I'm making a slime farm and since the wiki article comments argue slimes are quite picky when it comes to collisions, I plan "alternative lighting", like embedding the jack-o-lanterns in the spawning floors. Now, can I reasonably expect no common mob can spawn on them due to light level but since slimes ignore light level, I hope embedding the lanterns in the spawn floors will suffice.
Still, slimes share the same requirement as other mobs for underlying block: it must be 'opaque' by game mechanics, or stairs or upper slab, and the wiki says "Transparency: Partial (blocks light)" about jack-o-lanterns. Considering it generates light 15 and there are no stronger light sources in the game, that claim is rather bogus (unless you count "magical" properties of sunlight). But are they considered opaque from mob (slime) spawning point of view?
Best Answer
Jack-o-lanterns do allow mobs to spawn on them (so long as they can spawn at any light level).
Jack-o-lanters are both solid and opaque, allowing mobs such as slimes and pigmen to spawn on them. Compare this to Glowstone which is solid but not opaque (it is transparent), thus preventing spawning.
Unless they changed this since version 1.4, jack-o-lanterns are identical to regular pumpkins aside from glowing, meaning they are in no way, shape or form transparent. The fact from the Wiki on them being "partially transparent" is entirely incorrect.