Neutral Density (ND) filters are designed to reduce transmission evenly across a portion of a specific spectrum. ND filters are typically defined by their Optical Density (OD) which describes the amount of energy blocked by the filter. A high optical density value indicates very low transmission, and low optical density indicates high transmission (Equations 1 – 2). ND filters can be stacked to achieve a custom optical density. To calculate the final system OD, simply add the OD of each filter together.
| ATTRIBUTE | SPECIFICATIONS |
| material | NG1, NG4, NG9, NG11 or equivalents |
| wavelength range | 400-700nm |
| dimension tolerance | +0.1, -0.1mm |
| thickness tolerance | +0.2, -0.2mm |
| CLEAR APERTURE | >80% |
| flatness | < 1λ, subject to the size |
| SURFACE FIGURE: IRREGULARITY(N) | <0.5 fringe |
| od tolerance | +/-5% |
| COATING | With AR coating on outside surfaces |
