project: Jose Mata Gooptar, Katherine Kristofek

This project is an extension of our ongoing characterization of black and white materials. The reflectance of black and white samples are important to astronomical instrumentation design. Black materials are used to "black out" instruments to reduce stray light. Black materials are characterized via total and specular reflectance measurements. Low reflectance ensures that light is not bouncing off the material and is instead absorbed. White materials are used for flat field screens, which are white screens that reflect a smooth white plane and allow for telescopes and instruments to take flat fields for calibration. White materials are characterized for total reflectance and deviation from a Lambertian surface. A Lambertian surface has the same radiance when viewed from any angle.

Design

A reflective sphere is placed on top of a sample. Light enters the sphere and hits the sample from various angles, selected by a motorized arm. The light bounces off the sample and reflects off the sphere. A mirror in the sphere collects the light and reflects it into a detector outside of the sphere.

refl_sphere_SW_design_trans
SolidWorks design of the reflectivity sphere (sphere is transparent in this view). The motor moves the arm that mounts the light source so that the light can have a range of incident angles. The center hole holds the sample. The offset hole has a convex mirror. The hole in the sphere allows the detector to see the mirror.
The back of the reflectivity sphere. The sample is held by the back piece.
refl_sphere_SW_back_piece
The sample snaps into the back piece, which is installed on the back of the reflectivity sphere.