What is the UV camera showing in your video?

The video uses a UV camera to demonstrate the difference between mineral sunscreen and chemical sunscreen mode of action.
Cameras work by sensing light reflecting off objects. The less light reflecting into a camera, the darker the object will appear. A UV camera works the same way, but with a special filter that only lets in UV light.

By design, Chemical Sunscreens absorb UV light and convert that energy to heat. Because of this exchange, there is no UV light reflecting away from the sunscreen. With no reflected light, the object appears black through the UV camera.

In contrast, Mineral Sunscreens reflect/refract/scatter UV light away from the skin. This means the UV light is reflecting from the sunscreen to the camera, and the object will appear lighter.