I'm not actually sure that this gives you the light levels, but the most detailed command that I've been able to find for individual interfaces is 'show system state filter-pretty sys.sx.py.stats' where X is the slot number and Y is the port number.
I would check with your SE if there is an active feature request to add this feature into PAN-OS. If there is post it here so that others can add their votes to it ;-)
Click Accept as Solution to acknowledge that the answer to your question has been provided.
The button appears next to the replies on topics you’ve started. The member who gave the solution and all future visitors to this topic will appreciate it!
These simple actions take just seconds of your time, but go a long way in showing appreciation for community members and the LIVEcommunity as a whole!
The LIVEcommunity thanks you for your participation!