Novel photographic pentamethine cyanine dyes

Novel pentamethine cyanine dyes which act as spectral sensitizers for silver halide in the infrared region of the spectrum are of the formula:- where R1 and R2 are each alkyl or together form a saturated ring, R3 and R4 are each alkyl or together form a saturated ring, R5 and R7 are alkyl, substituted alkyl or aralkyl, R6 is hydrogen or an alkyl group comprising 1 to 4 carbon atoms, A represents the atoms necessary to complete an optionally substituted 5 or 6 membered heterocyclic ring, and X is anion, however when R5 is a sulphoalkyl group the dye may be obtained as a zwitterion in which case X is absent.