Didymium filters, also known as the “poor man’s sky glow filter” are used as safety goggles by glass blowers. Didymium glass selectively blocks the radiation component emitted by sodium in glass (589 nm), which is also the predominant component of regular sodium-vapor street lamps.

Sodium-vapor street lamps are a major contributor to light pollution, which is why one sees very few stars in urban areas. Long exposure photographs from cameras pointed in the direction of urban areas also develop an orange cast which may not be visible to the naked eye.

An example of what the filter can do is shown on the left.

Both photographs were taken at San Luis reservoir on Hwy 152, and are 20-second exposures at ISO 1600 with long-exposure NR enabled on camera. The one on the right is taken with the Nikkor 20mm AF-D lens at f/2.8 without any on-camera filters, whereas the one on the left is the same lens, but with the Hoya 62mm Red Enhancer (Intensifier) filter. The sky glow on the right is from the city of Los Banos, and the filter does a very good job in reducing the sky glow intensity and improving the contrast of the stars close to the horizon.