what's an "ND Filter wheel"?

Question

I saw this term but don't know what it is.

Answer 1

A ND filter is a "neutral density" filter used to uniformly cut (i.e. it effects all visible light frequencies equally) the amount of light through a lens. They are available in various optical densities to cut light by specified amounts. See the link.

They are used on home theatre projectors to cut the amount of light reaching a screen from high power projectors to prevent "hot spotting" (too bright an image).

Some HT enthusiasts --- who like bright images -- use this type of setup to get both a bright image and long bulb life (i.e. by using different strength ND filters as the bulb ages until the bulb is eventually replaced after all filters are removed).

A ND filter wheel is a set of ND filters of different optical densities mounted in a single wheel which can be rotated around a fixed point so that different density filters can be selected in a light path. They would be useful (although expensive) way for the HT enthusiast who wants to be able to experiment with the technique described above, and doesn't want to buy many large photographic ND filters.

Hope this helps.