A forensic detector for remotely sensing a porphyrin-containing substance on a surface of an object is provided. The forensic detector includes an excitation light source for generating an excitation light beam, plural optical elements, one or more sensors for measuring a first and second radiation powers, and a computer processor. The excitation light beam has an excitation wavelength capable of exciting porphyrin to luminesce at a target wavelength centered between 665-675 nm. The first and second radiation powers are in non-overlapping wavelength regions of the object-originated light beam. The computer processor is configured to compute a SNR according to the first radiation power and a background noise power level, and to determine if the porphyrin-containing substance is present on the surface according to the SNR. The background noise power level is obtained from the one or more second radiation powers.