Starscape - Light Pollution

This filter has many names; Starscape, Intensifier, Red Intensifier, Ra54, Light Pollution filter, "didymium" filter (technical term) and so on. In today's digital photography world it is most commonly used by astrophotographers to reduce the orange/yellow glow caused by city lights. 

The filter targets specific wavelengths of light emitted by common Sodium and Mercury-vapor street lamps and blocks a portion of that orange/yellow glow as it passes through the filter. The effect can be dramatic or subtle depending on how strong the light source is and how much atmospheric haze is in the air.

Remember, the filter is not adding blue or changing the color of the scene. It is literally stopping specific areas of the color spectrum from passing through the filter. The camera never sees that light or color.

The end result is a "cleaner" RAW file without the orange/yellow glow polluting your image. It allows you to color correct and enhance colors without needing to compensate for the "pollution".

The filter can also be used during the daylight hours to enhance red, orange and yellow colors. It was originally created for film photographers to "intensify" warm fall colors on overcast and cloudy days. The filter is still used today by digital and film photographers to target those specific colors for enhanced saturation while leaving the blue and green colors alone. Add a circular polarizer to this and you will have an excellent filter combination to capture fall colors.

NOTE: The filter has a base filter factor (light loss) of 1/2 stop, but you may see a 1-2 stop difference in exposure depending on how much of your scene is lit by Sodium or Mercury Vapor lights.