Here's a page with images and some description

(http://architectures.danlockton.co.uk/2006/10/28/a-vein-atte...)

It might be the type of lighting that makes it much harder to find a vein to inject, making it much less useful for taking drugs.

Certain lighting (most often used in public bathrooms) makes it very hard to see your veins & shoot up.

UV fluorescents make it harder to find a vein.