This 55mm RA56 Green Enhancer/Green Field Filter from Hoya is designed to enhance colors in the green spectrum without changing or having negative effects on other colors. Combining the Green Enhancer with a circular polarizer or a UV filter will also increase the saturation of the color and the sharpness of an image, such as foliage in a landscape shot.
The glass filter has a filter factor of 2.5, so you should increase your exposure by 1 and 1/3 of a stop to compensate. Transmission of light passes easier through the filter because of a single type of coating, which has been placed on both sides of the glass, reducing reflections off the surface of the filter. The ring is made of aluminum, adding strength and protection to the lens while it is screwed on.