For decades SCHOTT has been using neutral density glass filters in the Vis spectrum for testing the photometric accuracy and linearity.

They provide a relatively constant transmission overthe visible wavelength spectrum and are calibrated at the following wavelengths: 440.0 nm, 465.0 nm, 546.1 nm, 590.0 nm, 635.0 nm

If desired, other wavelengths are also possible. The thickness of the filters is determined such that there is a stepping of the absorption values from approx. 0.25 A, 0.5 A and 1 A.