In my photonics class, we conducted an experiment to observe the Beer-Lambert law and the effects of filters(absorption, transmittance, and reflection). We shined a red(632nm) 5mW Helium-Neon laser through combinations of 3 filters: red, green, and blue. We measured the output beam power using a highly sensitive ThorLabs power meter that can detect nanowatt levels.
Distance between the filters = 3.81 cm Distance between the laser and the 1st filter = 25.4 cm
Here our the surprising results:
What we found is that when we used all 3 filters, the
In table 1: x = thickness of the filter and a = absorption coefficient. Now as you can see from table 2, the actual transmitted power output measured from using combinations of filters is orders of magnitude larger than predicted. For the predicted values the books told us to take the product of the individual filter transmittances.
Also, when using combinations of all 3 filters: the ORDER in which the filters were placed made a big difference. Using the RGB combination(left to right = first to last) got the highest transmittance. To me this suggests some Nonlinear optical phenomenon occurring between the filters.
But what I find curious is how when a blue or green filter was placed in front of the red filter the measured power of the output beam was greater than that of using only the red filter. My guess is there is some back reflection leading to standing wave resonant modes. But also the blue/green filter(s) seem to be interacting with the red filter to increase the red filters transmittance. Is it possible that there is some nonlocality going on here too?