There is a perceived need to measure the UV-INDEX exposure to people in a manner analogous to the dosimeter worn by X-Ray technicians, but two basic problems interfere: The formal UV-Index measurement definition is not an “exposure” measurement and UV radiation is easily blocked from the dosimeter.
The formal UV-Index specification created by the UN's World Health Organization and World Meteorological Organization in 1994 provides a measurement of the UV hazard to skin in a geographical site. It evaluates the hazard to a person wondering around in this site with no attention to what task the person is doing. e.g.: walking, lying, swimming. It also designed to measure the hazard in a fixed manner, requiring a cosine response with respect to the vertical.
This UV index definition was not intended to be implemented by a wearable or any device not aimed vertically, and one can’t claim that such devices are measuring UV index unless at the very least it is aimed at zenith when it makes the measurement and has an unobstructed view of the sky.
What can be salvaged from the UV-Index definition and used by a “UV Dosimeters”, is the shape and sensitivity of the erythema curve. A UV-Index measuring device. such as the Si1133, can be worn on the wrist and be changing directions constantly, always accumulating exposure readings. The wearable’s MCU can be programmed to use a suitable integration algorithm to give UVIH (UV-Index Hours) readings which are weighted so that despite the location of the sensor (the wrist), a user in a UV-Index environment of 5 for one hour would get a reading of aproximately 5 UVIH.
There are some obvious statistical difficulties to overcome with this concept, but they are not insurmountable.
One problem with this approach in that there are many cases where most of the time the sensor is aimed in a constant direction. The wearer could be walking down a beach with the sun at 45 degrees and the device aimed away from the sun almost the entire time. This is overcome by putting a sensor on both the wearable face and the opposing band allowing normal hand movements to briefly expose the sensor. The sensor in this case would operate in a peak detection mode over short intervals of about a minute.