I have a 5mm High Brightness UV Led that I bought from Radio Shack that states Wavelength 405nm ( typ) View angle 20 Degrees FW current 20mA FW supply 3.3V ( typical), 4.0V (max.) part # 276-0014 pkg. of two

I have put this Led in a key chain light that has two 3V batteries in series making 6V output to the LED. 2V over Max. Will this change the wavelength of the led and intensity ? Is it hazardous to the eyes looking directly at it from a few feet away or looking at its reflection off objects with unprotected eyes. Thanks Light has a purple violet tone

  • \$\begingroup\$ What battery type? \$\endgroup\$ Sep 25, 2014 at 4:57
  • \$\begingroup\$ Two Three Volt 2016 lithium button cell batteries in series \$\endgroup\$
    – SST
    Sep 26, 2014 at 5:20

1 Answer 1


LEDS need a series resistance to control the current passed through them. The resistance 'drops' the excess voltage. Without that even a slight over-voltage will cause current to exceed the rating, and destroy the LED. I have disassembled some keychain lights. The better ones have a resistance or even an electronic current control chip, but cheap ones rely on the tiny batteries having enough internal resistance to limit the current to a value safe for the LED. If your light is one of these, the LED will probably be over-run for the first few minutes of operation, until the batteries deteriorate and reduce their available current. Overrunning a LED tends to make the emission spectrum broader, with a slightly longer dominant wavelength.

The LED will emit a band of wavelengths, with the highest output being at 405nm, which is visible violet light. (Most of the inexpensive so-called UV leds are actually visible violet emitters, which have an emission spectrum that may include some near-UV). Both the UV and visible violet will excite fluorescent or phosphorescent materials. Visible violet and near-ultraviolet light have slight tanning effects on skin, and are suggested to be involved in premature ageing of skin. Those wavelengths are also have some germicidal and photochemical effect, so are potentially harmful to unprotected living tissue, like eyes. However, sunlight contains a significant amount of 405nm radiation, so limited or low-intensity exposure is not considered hazardous. This type of LED emits so much visible light that discomfort and blinking is likely to prevent excessive eye exposure at close range. At a metre or more distance the hazard is negligible. Prolonged close exposure might eventually cause injury.


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