Use of the Melles-Griot Reflectometer

Updated 29-Aug-02 E. Falco and M. Calkins

  • Unpack instrument with the exception of filters and spare light bulbs. Determine the most convenient layout of instrument to conduct measurements. Note - If measuring samples on a tabletop, the power supply can be a source of vibrations which may adversely affect measurements. Position power supply accordingly.

  • Attach test light source to power supply and power up. The test light source is a 6 V, 20 W bulb. Do not exceed that voltage and power. It takes a few minutes for the power supply to stabilize.

  • Use an RG58 coax cable to connect the light detector to the Melles Griot Large Dynamic Range Amplifier (Input side). Note - The amplifier can either be operated with internal batteries or with external power transformer included in the case.

  • Measure direct (i.e., unreflected) light intensity. This can be somewhat complicated depending on the location where the measurements are taking place. Since the test light source is essentially a traditional filament type bulb, the angle at which it is mounted may cause the filament to sag, affecting output intensity. Included with the reflectometer is an adjustable rig mounted to a C-clamp. Secure the C-clamp to a solid surface and use an allen wrench to rotate the rig to a position approximating as close as possible the position where the measurements will take place. Insert the light source at the end of the rig appropriate to test conditions. Attach desired filter to end of light detector making sure to align hash marks. Once the rig is secure you are ready to measure the direct light intensity. Use the dial on the Dynamic Range Amplifier to select appropiate range and read measurement from LED.

  • Measure reflectivity of desired surface. Place the triangular plate over surface to be measured. Insert test light source into orifice on "base" side" of triangular plate. Insert desired filter into other orifice and light detector into filter. Align the hash marks between filter and detector. Select range on amplifier (should be the same as unobstructed measurement). Take reading from LED. You should adjust the height of the feet of the triangular plate until the reading is maximized, otherwise your readings may be spurious. Usually you only need adjust the foot at the tip of the triangular plate to achieve happiness.

  • Repeat for all desired filters and surfaces.

    • Note: the Melles-Griot reflectometer that we used in 2001-2002 was known to have suffered degradation (by August 2001) in its sensitivity at wavelengths short of 500 nm. That explains the greater decline in measured reflectivity than suggested by the trends in the plots. Bill Kindred refurbished it, as of Jul 2003.