Akari is a logic puzzle where you place bulbs on a white grid so that every white cell is lit and no two bulbs can see each other across an open row or column. Numbered black walls force an exact orthogonal bulb count, and only walls — never X marks — stop a beam. The Light Up (Akari) tool ships a single fixed 5×5 fixture with 25 cells: 20 white cells, four numbered walls, and one unnumbered wall, and that fixture has been independently verified to contain exactly one valid arrangement of five bulbs. Because the board is fixed, the score is also fixed: a strictly valid full submission finishes at exactly 1,000 points, and a second invalid full submission deadlocks the run until you Restart. The whole game runs in your browser with no account, no server sync, and no daily download, which makes the same fixture reproducible across every device and session.
What Is an Akari Puzzle on a 5×5 Board?
Akari is the Japanese name for the placement puzzle known in English as Light Up. The ruleset was introduced by Nikoli in 2001 and has since been re-implemented by many puzzle authors, including Simon Tatham's maintained Light Up documentation, which describes the same three constraints that govern every Akari puzzle. The grid is a mix of white cells and black walls; some black walls carry a printed number, while others are blank and serve only as beam blockers.
The browser board in this tool is a single compact 5×5 fixture rather than an infinite generator. The fixed layout has 25 cells in total: 20 white cells, four numbered walls, and one unnumbered wall. Because the fixture is product-authored and reproducible, the checker, the scoring formula, the solution count, and the error behavior are all reproducible too. The unique completed arrangement uses exactly five bulbs, and that uniqueness was proven by an enumerator that was asked to find up to two distinct solutions and received exactly one. Nikoli is cited only for the rule set; this particular board is not described as an official, historic, daily, or published Nikoli puzzle.
The Three Rules That Define Every Light Up (Akari) Game
Every Akari puzzle, regardless of size, is governed by exactly three rules. They are independent of grid size, number of walls, and difficulty, and they are the rules that the in-game checker enforces before awarding the final score.
- Full coverage: Every white cell on the board must be lit. A bulb placed on a white cell illuminates its own cell and every other white cell in the same row and column until a black wall or the edge of the board stops the ray.
- No mutual visibility: No two bulbs may share an unobstructed row or column. If a wall does not sit between them, the configuration is rejected.
- Exact numbered-wall count: Each black wall that carries a number must touch exactly that many bulbs in its four orthogonal neighbors (up, down, left, and right). Diagonal cells are never counted.
The Nikoli description of Akari lists the same three constraints as the defining ruleset, and Simon Tatham's documentation mirrors them. The 5×5 board in this tool uses those rules exactly as published, with no hidden modifiers or alternative scoring: every other detail of the game — the unique solution, the exact 1,000-point finish, the two-error deadlock, and the keyboard and touch controls — is a product fixture rather than a rule from the original puzzle family.
How to Play the Fixed 5×5 Light Up (Akari) Board
The interface supports both touch and keyboard play, and you can mix them in a single session. Every action below is allowed in any order, and the board never auto-checks, so you can experiment freely before submitting a full attempt.
- Open the board and inspect the clues. Read the four numbered walls and the position of the unnumbered wall. Each number fixes the bulb count in its orthogonal neighbors, which gives you the tightest constraints to start with.
- Mark obvious empties. Tap a white cell to cycle it through clear, bulb, and X, or use the dedicated Mark empty and Place light buttons below the grid. An X is a personal declaration that the cell stays empty; it does not block any beam, so do not treat it as a wall.
- Place bulbs that satisfy the numbered walls. On the keyboard, the Arrow keys move the active cursor, B or Space toggles a bulb on the selected white cell, X toggles an empty mark, and Delete or Backspace clears the cell. The cursor is allowed to move onto a wall so it follows a predictable rectangular path, but placement controls are disabled there.
- Trace each dark corridor to find its only possible illumination source. A corridor is a sequence of white cells bordered on both sides by walls or by the board edge. Wherever two candidate bulbs would share an open row or column, the mutual-visibility rule rejects one of them, so an X at the conflicting cell usually resolves the ambiguity.
- Cover every white cell. Walk row by row and column by column until no white cell is left clear. Every cell must hold either a bulb or an X before the checker will accept the board.
- Check the board. Press Enter or choose Check board. The checker requires all 20 white cells to be marked first; an incomplete submission reports that the attempt is incomplete and counts no mistake. A valid full board ends the run at exactly 1,000 points, and the final result freezes.
