Which of the following method is used to detect semen stains on clothes?

Semen stains on cloth can be identified by ultraviolet light. When examined under UV light seminal stains show a strong bluish white fluorescence. Stains mixed with blood does not fluorescence. If the seminal stain does not fluoresce on account of its being mixed with blood, this can be overcome by spraying the stained area with 1% alpha naphthol phosphate with 1% sodium hydroxide solution two minutes later. Ref: Forensic Medicine By P.V. Guharaj 2nd edn, page 281.