As you know, Reflective ND filters consist of metallic thin film optical coatings.
In the first step, I am looking for which metallic material is better use for deposition? In the Second step, I would like to know one of the best method that I can check in its QC (Quality Control).
Dear Sepideh,
if you have a possibility to access a physical vapor deposition facility, it will be very simple to evaporate a thin metallic layer on a glass substrate, thus obtaining the ND filter. You could also play with the time of evaporation to decide which ND degree your filter will be. About the metal, You could use gold, silver or aluminum. The last two tend to easily oxidize though but this should not severely change the properties of your filter.
Cheers, Roberto
Roberto's suggestions are correct. I would note, however, that if you want control over the filter transmission, you need very precise control over your evaporation parameters, as the film very quickly becomes 100% reflecting (as far as I remember some 10s-100s of nm).
Dear Roberto,
Thank you so much for your consideration. I would like to control and modify the intensity of all wavelengths without changing the color. So it is better can use a material that can make colorless filter or transparent (light smoky). These metals which you mentioned, do not have colorless.
I have made near-UV NDF's from evaporating metal onto quartz, but they will only have good reflectance over a limited wavelength range. You can just do thermal evaporation onto a glass or quartz substrate. Obviously the thermal control needs to be precise; the higher the temperature the higher the evaporation rate. The evaporation is usually anisotropic so you would need an apparatus that can spin the substrate during evaporation, this will help make the deposition uniform. There should be some prebuilt systems on the market that can do this.
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