Hey it almost works

code/modules/lighting/lighting_overlay.dm

@@ -7,13 +7,12 @@
 
 	icon             = LIGHTING_ICON
 	color            = LIGHTING_BASE_MATRIX
-	#warn [insert crashing airplane noise here]
 	plane            = LIGHTING_PLANE
 	mouse_opacity    = 0
 	layer            = LIGHTING_LAYER
 	invisibility     = INVISIBILITY_LIGHTING
 
-	blend_mode       = BLEND_MULTIPLY
+	blend_mode       = BLEND_OVERLAY
 
 	var/needs_update = FALSE
 
@@ -80,10 +79,10 @@
 	var/max = max(cr.cache_mx, cg.cache_mx, cb.cache_mx, ca.cache_mx)
 
 	color  = list(
-		cr.cache_r, cr.cache_g, cr.cache_b, 0,
-		cg.cache_r, cg.cache_g, cg.cache_b, 0,
-		cb.cache_r, cb.cache_g, cb.cache_b, 0,
-		ca.cache_r, ca.cache_g, ca.cache_b, 0,
+		1-cr.cache_r, 1-cr.cache_g, 1-cr.cache_b, 0,
+		1-cg.cache_r, 1-cg.cache_g, 1-cg.cache_b, 0,
+		1-cb.cache_r, 1-cb.cache_g, 1-cb.cache_b, 0,
+		1-ca.cache_r, 1-ca.cache_g, 1-ca.cache_b, 0,
 		0, 0, 0, 1
 	)
 	luminosity = max > LIGHTING_SOFT_THRESHOLD

code/modules/lighting/lighting_source.dm

@@ -92,10 +92,7 @@
 		top_atom = new_top_atom
 
 		if (top_atom != source_atom)
-			if(!top_atom.light_sources)
-				top_atom.light_sources = list()
-
-			top_atom.light_sources += src // Add ourselves to the light sources of our new top atom.
+			LAZYADD(src, top_atom.light_sources) // Add ourselves to the light sources of our new top atom.
 
 	EFFECT_UPDATE
 
@@ -121,7 +118,7 @@
 		top_atom = source_atom
 		. = 1
 
-	if (istype(top_atom, /turf))
+	if (isturf(top_atom))
 		if (source_turf != top_atom)
 			source_turf = top_atom
 			. = 1

code/modules/lighting/lighting_turf.dm

@@ -61,6 +61,16 @@
 
 	return CLAMP01(totallums)
 
+// Returns a boolean whether the turf is on soft lighting.
+// Soft lighting being the threshold at which point the overlay considers
+// itself as too dark to allow sight and see_in_dark becomes useful.
+// So basically if this returns true the tile is unlit black.
+/turf/proc/is_softly_lit()
+	if (!lighting_overlay)
+		return FALSE
+	
+	return !lighting_overlay.luminosity
+
 // Can't think of a good name, this proc will recalculate the has_opaque_atom variable.
 /turf/proc/recalc_atom_opacity()
 	has_opaque_atom = FALSE