Overview and Theory

The test station uses a Quartz-Tungsten Halogen (QTH) lamp that provides light to a Newport monochromator. The monochromator split the light up into different wavelengths with a diffraction grating set up in a Czerny-Turner configuration. The wavelength that is output from the monochromator is set using either the software that is provided for it, or using drivers to create your own program.

After the monochromator optics, the light passes through a slit (to ensure only the desired wavelength is output) and two collimators (to create a beam that is paralell to the table surface). The light then passes through a beamsplitter where 50% of the light is directed to a reference photodiode and 50% of the light is directed to the mirror under test. The mirror under test is aligned so the light reflecting off of it is directed to a second measurement photodiode.

The two photodiodes' current response was measured with a Keithley 6571B electrometer read out using our DAQ program. The DAQ program also performed all of the calculations on the current measurements to get the percentage of light that the mirror reflects at that wavelength.

Because the power of the QTH lamp changed over time as it heated up, we had to perform calibration runs at the start and end of each day where the light was directed straight to the measurement photodiode after the beamsplitter instead directing it to reflect off of the mirror. These results are then used to create a calbration constant that lets the final measurement results be independed of the QTH lamp power.

Recommendations

If a similar test station is set up, it is strongly recommended to look into a way to automate the alignment of the mirror and photodiodes. The alignment is very tricky with this setup and took a lot of time to perfect for each spot to be measured. If full automation is not an option, it would be strongly recommended to set up equipment is securely fixed and cannot move once it is properly aligned.