Neutral density filters attenuate light by a constant amount that is, to first order, independent of the wavelength. Gelatin ND filters are the cheapest but tend to have more fluctuation with wavelength and a narrower wavelength range than glass ND filters and metallic types.  Gelatine neutral density filters work by absorption so care must be taken to avoid overheating which can lead to damage to the filter.  Gelatine ND filters can also be cut easily to bespoke custom sizes in your own laboratory which can be extremely useful.

Wratten neutral density gelatin filters are now being replaced by Wratten 2 type filters which are manufactured using a ridged polymer.  Wratten 2 filters are more robust and unaffected by water and moisture as gelatine type.