Akari (Light Up)

Akari, also known as Light Up, is a logic puzzle played on a rectangular grid containing white cells and black cells (walls). Some black cells display a number. The player must place light bulbs in white cells so that every white cell is illuminated. A bulb shines horizontally and vertically from its position, illuminating all cells in its row and column until blocked by a wall. No two bulbs may illuminate each other (no bulb in the direct line of sight of another), and each numbered wall must have exactly that many bulbs in its orthogonally adjacent cells.

Rules

1. The grid contains white (empty) cells and black (wall) cells.
2. Some black cells contain a number from 0 to 4.
3. Place light bulbs in white cells. A bulb illuminates its own cell and all white cells in the four cardinal directions until a black cell blocks the light.
4. Every white cell must be illuminated by at least one bulb.
5. No bulb may be illuminated by another bulb — no two bulbs may share a row or column without a wall between them.
6. A numbered black cell must have exactly that many bulbs placed in its orthogonally adjacent (up, down, left, right) white cells.
7. Black cells without numbers may have any number of adjacent bulbs (including zero).

Strategies

• Zero Walls: A wall numbered 0 means none of its adjacent white cells may contain a bulb. Mark them as "no bulb."
• Forced Placement on Numbered Walls: A wall numbered 4 requires bulbs in all four adjacent cells. A wall numbered 3 in a corner needs bulbs in all three available neighbors.
• Counting Remaining Spaces: If a numbered wall already has N-1 of its N bulbs placed, the last bulb's position is forced among remaining available neighbors.
• Unreachable Cells: If a white cell can only be illuminated from one direction (all other directions are blocked by walls or already have constraints preventing bulb placement), a bulb must be placed along that path.
• Shadow Analysis: If placing a bulb in a certain cell would leave another cell permanently unilluminable, that placement is invalid.
• Mutual Exclusion: If two white cells see each other (same row or column with no wall between), at most one can hold a bulb. Combined with numbered wall constraints, this often forces decisions.

Akari was created by the Japanese puzzle publisher Nikoli and first appeared in their magazine "Puzzle Communication Nikoli" in 2001. The name "Akari" (あかり) is the Japanese word for "light" or "illumination," directly reflecting the puzzle's theme. The puzzle is also widely known by its English name "Light Up."

The puzzle was part of Nikoli's ongoing effort to create logic puzzles with intuitive, visually engaging themes. The concept of placing lights to illuminate a floor plan resonated with solvers who appreciated the real-world metaphor. The mechanic of light rays being blocked by walls gives the puzzle a spatial, almost physical quality that distinguishes it from purely abstract number puzzles.

Akari gained international prominence through the World Puzzle Championship, where it has been featured regularly since the mid-2000s. The puzzle was also included in Simon Tatham's Portable Puzzle Collection (under the name "Light Up"), which brought it to a large audience of casual puzzle solvers on desktop computers and mobile devices.

The puzzle became particularly popular in the competitive puzzle-solving community due to its balanced difficulty curve: simple puzzles can be solved quickly with basic strategies, while complex ones require deep chains of logical reasoning. The interaction between numbered walls, light coverage, and the no-mutual-illumination rule creates a rich space for puzzle construction. Today, Akari is recognized as one of Nikoli's finest puzzle inventions and appears in puzzle books, newspapers, apps, and competitions around the world.