Frederick O. Bartell

The Theory And Measurement Of Bidirectional Reflectance Distribution Function (Brdf) And Bidirectional Transmittance Distribution Function (BTDF)

The concepts of BRDF (Bidirectional Reflectance Distribution Function) and BTDF (Bidirectional Transmittance Distribution Function) are defined and discussed as being the ratios of differential outputs of radiance divided by differential inputs of irradiance. Appropriate measurement arrangements and procedures are presented, and the problems are described that are involved in going from the differentials of theory to the finite quantities of measurement. Finally appropriate data reduction schemes are given for determining BRDF and BTDF from these measurements.

Bidirectional reflectance distribution function of the Infrared Astronomical Satellite solar-shield material

The BRDF values of a special, striated gold sample at 10.6 and 118 microm were measured. For 118-microm radiation the values decrease from beta - beta(0) = 0.1 to 1 for both orientations, but the vertical orientation of stria always produces a higher value. At 10.6 microm the horizontal orientation produces BRDF values that start at approximately 10(-2) at 0.1 and decrease to approximately 5 x 10(-4) at 1. The vertical orientation generates curves that are considerably higher.

Scattering characteristics of Martin Black at 118 μm

BRDF (bidirectional reflectance distribution function) values for 0.000118 m radiation at different angles of incidence and different scattering angles from the Infrared Astronomical Satellite telescope baffle coated with Martin Black are presented. Data from scatterometer experiments are collected and the BRDF and beta - beta sub 0 (sin theta sub s - sin theta sub 0) values are calculated based on the geometry, the voltage readings, the attenuators in the beam, and the calculated reference levels. A composite curve of forward and backward scattering data for several angles of incidence shows a peak near the specular direction (beta - beta sub 0 = 0), which is the instrument profile reduced by the 20% specular reflection of the Martin Black. The nonspecular part of the reflectivity indicates the slightly specular but largely Lambertian character of the coating. Data for the specular reflectivity as a function of the incidence angle unexpectedly shows a decrease in the specular reflectance with increasing angle of incidence.

Measurements of Martin Black at ~10 μm

The BRDF values of several samples of Martin Black have been measured at 10.6 μm and several angles of incidence. The values are in good agreement with data taken recently on an identical sample with a different instrument and with data from an identical sample taken several years ago with a similar instrument.The BRDF values of several samples of Martin Black have been measured at 10.6 microm and several angles of incidence. The values are in good agreement with data taken recently on an identical sample with a different instrument and with data from an identical sample taken several years ago with a similar instrument.

Description And Limitations Of An Automated Scatterometer

The methods, capabilities and limitations of a multiwavelength scatterometer are described.

An Unheated Four-Bar Infrared Test Target

A four-bar infrared test target has been built to test the feasibility of using perforated ambient temperature plates for the four bars instead of the customary plates operated at temperatures different from ambient. Six different sizes of four-bar arrays provide six different spatial frequencies. Front and back arrays of perforated plates are moved relative to one another by a motor drive to expose more or less of a sky reflector backplate, thereby producing a varying apparent temperature differential between the bars and their interstices. The target system also features a radiometric monitor which mechanically servos the bar patterns to compensate for changes such as dust or clouds. Qualitative tests with infrared imaging systems show satisfactory bar patterns which disappear when the perforations are closed. Quantitative tests show that differential temperature accuracies of about one tenth of a kelvin can be attained.