Astro optics pvt. Ltd. An iso 9001 : 2000 certified company manufactures various types of optical components and lenses are one of them. Astro optics pvt. Ltd. Presents a range of filters which are used to `filter` out light, which means to separate out wavelengths. As a part of that coated neutral density filters are like the absorbing glass neutral density filters but the only difference is that these filters are made by coating inconel or nichrome on optical glass.