What coating do old non-HMC Hoya filters have?

Question

Recently I was looking for some used 85A filters to shoot tungsten film outside. My primary consideration are ones made by Hoya because they are good quality, abundant and relatively inexpensive. But I would prefer those that have multi-coating.

Some older Hoya filters (those with older Hoya logos) have the HMC marking on them, like modern ones, but most others do not.

Any idea what type of coating do older non-HMC-marked Hoya filters have?

Hoya had many different packaging designs and several different logos over the years. Some filters have HMC marking, some don't, and some have things like "This filter is coated on both sides" written on the packaging. It's all a little confusing. Were any of the non-HMC-marked filters actually multi-coated?

Answer 1

"Multi-coated" doesn't primarily reference a filter that has a coating on more than one side. It means each coated surface has multiple coating layers on the same surface.

Each of these multiple coatings are used to control reflections of the various wavelengths of light. Just as refractive lenses bend light slightly more or less depending on the different wavelengths of light, different coating materials will be more effective at certain wavelengths and less effective at other wavelengths. By using multiple layers that are each optimized for different ranges of the wavelengths of visible light, a multi-coated surface can suppress reflections more or less equally across the entire visible spectrum.

A filter could have a single layer coating on both surfaces and would still not be considered a true 'multi-coated' filter.

Were any of the non-HMC-marked filters actually multi-coated?

Probably not. If the coated surfaces have more than one coating layer Hoya would almost certainly have labeled the filter as 'HMC' rather than "...coated on both sides."