I bought some cheap LEDs from a vendor on eBay, and found that most of them had air bubbles inside of the clear epoxy that makes the lens. This causes the light to scatter, and the LED will not meet the specifications that are given for the light beam output. They still put out bright light, but if the LED is used in a flashlight for example, the beam will not be narrow as it was intended to be, and the light will not be as bright within the light beam.
I complained to the vendor, and sent a picture of the air bubbles. The vendor replaced the defective LEDs, but I got to keep the defective ones, so this must be one of those. Some vendors are unscrupulous, and will not acknowledge that the LED is defective, since it puts out light. I had one eBay vendor tell me that if I filed a dispute she would make sure to tell other vendors and that no other vendor would then deal with me (nasty witch!). I had to go through the resolution process and send the LEDs back and pay for shipping, and I finally got my money back.
This LED happens to be a blue LED. It seems that the blue LEDs are somewhat more expensive than other colors, and so the vendors are more prone to buy the cheaper rejects, and try to get them to slip through, hoping the buyer doesn’t notice the flaws. But all one has to do is look into the front of the lens to see the air bubble.
Back to experimenting…