Retroreflectance measurements of photometric standards and coatings

Using a technique that we have developed, the opposition effect (brightening in the retroreflection direction) has been measured for MgCO(3), BaSO(4) paint, and sulfur in the visual region with incandescent illumination and found to be 1.3,1.5, and 1.3, respectively, independent of wavelength. Nextel red, blue, white, and black paints are generally similar. However, in comparison to incoherent illumination, 0.6328-microM laser illumination shows a reduction in diffuse reflectance for angles less than 40 degrees from the incident direction. In addition, the coherent opposition effect may be very large for dark paints. The opposition effect in photometric standards can lead to calibration errors at near opposition and spurious responses in integrating spheres coated with such materials.