Place light bulbs in white cells to illuminate every white cell.
A bulb shines horizontally and vertically until a black wall or the board edge blocks it.
Light bulbs cannot shine on one another.
A numbered black wall must have exactly that many bulbs in its orthogonally adjacent white cells.
Mark a white cell with a dot when you know it cannot contain a bulb. A numbered wall turns green when its bulb count is met and red when the clue can no longer be satisfied.
A 0 wall has no bulbs beside it at all, and a 4 wall has one in every direction. Both are certain before any other deduction.
Example: Dot the cells above, below, left, and right of every 0. Put a bulb in all four white neighbours of a 4.
Once a numbered wall has its full count of adjacent bulbs — the clue turns green — every remaining white cell beside it is a no-bulb.
Example: A 1 wall already has a bulb below it. Dot its other three neighbours.
Count how many bulbs a clue still needs, then count the neighbouring cells that are still undecided. If the numbers match, every one of them is a bulb.
Example: A 2 wall on an edge has three white neighbours, one already dotted. The other two must both hold bulbs.
Every white cell must be lit by something in its own sightline, including itself. When only one cell in that sightline can still take a bulb, that is where the bulb goes.
Example: A dotted cell in a short corridor has walls on three sides and only one undotted cell left in view. Place the bulb there.
Two nearby clues often share the same white cells. What one clue forces immediately constrains the other, so read them as a pair rather than one at a time.
Example: A 1 and a 2 wall share a single white cell between them. Deciding that cell settles how many bulbs each clue still needs from its other neighbours.
Clues set diagonally from each other pin down their shared corner cells. A high clue diagonally beside a 0 pushes its bulbs away from the 0’s neighbours.
Example: A 3 wall diagonally adjacent to a 0 wall cannot use the two cells the 0 has already ruled out, so its three bulbs are forced into the remaining directions.
An unlit white cell is not an error — it just means the puzzle is unfinished. Work from the clues first, then sweep the board for cells nothing reaches yet.
Example: With every clue green, look for the remaining dark cells and place the bulbs that only they still need.