Blue LED lights have appeared in recent years on anything from Christmas tree lights to police cars. However, they're incredibly difficult to focus on compared to any other light source, often appearing somewhat of a blur. Why is this?
I think it is a combination of being at the blue/indigo/violet end of the spectrum and the way some people's eyes work, especially astigmatism. Black lights are very hard to look at - the purple glow.
My dad had an adventure with blue Christmas lights back in the 60's before LED lights. He went into a store in Chicago and asked for some of the blue lights with the little red end. The clerk had no idea what he was talking about, even when he pointed to lights right there. He (being the kind of person he was) started asking the people waiting behind him, then people passing in the aisle and found some fraction saw little red tabs and thought they were part of the bulb design and the rest didn't see anything. He brought a set home and I could see them and I know I have astigmatism (distortion of lens shape)
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