The Shade sensor tells you not only the instantaneous UV Index being experienced by you, but also calculates your accumulated UV exposure through the course of the day.
The UV Index is a clinically accepted measure of the strength of UV. This was developed by doctors studying sunburn in 1987. They discovered that high-energy UV (UVB) is able to cause more damage than low-energy UV (UVA). The UV Index metric was built to reflect this.
Being exposed to UV over a certain amount of time results in UV exposure (also known as UV dose). On the Shade app, this is measured in UV Units. At a UV Index of 1, it takes 40 seconds to accumulate 1 UV Unit. At a UV Index of 2, it takes 20 seconds to accumulate the same 1 UV Unit.
The higher the UV Index, the quicker the UV exposure accumulates. For more advanced readers, 1 UV Index is equivalent to 25mW/m² of UV intensity. 1 UV Unit is 1 J/m² of UV energy. The relation between UV exposure (UVx) and UV Index (UVI) can be summarized for a period of t seconds as: