The HoDi liquid filter consists of a solution of holmium and didymium (praseodymium and neodymium) in perchloric acid. This filter features an especially broad wavelength spectrum and is therefore ideally suited to checking the wavelength accuracy of spectrophotometers in the UV and visible range.

It has a broad spectrum with a variety of characteristic, very well-defined peaks in the range between 241 nm and 864 nm. Depending on the performance of the spectrophotometers used, up to 22 peaks can be detected at a slit width of 1 nm.