An infrared dye, characterised in that the dye comprises two bridged diarylpolymethine type dyes or derivatives thereof connected together at either the 3, 4, 5 or 6 position by a central moiety such that the two dyes are located on each side of the central moiety, wherein the infrared dye absorbs strongly in the near infrared region of the spectrum but poorly in the visible region of the spectrum.