The holmium glass filter 666-F1 has a range of narrow, well defined peaks in the UV and visible range, making holmium an excellent choice for checking the wavelength scale of spectrophotometers. In comparison to filters that use holmium solution (Holmium liquid filter), the holmium glass filter has a weaker spectrum with fewer peaks. In the low UV range the absorbance behavior of the glass matrix is superimposed on the holmium peaks. The main advantage of using a glass filter over a liquid filter is that it is more robust.