Scoring: Why a Valid Board Finishes at Exactly 1,000 Points
The scoring system is deterministic and rule-driven, not time-based or move-count-based. Once every white cell is marked, the checker rebuilds the bulb set and applies all three rules: legal placement, mutual visibility, exact numbered-wall adjacency, and full coverage of all 20 white cells. A strictly valid full board finishes immediately at 1,000 points. A full but invalid board records exactly one mistake and remains editable, so the highlighted conflict outlines can guide your revision.
| Board state at check | What happens | Score or status |
|---|---|---|
| Any white cell still clear | Submission reported as incomplete | No mistake counted, board still editable |
| All 20 white cells marked, valid | Run freezes at completion | Exactly 1,000 points |
| All 20 white cells marked, invalid (first time) | One mistake recorded, conflicts highlighted | Board editable, score below 1,000 |
| All 20 white cells marked, invalid (second time) | Run freezes at deadlock | Until Restart is selected |
Completion and deadlock are terminal states. Once the run freezes, no later click, key press, cursor move, or board check changes the result, so a stale attempt cannot be retroactively rescued by extra input. Restart restores the authored clues, a clean 20-cell workspace, zero mistakes, zero score, and the initial cursor position, so you always return to a known starting point. The double-Escape gesture opens a boss screen during play and restores the puzzle with the same gesture, which is useful when you need to hide the board quickly without losing progress.
Solving Strategy: Where to Start on a 5×5 Light Up
A useful approach is to begin with the numbered walls, because they fix the bulb count in their immediate neighborhood. For each numbered wall, list which of its up to four orthogonal white cells are still available to hold a bulb, and reject any candidate that would exceed the printed count. Mark every cell that cannot hold a bulb with an X so you do not waste time revisiting it later. A 1 clue, in particular, pins the bulb count in its orthogonal neighbors to exactly one, which makes it the strongest single move on a small board.
Next, trace each dark corridor: a sequence of white cells bordered on both sides by walls or by the board edge. A corridor that is fully isolated from other corridors must be lit by bulbs placed inside it, and a corridor with a tight bottleneck often leaves only one cell that can shine onto the rest of the run. Whenever two candidate bulbs would share an open row or column, the mutual-visibility rule rejects one of them, so an X at the conflicting cell usually resolves the ambiguity. You can keep placing tentative marks as long as the board is incomplete, because the checker does not count a mistake until every white cell is marked. This protects exploratory play and lets you use the danger outlines that appear around a bulb or numbered wall involved in a visible conflict before you commit to a full check.
It also helps to remember that X marks are working notes, not walls. They are easy to clear with Delete or Backspace or with a dedicated Clear control, so feel free to mark many candidates as empty early on. On a 5×5 fixture, the puzzle usually resolves once the 1 and 2 clues are fully exploited, because they pin most of the bulb positions directly.
How the Unique Solution Is Mechanically Verified
The released fixture is verified without trusting a hidden answer comparison. A constraint enumerator starts from any unlit white cell, considers every white position capable of illuminating it, and rejects any candidate bulb that would see an already placed bulb or that would overflow a numbered wall's adjacency count. When every white cell is illuminated, the enumerator independently checks exact clue equality, mutual visibility, legal placement, and full coverage. The release test asks the enumerator for up to two distinct solutions and receives exactly one, which is why the board is described as uniquely solved.
Automated golden checks encode the published rules separately from the board's displayed answer. They verify that a black wall blocks a beam, an unobstructed row is illuminated, a mutually visible bulb pair is rejected, numbered walls use orthogonal rather than diagonal neighbors, an overfilled clue fails, all white cells are covered, and the constraint count is exactly one. Product-state checks separately confirm that an incomplete submission records no mistake, the unique fully marked board scores exactly 1,000, two wrong full submissions deadlock, Restart restores a clean state, and both terminal outcomes resist later input. Because both the rule checks and the product-state checks are independent of the displayed answer, the fixture's correctness does not depend on comparing a player submission against a hidden string.
Everything runs locally in the browser. The board sends no state to a server, performs no upload, requests no account, downloads no daily puzzle, uses no random number generator, and adds no package dependency. Native buttons expose row, column, wall, clue, mark, and illumination labels for assistive technology, and the 5×5 grid scales to narrow screens with touch-friendly targets that do not depend on drag gestures or hover-only information. This is a short recreational logic puzzle, not a standardized intelligence test, cognitive assessment, educational placement measure, health screening, or employment benchmark. The fixed board becomes familiar after repeated play, and neither completion time nor the number of tentative marks changes the 1,000-point total.
If you're weighing options, Easy Lights Out Puzzles: A 5-Move 4×4 Walkthrough covers this in detail.