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Light Up Rules: How to Play Akari (With Examples)

Light Up (also called Akari) is a logic puzzle about lighting a room. The grid has white squares and black squares, and you place light bulbs in the white ones until every white square is lit. A bulb’s light travels along its row and column until a black square stops it.

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The rules

Place bulbs in white squares so that:

  1. Every white square is lit. A bulb lights its own square, and every square in its row and column until a black square or the edge of the grid.
  2. No bulb shines on another bulb. Two bulbs can’t be in the same row or column unless a black square stands between them.
  3. A number on a black square says how many bulbs touch it, side by side (not diagonally). A 0 has none beside it; a 4 has one on every side.

Black squares without a number can have any number of bulbs beside them.

A bulb lights its row and column A 5 by 5 grid with a bulb in the middle. Its whole row glows, and its column below it. The black square just above the bulb stops the light going up, so the top of its column stays dark.
A bulb lights its row and its column. The black square just above it stops the light going up.

What breaks a rule

Two bulbs that see each other. Nothing stands between these two, so each lights the other:

Two bulbs that light each other: not allowed The same grid with bulbs at both ends of the middle row. Nothing stands between them, so each lights the other (ringed in red).
Two bulbs in an open row light each other: not allowed.

A number with the wrong count. Here the 2 has its two bulbs and the 0 has none, so both are happy:

A number counts the bulbs beside it A 5 by 5 grid with a 2 and a 0 on black squares. The 2 has two bulbs touching it, above and to the left. The 0 has none. 2 0
The 2 touches two bulbs; the 0 touches none.

In the game, bulbs that light each other are ringed in red, and a number turns red when it has too many bulbs or too little room left. A number dims once it has all its bulbs.

A puzzle from the game, solved

This is the first level in the game: Easy level 1, a 7×7 with 10 black squares, 3 of them numbered.

Easy level 1, before you start A 7 by 7 grid with 10 black squares, 3 of them numbered. 2 2 2 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Easy level 1, before you start.

It takes 11 steps. Here are the first three.

Step 1: a 2 with only two squares beside it

The 2 in row 1, column 5 sits on the top edge with a black square to its right. Only two white squares touch it, the one to its left and the one below, and it needs two bulbs. So both get one.

Easy level 1, step 1 number-needs-all: bulb row 2, column 5 & row 1, column 4 (from row 1, column 5) 2 2 2 1 2 3 4 5 6 7 1 2 3 4 5 6 7
The 2 has only two white squares beside it: both get bulbs.

Steps 2 and 3: no bulb in the light

The new bulb in row 1, column 4 lights 9 squares along its row and column, and the one in row 2, column 5 lights 5 more that weren’t already ruled out. No other bulb can stand in that light, so those squares get dots.

Easy level 1, step 2 lit-squares: no bulb on 9 (from row 1, column 4) 2 2 2 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Step 2: the squares lit by the bulb in row 1, column 4 can’t have bulbs.
Easy level 1, step 3 lit-squares: no bulb on 5 (from row 2, column 5) 2 2 2 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Step 3: the same for the bulb in row 2, column 5.

The rest

From there, it’s the same ideas again. In all, it takes 3 steps where a number needs every square left beside it, 6 where a bulb’s light rules squares out, and 2 where a dark square has only one place left that could light it.

Easy level 1, solved Every white square lit, no bulb lighting another, and every number with its bulbs. 2 2 2 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Easy level 1, solved.

Where to start in any puzzle

Look for a 0: none of the squares touching it can have a bulb. Then look for numbers with just enough room: a 4, a 3 on an edge, a 2 in a corner, or any number with black squares or walls around it. On the game’s levels, a 0 is the first move on 38 of the 50 Easy levels, 45 Medium and all 50 Hard; a number that needs every square beside it is the first move on 8 Easy levels and 4 Medium.

Common mix-ups

  • Light goes in straight lines only. A bulb lights its row and column, not its diagonals.
  • Numbers count touching bulbs only. Side by side, not diagonally, and not bulbs further along the row.
  • Black squares stop light. Two bulbs in the same row are fine if a black square stands between them.

Next steps

Questions

Is Light Up the same as Akari?

Yes. Akari is the Japanese name (it means light), and Light Up the English one. The rules are the same.

Does a bulb light the square it's on?

Yes. A bulb lights its own square, and every square in its row and column until a black square or the edge of the grid.

Do black squares without a number matter?

Only as walls: they stop light. They can have any number of bulbs beside them.

Does every puzzle have one solution?

Yes. Every puzzle on PeerPuzzles has exactly one solution, and can be finished by logic alone, without guessing.