Transmission of blue light through ceramics and surface roughness effects

[ccdani]

Hey :)

I measured the transmission of blue visible light (350-550nm) through lithiumdisilicate ceramics with an ulbricht ball and an spectrometer. The light source was a led dental curing unit (bluephase style). The light guide was positioned direct on the ceramics.
Now I wanted to test different surface conditions. The transmission was lower when the roughness of the ceramic is higher. Then I tested the transmission with a thin film of immersion oil (refractive index 1.51) between the light guide and the ceramics. I thought that the degree of transmission has to be higher because of the lower reflection on the surface.
But it was 3% lower. I thought that there is less reflection at the surface with immersion oil because of the similar refractive index of immersion oil than of air.
Can you explain this phenomenon? Is it because of thin film interference?

Thank you very much!

 

[LURCH]

Did you try this on both rough and smooth surfaces? Was the decrease in the degree of transmission consistent for both?

 

[ccdani]

I've tried it on both (also glasured ceramics) also on other thicknesses. When I measured the same thickness there was the same decrease of Transmission. The higher the thickness the lower is the difference between with immersion oil or with no Immersion oil.