BRDF bench setup

The following is a drawing of the BRDF setup used to get the data presented in this section. It consists of a light source at 650 nm that illuminates the screen through a beam expander that creates a collimated beam of 4 x 1.5 cm on the screen. This is necessary because the original beam size (1 x 0.3 cm) about the same size as the texture of the screens and creates local variations in the reflectivity that are not representative of the bulk effect.

The main photodiode is fixed on the table and is placed 50 cm in front of the screen. It measures the light reflected by the screen. The 1cm diameter aperture of the photodiode collects light in a 1.14 degree angle. .

The reference photodiode measures the light reflected of the first lens of the Beam expander to correct for source fluctuation and drift with time. Both photodiodes are measured with a Bias of 5V.

Signals from both photodiodes are collected by a Analog to Digital converter card (Measurement Computing, model 1608-FS) using the Labview program 1608FSV3SaveButton with an acquisition rate of 1ks/s and a duration of 1 second for each measurement,

Two angles can be changed with this experimental setup:

1: The screen tilt angle changes the orientation of the screen relative to the Main Photodiode. A tilt angle of 0 corresponds to the screen facing the photodiode directly. This angle can be changed horizontally (as shown on the drawing) or vertically

2: The Incident angle is the angle at which the incident light hits the screen. The source, the beam expander and the reference photodiode are mounted on a rail that pivots along the same axis as the screen and allows the whole unit to be moved while keeping the light aligned on the same spot on the screen.

Not shown on this drawing are a series of baffles that are required to make an accurate measurement. Since the main detector receives approximately 1/1000^th of the light incident on the screen, great care is required to block out stray reflections, especially on posts and the table surface.

Specific limitations:

The minimal angle allowed for incident light is 2.4 degrees and is limited by the finite size of the photodiode detector. The main photodiode has to be placed closer to the screen that the laser beam to eliminate stray light from the beam expander to contaminate the results. There is no limit on the max angle.