Where are you located?
I have an apogee sensor SQ-110. This is calibrated for sun, but can be converted for LED. I do not have a special meter, just use a voltmeter. Willing to lend it out to BAR members. But you can also buy these for only $139...
One of mine did this; it’s a very common problem. The newer koralias have an “anti-reverse” feature on the cage that’s supposed to prevent the issue but mine had broken off in regular use (less than 6 months).
You might be able to fix it with a dab of cyanoacrylate. *Carefully* take off...