UV Index is a measurement system used by the World Health Organization to determine the likelihood of getting a sunburn. It measures the wavelengths of light that come from the sun that are in the range that cause sunburn. As these are the same wavelengths that we use to synthesize vitamin D3, the reptile community has adopted this measurement system for our UVB needs.
Previously, we used a system that measured the energy hitting a certain area and that is where you see the units of micro-watts per square centimeter (µw/cm^2). When you review the past works of Dr. Gary Ferguson and other scientists you’ll find all measurements done in this system. The meters that are used to measure UVI and µw/cm^2 measure different wavelength spans so there is no direct conversion between the two.
For context, here are some prominent UVI levels
UVI 0 = Nighttime
UVI 3 = target basking for Veiled & Panther Chameleons
UVI 11 = Human skin burns in 10 minutes
UVI 13 = typical daytime maximum for lowland Madagascar
UVI 43 = maximum natural UVI recorded on Earth
UVB technology today can expose chameleons to UV Index levels from UVI 0 to UVI 160 depending on the distance from the bulb and the filtering. It is important that we use UVB correctly!