Special UV cameras use a filter that captures only the UV light in an image. Since sunscreen effectively protects skin against UV light, the cameras can also identify traces of sunscreen application: Any surface area you cover will appear darker in color (usually a dark blue), while unprotected areas will appear light. Neat, no? 

Big, big caveat: Because different types of sunscreens have different mechanisms on the skin, they will show up differently in the UV imaging.

See, chemical sunscreens work by absorbing UV rays, triggering a chemical reaction, which then transforms the UV into heat. “Chemical sunscreens are visualized well on UV cameras,” says board-certified cosmetic and medical dermatologist Blair Murphy-Rose, M.D., FAAD, as the protected areas will appear quite dark on-screen. 

Mineral sunscreens, on the other hand, manually block UV rays by creating a barrier on the skin. They do some absorbing like their chemical counterparts, but they also deflect light—that’s why you may notice a light blue or gray color under the UV camera as opposed to a uniform darker hue. 

That doesn’t mean the mineral sunscreen is ineffective, contrary to what some users on TikTok seem to believe. Just make sure you know what color to expect from your UV camera before slathering on, and that will help inform your application.