Phosphorescence is a type of lighting that works because one light kept shining on a block, and then the lights went out, and it starts to glow. Phosphorescence seems to affect zombie spawns because of the light levels produced.
Mechanic[]
Phosphorescence in Minecraft comes in the form of zinc sulfate. Zinc sulfate has a maximum phosphorescent light level of 7 at the highest band in the visible spectrum. The bias of light on the other two visible bands are 80% on the blue band and 10% on the red band. This means that light from a block of zinc sulfate has light levels 80% less on the blue band than on the red band. According to the 1.25 phosphorescence mechanic, the block can have eight levels (3 bits) of phosphorescence. For one block to go up one level, a light of level 15 on any visible band has to strike the block for 100 seconds. For a block to become fully charged, it must be exposed to light level 15 light on any band for 13 minutes and 20 seconds. For lower levels to activate phosphorescence, it will take 15/x times longer for the process to work (x=light level striking block). Level 0 cannot charge a block. If the visible ambient light level on any band is lower than 3 levels above the state of the block, the block will glow.
Faces[]
If the glowing requirements above are satisfied on one face, the block will glow on the face, but not the others not satisfying the glowing requirement. Every lit face is equivalent to a sixth of the "phosphorescence energy release rate" of the block.
Energy in a Block[]
Every 20th of a second, the phosphorescent block receives 1 "energy" point. A block can have a maximum of 16000 energy points. The phosphorescence energy release rate of the block is 40 points per second. One ligt level is equivalent to 2000 points.