The light meters in my opinion are only there to "look like" that the light offerings are consistent around the world.
And honestly, this idea of comparing the light to the first reading taken (when the batsmen first came off) is epic bull crap. I wonder what they're going to do if a team is 391/2, and continues to bat till its pitch black, and then the next innings a team is struggling 130/9, and the light is "pretty bad" but not pitch black, like in the first innings.
Then what? Complete farce, to be honest. Just let common sense prevail, and please do not give Asoka de Silva the power to judge the light

This is test cricket, umpired by two neutral (or should be) umpires. I think it is fair to say that if the players respect the umpires as they are suppoused to, the judgement of coming off should lie with the umpires' senses, and not technology. Has this game become that dependent on technology that we have to use a meter to judge the light?
But then again, remember the pictures you see on TV is not the actual light - the lenses are enhanced in those feeds. I think only the stump cam does not have that "enhancer" which actually depicts the light.