Light Up Rules: How to Play, Strategies and Variations

What is Light Up?

Light Up is a logic puzzle played on a grid of white and black cells. You place light bulbs in white cells. Each bulb shines along its row and its column in all four directions, lighting every white cell until the beam hits a black cell or the edge of the grid.

The goal is to light every white cell in the grid, with two catches. No bulb may shine on another bulb, and some black cells carry a number from 0 to 4 that tells you exactly how many bulbs sit directly next to it, above, below, left or right.

The puzzle is also widely known as Akari, the Japanese word for light. It is one of the friendliest logic puzzles to learn: the rules fit in two sentences, the first moves are usually obvious, and the grid fills with a satisfying glow as you go.

A short history

Light Up was published by the Japanese puzzle company Nikoli, the same publisher that popularized Sudoku and created many of the grid puzzles on this site. Its Japanese name is Bijutsukan, meaning "art gallery", fitting for a puzzle about lighting every corner of a room. Nikoli also uses the name Akari, and Light Up is the common English title.

The puzzle grew a strong following outside Japan through puzzle books and computer puzzle collections, helped by how quickly the rules can be explained. Nikoli has published books made up entirely of Light Up puzzles.

It has also caught the attention of computer scientists. In 2005 Brandon McPhail showed that deciding whether a general Light Up grid has a solution is NP-complete, which puts it in the same complexity class as many famously hard problems. You can read more in the Wikipedia article on Light Up.

The core rules

A standard Light Up has five rules. The first three describe how bulbs work, the last two describe the goal and the clues.

1. 1Bulbs go in white cellsPlace light bulbs in some of the white cells. Black cells never hold a bulb.
2. 2Light travels in straight linesA bulb lights its own cell and every white cell in its row and column, in all four directions, until the beam is stopped by a black cell or the grid edge.
3. 3No two bulbs see each otherTwo bulbs may never shine on each other. Two bulbs in the same row or column are only allowed if a black cell sits between them.
4. 4Light every white cellWhen the puzzle is finished, every white cell is lit by at least one bulb. A cell may be lit by more than one.
5. 5Numbers count adjacent bulbsA black cell with a number has exactly that many bulbs in the cells directly above, below, left and right of it. Diagonal neighbors do not count. Black cells without a number can have any number of bulbs around them.

The rules pull in opposite directions, and that is where the logic comes from. Lighting everything pushes you to add bulbs, while the no-clash rule and the numbers hold you back. Every correct solution sits exactly on the balance point.

A solved example

Here is a 7×7 Light Up with twelve black cells, five of them numbered. It has exactly one solution. Try it before looking at the lit grid on the right.

The 4 is the way in: all four of its neighbors must hold bulbs. The 3 near the top left has a black cell below it, so its other three neighbors are all bulbs. The two 0 clues rule out bulbs next to them, and the 1 on the left edge is already satisfied by the bulb above it, so the cell below it stays empty. From there, look for unlit cells that only one empty cell can still reach: the bottom left corner area and the right-hand column each have one forced spot, and the grid lights up completely.

How to play: a step-by-step method

This routine gets you started on any Light Up and finishes most small grids on its own.

1. Fill the 4s and the 0s first. A 4 means a bulb on all four sides. A 0 means none of its neighbors can hold a bulb, so mark them with a small dot.
2. Check numbers against walls and edges. A 3 on the grid edge, a 2 in a corner, or any number with exactly as many open neighbors as its value is fully determined. Place all of those bulbs.
3. Light the beams as you go. After each bulb, shade or mark every cell it lights. Lit cells cannot take another bulb, which removes options for nearby numbers.
4. Mark cells that cannot hold a bulb. Use dots for cells that are lit, next to a satisfied number, or would break a clue. Dots shrink the search as much as bulbs do.
5. Hunt for lonely dark cells. Look at each unlit cell and ask which empty cells could still light it, including itself. If only one candidate is left, it must hold a bulb.
6. Re-check every number. When a number has as many bulbs as its value, dot the rest of its neighbors. When its open neighbors exactly match what it still needs, fill them all.

Strategies experienced solvers use

These patterns turn up constantly. Spotting them at a glance is what separates a quick solve from a slow one.

Light Up rewards a steady rhythm: fix the numbers, light the beams, then sweep for dark cells with only one rescuer. If you want a fresh grid every day, the Light Up (Akari) puzzles at LogicPuzzles.io include a daily 7×7 plus unlimited 5×5, 7×7 and 10×10 boards for practice.

Popular variations

The standard rules are by far the most common, but a few variations change the feel of the puzzle.

• No numbers. Grids with only plain black cells. The logic relies entirely on corridors and lighting every cell, and these puzzles are usually easy.
• Larger grids. Beyond 10×10, puzzles rely less on quick number wins and more on long chains through corridors and the lonely-cell technique.
• Rectangular and irregular boards. Grids do not have to be square. Wide, tall or unusually shaped boards change which edge patterns appear.

Common mistakes

The most common mistake is counting diagonal bulbs toward a number. Only the four cells directly above, below, left and right of a numbered black cell count. A bulb sitting diagonally next to a 2 does nothing for that 2.

Second, beginners sometimes let two bulbs see each other across a long gap. It is easy to miss a clash when the bulbs are six or seven cells apart, so always trace the full row and column of a new bulb out to the nearest black cell or edge.

Third, solvers often forget that bulbs light themselves. A cell with a bulb in it counts as lit, and an isolated white cell surrounded by black cells is usually solved by placing a bulb right in it.

Finally, watch out for reading unnumbered black cells as 0. A black cell without a number has no restriction at all. Any number of bulbs, from none to four, may sit around it.

How Light Up compares to other puzzles

Light Up is a placement puzzle, much like Star Battle: you decide where objects go, and objects in the same line constrain each other. The difference is that in Light Up a black cell blocks the line of sight, so the same row can hold two bulbs as long as a wall separates them.

Its number clues feel a little like Minesweeper or the counting walls in Nurikabe, while the need to cover the whole grid gives it a different ending: you are not done until the last dark cell glows. If you like that visual payoff, Aquarium is another puzzle where a simple physical rule does most of the work.

Frequently asked questions