Physical developer (PD) is an aqueous, silver-based reagent that is used to develop prints on porous (mostly paper) and semi-porous items. It acts on components of the fingerprints that are "greasy" and are not easily washed out with water (like amino acids). It often develops prints that were not detected or developed weakly with amino acid reagents like indanedione and ninhydrin.
Because after the use of PD other fingerprint reagents cannot be used anymore it should be used last in a sequence (e.g. ninhydrin and then PD). When a paper item has been wet or a long time in a high humidity environment so that amino acid reagents are useless it can be used as the first (and last) reagent.
Partial fingermarks developed with PD on notepad paper
Detail of a partial fingermark developed with PD on a piece of paper from a notepad