This is the FND-25C02-20, a 25 mm diameter, unmounted Neutral Density Filter from OptoSigma. It is designed for use in the visible light spectrum, specifically between 400 and 700 nanometers. The filter is made from N-BK7 Schott, a high-quality borosilicate glass known for its excellent optical properties.
The filter features a reflective coating on both the front and back surfaces, identified as 'opt fnd-20'. This coating is designed to provide a neutral density effect, reducing the intensity of light without altering its color balance. The thickness of the filter is 2 millimeters, contributing to its overall durability and stability.
The FND-25C02-20 is shaped as a thin lens, making it suitable for a wide range of optical applications. It is currently in stock and available for purchase through the OptoSigma website, as indicated by its 'Valid - in stock' web link status.