The Meprolight M21, best known for being the favorite optic of many IDF soldiers, has one flaw: if you use it in relative darkness and are aiming at a very well lit target, the reticle brightness can be on the dim side, making it hard to aim. The M21 is lit by a fiber optic collector, with additional light coming from a tritium lamp, not by batteries, so it works well both during the night and day just so long as the target is not lit up brightly relative to the optic/operator.
This problem is well known and would not stop me buying a M21, in fact I wanted to buy one earlier this year but when the company stopped returning my emails I gave up and spent my money elsewhere, but some ingenious person came up with a solution: a little battery powered light that attaches to the M21’s outer-case to shine light into the the fiber optic collector.
Go ahead and read the review of the WOR (Wash Out Remover) at The Bang Switch.
Unfortunately The Bang Switch did not say who makes the WOR or where you can guy it. My Googling did not turn up any retailers selling it. If you know where it can be bought, please post a link in the comments below.