I do not mean to be negative and I'm sure it is a cool LED, but 400 nm is the wavelength for UVA also known as Ultra-violet A which is the closest UV to the visible spectrum. A good purple, is around 420 nm. Still, 400 nm is on the purple side of UV so probably better visually than some of the lower types eg. UVB, UBC, FUV, SUV (I kid you not :D), and EUV. I have tested LEDs in that range (400-405) in the past. In fact they are commonly used for Fish Tanks, believe it or not :). However, for our purposes they are not as bright as colors that are completely in the visible spectrum. Whether they are harmful to the eyes or not, I do not know.