I want to operate a UV LED at continuous 20mA for one week. However, when I measure the LED intensity with a photodiode (see image), I find that the light intensity changes continuously in time. I would like to ask you how I can reduce the light intensity drift that I am observing.
The LED data sheet is: http://static.vcclite.com/pdf/VAOL-5EUV8T4-LED-5mm-UV.pdf. I am operating it in series with a 461.6 ohm resistor and a 12 V AC-DC power supply. The voltage drop around the LED is 3.2 V, the current is 19 mA.
I am measuring the light intensity with a photodiode connected to an Arduino. I attach a picture of the circuit and the graph of light intensity (photodiode voltage output) vs time (hours).