The didymium glass filter 666-F7 made of SCHOTT glass BG 36 can be supplied to check the wavelength accuracy in both the UV and visible range.

This filter can additionally be used to check the photometric accuracy at wavelengths of 270 nm, 280 nm, 300 nm, 320 nm and 340 nm. Didymium glass shows a spectrum with a multitude of highly defined peaks. The heights and positions of these peaks are dependent on and will vary with the chosen slit width of the measuring instrument.