How to play Light Up
A grid of white cells and black walls, some of the walls numbered. Put light bulbs in white cells until every one of them is lit. A bulb lights its whole row and column, both ways, until a wall stops the beam.
The puzzle is also known as Akari, and in Japan as Bijutsukan, “art gallery”. Boards come in three sizes, 7 × 7 to 14 × 14. Every one is generated in your browser and has exactly one answer, which you can reach without guessing.
Rules
- Bulbs go in white cells only. Each one lights its own cell and every white cell in its row and column up to the nearest wall or the edge.
- Every white cell must end up lit.
- No bulb may shine on another: two bulbs in the same row or column need a wall between them.
- A numbered wall has exactly that many bulbs in the four cells beside it; diagonals do not count. A 0 has none. A wall without a number can have any number of bulbs beside it.
- Dots are your own notes for “no bulb here”, and the game ignores them when it checks the board. Bulbs that shine on each other turn red, and so does a number with too many bulbs or too little room left. A met number fades.
- The clock stops when the last cell lights up with every rule kept. Each size remembers its own board, and the clock pauses while the tab is hidden.
Controls
- Click / tap
- Cycle a white cell: empty → bulb → dot → empty
- Right-click / long-press
- Toggle a dot
- Drag
- Dot (or clear dots from) every cell you pass over
- Arrows / WASD
- Move the keyboard cursor
- Space / Enter
- Cycle the cell under the cursor
- B / X
- Toggle a bulb / a dot
- Backspace
- Clear the cell
- Z / Ctrl+Z
- Undo
- P
- Pause
Tips
- Start with the numbers that leave no choice. A 4 takes bulbs on all four sides, a 3 against the edge or another wall fills its three open sides, and a 0 rules out every cell beside it.
- A lit cell can never take a bulb. Look for a dark cell that only one cell can still light, either itself or a cell that sees it: the bulb goes there.
- The cells diagonal to a 3 can never hold a bulb. One there would see two of the 3’s neighbours at once and leave room for only two bulbs.
- Dot what you rule out. A dark cell surrounded by dots shows at a glance where its light has to come from.
Light Up strategy
Read every number against its room
A number’s room is its white neighbours that are still dark and empty. When the room equals the bulbs it still needs, every one of them takes a bulb; once the number is met, dot the rest. So a 4, a 3 beside one wall or the edge, a 2 in a corner and a 1 with a single open side all fill at once, and a 0 dots its whole neighbourhood.
Diagonals of 3s and 2s
A bulb diagonal to a 3 would light two of the 3’s neighbours and leave room for only two bulbs, so those diagonal cells never hold one. The same goes for a 2 with exactly three open sides: the diagonal cell between two of them is out, since a bulb there would leave only the third side. Dot these early; they often settle the cells around them.
Follow the dark cells
Every dark cell needs a bulb on itself or on a cell it can see. Count those candidates. When a dotted cell is down to one cell that could still light it, that cell takes the bulb. This is where dots pay off, and long corridors fill in this way once their ends are settled.
A short what-if on the big boards
On 10 × 10 and 14 × 14 the generator sometimes relies on a one-step test: put a bulb on a cell and follow the direct rules. If that leaves a cell nothing can light, or a number with too many bulbs or too little room, the cell is a dot. Undo back and mark it. The test never needs to go deeper than one bulb, and since each board has one solution, a contradiction is proof.
About Light Up
Light Up comes from the Japanese puzzle publisher Nikoli, which named it Bijutsukan, “art gallery”: picture a museum that needs every corner lit. It first appeared in issue 95 of the magazine Puzzle Communication Nikoli, and outside Japan it is also called Akari, Japanese for “light”.
Here the walls are laid out with the half-turn symmetry of Nikoli’s grids, and a random set of bulbs that lights the whole board is numbered onto every wall. Then numbers are taken away one by one, in random order, as long as a logic solver can still reach the whole answer. On 7 × 7 that solver only reads numbers and dark cells directly; on the larger boards it may also try a bulb and keep the opposite when the try runs into a contradiction.
Questions
Can I solve it without guessing?
Yes. The generator keeps a board only if its logic solver can finish it, and that solver never guesses. On 7 × 7 every step is a direct reading. On 10 × 10 and 14 × 14 you may sometimes need a short what-if: place a bulb, see that it leaves a cell nothing can light or breaks a number, and undo.
Do numbers count diagonal bulbs?
No. A number counts only the four cells directly above, below, left and right of its wall.
Is this the same as Lights Out?
No. In Lights Out each press flips a light and its neighbours, and the aim is to switch the whole grid off. Light Up is a pencil puzzle from Nikoli: you place bulbs, and their light runs along rows and columns to the walls.
Is my board saved if I leave?
Yes, in your browser, one board per size. The clock pauses while the tab is hidden.
