Phosphor-persistence (called Afterglow in other shaders) is a crt effect available in just some crt shaders: crt-geom-deluxe, crt-guest-advanced, glow-trails, for example.
I’ve detached the code from crt-geom-deluxe, which I think is simple enough and made a preset called ‘phosphor-persistence’ inside crt folder (already uploaded into the oficial repo). This preset can be prepended to any other crt-shader to add the effect. It’s a bit different compared to the glow-trails implementation.
One odd thing about phosphor persistence is that you can’t take screenshots of the effect, because it’s a temporal effect. Games where you can easily spot the effect are: Rtype-3 (title screen, snes), Thunder Force 3 (genesis), vector games.
It’s true that you can only ‘see’ the effect in a few games, though I think it manifests on most of them. At first, I thought it wasn’t real until I tested R-Type 3 on my own real CRT TV and can confirm it’s real and the crt-geom-deluxe params is set to get the exact tail size of the asteroids/comets passing through this game title scene.
This subject was always discussed in side comments of other threads, so I thought it deserved its own topic.