Spectral transmittance of Christiansen filters - Experimental observations

Powder of the optical glass K5 has been immersed into methyl benzoate as re fractice index matching fluid to fabricate Christiansen filters. The intern al spectral transmittance of these filters has been investigated in the vis ible spectral region as a function of the filter thickness, the mean diamet er of the powder grains, and the difference between the refractive indices of the fluid and the K5 glass. The minimum internal spectral extinction of the filter curve is approximate ly proportional to the thickness of the filter. Furthermore, it scales inve rsely with the average diameter of the glass grains. Hence, one can deduce that the minimum spectral extinction is proportional to the total interface area between grains and immersion liquid. According to the experimental re sults it can be concluded further that this extinction is mainly due to Ray leigh scattering. The halfwidth of the spectral transmission passband decreases with increasi ng thickness of the filters and with decreasing average diameter of the gra ins. The spectral extinction at wavelengths sufficiently far from its minim um increases sublinearly with the filter thickness and the inverse mean dia meter of the grains. In the same spectral region, the extinction increases also sublinearly with the absolute difference between the refractive indice s of the material of the grains and of the immersion liquid. Until now, a t heory predicting all of these observations correctly seems to be still miss ing.