Black Light can be used for spectacular photography or just for having some photographic fun, but if you just want to try out a quick trick for testing your home for bacteria there is a way to do it for a couple of cents.
Turns out that a certain mix of sharpie ink will block all light but back light. The folks at Hefty.co made a quick tutorial on how it’s made.
You would need a blue Sharpie and a purple Sharpie and some tape. Applying two blue tape layers and one purple tape layer will act as a filter for the smartphone flash. In total darkness shining that flash onto anything will reflect any black light (or fluorescent emittance) from found objects.
The phenomenon is caused because some substance and bacteria tend to fluoresce in various colors when shown with black light. According to Wikipedia:
A Wood’s lamp [UV emitting light source, UT] is a diagnostic tool used in dermatology by which ultraviolet light is shone (at a wavelength of approximately 365 nanometers) onto the skin of the patient; a technician then observes any subsequent fluorescence. For example, porphyrins—associated with some skin diseases—will fluoresce pink. Though the technique for producing a source of ultraviolet light was devised by Robert Williams Wood in 1903 using “Wood’s glass”, it was in 1925 that the technique was used in dermatology by Margarot and Deveze for the detection of fungal infection of hair. It has many uses, both in distinguishing fluorescent conditions from other conditions and in locating the precise boundaries of the condition.
And the tape makes our phones act just as Wood’s lamp.
[tape on his smartphone and colors it in | h/t Rotem]