ND090 absorptive neutral density filters have an optical density of 0.9, or average 12.5% transmission across the visible spectrum. These filters include identical male and female threads on either end so that they can be stacked with other ND filters to achieve various optical densities. For example, the result from stacking two ND090 filters together would be almost identical to one (ND180) filter – transmitting approximately 1.6% of visible light.