UV Index monitor prototype #1

I don't know if I mentioned it here before, but I've been working on a personal UV (Index) monitor.
Folks who have skin cancer (or those at high risk) need to make sure they limit their sun exposure.

The general approach is to just lather up with sunscreen every time you leave the house, but this is impractical (plus you have to re-apply every couple of hours).  This becomes more of an annoyance when you consider spending hours riding in a car: Are the windows UV protected? How well? Do you have to lather up every time you drive?

You can get UV index forecasts on your smartphone, but these are just forecasts (for your area and for the whole day). When you are out in the sun, you'll need to know how much UV intensity is hitting you  "right now".

Another solution is to carry a UV monitor.

The only ones I've seen on the market are overkill (too large and complex) or vague (how does this work and is it reliable -- where is the sensor?) .
I am aiming at something so small that you'll always carry it with you, but also clear and as accurate as possible.  My target form factor is a key fob:

My target UI is based on colored LEDs. There are official colors for the UV index scale and I have an LED for each level.  I would like to have (at most) 2 buttons -- one for "instant read" (point at the sun and an LED will light up for 2 seconds indicating UV index level) and one for setting a countdown timer (for sunscreen re-application).

My current prototype has 1 button, 5 high-intensity LEDs (green, yellow, orange, red and blue/violet) and is a little bulkier than a key fob. Amazingly, the LEDs are quite readable in bright sunlight! If you are colorblind you can always read index based on which LED lights up (right?). The current layout ramps "upwards" depending on UV intensity.

It takes a single coin cell battery and is based on a very low power 8051 from SiLabs. It should get 3-5 years off the battery with casual usage.

I need to do lots of tuning/calibration and I know it won't be "demo worthy" for the rest of this summer, but I am making progress.  Apparently, the calculations done for UV Index forecasting aren't very practical for small single UV sensors. Somehow, the personal UV monitors make due though.  I think I'll use one of the better ones to aid in my calibration.

Maybe I'll have case design and a formal board spin ready for next summer?