Midwest Optics SKU Ni120 Vis+IR. Neutral Density filters reduce transmission uniformly over a specific portion of the spectrum. They are defined according to their Optical Density (OD). The higher the OD, the lower the transmission. These filters can be stacked to create filters with custom ODs. They are useful when trying to image extremely bright subjects or when trying to separate subject matter from a background by reducing depth of field.
nLearn more at http://midopt.com/filters/neutral-density/.