UV light is light in the "ultra violet" spectrum, beyond the violet part of visible light. Even though UV is not visible to human eyes it does effect the eyes and skin! UV light is what lets us produce vitamin D, which is essential for humans. But it can also burn skin - what we call a sunburn.

We can measure UV in mW per area, but that's a little tough to keep in your head, especially if the area value is in cm or m or ft. To make it easier for people to track UV safety, the WHO came up with the UV index.

The SI1145 is calibrated to transmit the UV index based on the visible & IR light received, but what do those numbers mean?

As you're probably aware of - if you have lighter skin, you are more likely to get burned. So it's important to watch the UV index and wear sunscreen, hats, and sunglasses when the UV index gets above 4 or so!

(UV index text below from epa.gov)

• Wear sunglasses on bright days.
• If you burn easily, cover up and use broad spectrum SPF 30+ sunscreen.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

3 to 5: Moderate

A UV Index reading of 3 to 5 means moderate risk of harm from unprotected sun exposure.

• Stay in shade near midday when the sun is strongest.
• If outdoors, wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

6 to 7: High

A UV Index reading of 6 to 7 means high risk of harm from unprotected sun exposure. Protection against skin and eye damage is needed.

• Reduce time in the sun between 10 a.m. and 4 p.m.
• If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

8 to 10: Very High

A UV Index reading of 8 to 10 means very high risk of harm from unprotected sun exposure. Take extra precautions because unprotected skin and eyes will be damaged and can burn quickly.

• Minimize sun exposure between 10 a.m. and 4 p.m.
• If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

11 or more: Extreme

A UV Index reading of 11 or more means extreme risk of harm from unprotected sun exposure. Take all precautions because unprotected skin and eyes can burn in minutes.

• Try to avoid sun exposure between 10 a.m. and 4 p.m.
• If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.