What is a Light Up?
Light Up (also called Akari) is a logic puzzle in which you place bulbs on the white squares of a grid so that every white square is lit, no bulb shines on another bulb, and each numbered black square has exactly that many bulbs touching its sides, where a bulb lights its whole row and column until a black square or the edge of the grid stops it.
Also known as: Akari, Bijutsukan.
How do you solve a Light Up?
- 01
Fill the walls that leave no choice
A wall marked four has a bulb on every side, and a wall whose number equals the white squares it touches, such as a three against the edge, has a bulb on each of them. Place these first.
- 02
Mark what a nought forbids
A wall marked nought has no bulb on any side, so mark those squares as empty; on this site clicking a white square cycles it from empty to bulb to a mark. A nought is often the most useful number on the board, because the squares it empties must be lit from somewhere else.
- 03
Mark everything a bulb already lights
No bulb may stand in another bulb’s beam, and bulbs do not block light, so every square a placed bulb reaches is off limits. Mark the whole beam; the remaining unlit squares then stand out, and each must be lit from a square that is still unmarked.
- 04
Finish numbers whose options have run out
When the unmarked squares around a numbered wall are down to the number it still needs, they all take bulbs; when a wall has its full count, its remaining sides are marked empty. The board flags a wall with too many bulbs at once, but one that is merely short is not flagged until the end, so keep counting.
- 05
Light the squares that can only be lit one way
Take an unlit square and look along its row and column to the nearest walls or edges. Every unmarked square in that reach, itself included, could be its bulb, and if only one is left, it takes the bulb.
- 06
Use corridors as the endgame
A run of white squares between two walls holds at most one bulb, because two would see each other, and a bulb placed there also lights its column; if that column already holds a bulb, the corridor must be lit from elsewhere. Every puzzle here has exactly one solution, so no guessing is needed.
Common Light Up mistakes
- Placing a bulb on a square that is already lit. It is in another bulb’s beam, and the board marks both bulbs; mark lit squares so the temptation is not there.
- Assuming bulbs block each other’s light. They do not, which is why two bulbs in one corridor are forbidden rather than merely wasteful.
- Counting diagonal squares against a numbered wall. A number counts only the four squares sharing an edge with the wall.
- Leaving a numbered wall short and not noticing. The board does not complain about too few bulbs until the end, so a three with two bulbs looks fine until nothing else fits.
Where does Light Up come from?
Light Up was created by the Japanese puzzle publisher Nikoli and first appeared in 2001 under the name Bijutsukan, meaning “art gallery”, a reference to the rooms and corridors the black squares carve out of the grid. Nikoli later settled on the name Akari, which means “light”.
It spread outside Japan with the Nikoli puzzles that followed Sudoku in the mid-2000s, and Light Up is the English name most sites use.
Is Light Up the same as Akari?
Yes. Akari is Nikoli’s name for the puzzle and Light Up is its usual English name; the earliest version was called Bijutsukan. The rules are identical.
How long does a 9×9 Light Up take?
Five to ten minutes is typical for a nine-by-nine. There is one board here a day, new at midnight UTC, free with no sign-up, and it plays on a phone by tapping squares.
Can a white square be lit by more than one bulb?
Yes, as long as no bulb is in another bulb’s beam. Two bulbs on different rows and columns can both light the square where their beams cross; what is forbidden is a bulb standing in a square another bulb lights.