Light Up Puzzle

Light Up is a shading puzzle. Black walls sit on a white grid, some of them numbered from 0 to 4. You place bulbs so every white cell is lit, no two bulbs see each other, and every number matches the bulbs beside it. You can play grids from 7×7 up to 10×10, and every puzzle here has exactly one solution that you can reach by logic, with no guessing.

Rules

The board is an n×n grid of white cells and black walls. Some walls show a number from 0 to 4. A wall with no number is still a wall: it blocks light and has no required count.

Light Up (sometimes called Akari) is solved when three rules all hold. A bulb lights its own cell and every white cell in the same row or column until a wall stops the line. Two bulbs must not see each other along such a line. A numbered wall must have exactly that many bulbs on the white cells that share an edge with it, across or down, not diagonally. Every white cell must end up lit, either by its own bulb or by a bulb looking at it.

A dot marks a white cell you have decided stays empty. The dot does not light anything. It is only a note.

Controls

Click or tap a white cell to cycle empty, then bulb, then a mark that the cell is empty, then empty again. Shift-click, right-click, or a long-press cycles the other way. Walls do not take a bulb; selecting one only moves the cursor.

On a keyboard, the arrow keys move around the grid. B, L, or 1 places a bulb. X, M, period, or 2 marks the cell empty. Backspace, Delete, or 0 clears it. Space or Enter cycles forward. Shift with Space or Enter cycles backward.

Two bulbs that see each other, and a number that already has too many bulbs or is finished with the wrong count, turn red at once. Cells that are still unlit turn red when you press Check. Press Hint for one cell the rules already force, with a sentence saying why. Undo and Redo step through your moves, and Reset clears every bulb and mark. When the grid is right, the bulbs turn teal.

Techniques

Easy grids follow from the numbers and from light that has only one place left:

• Zero: a 0 leaves no room for a bulb, so the white cells beside it stay empty.
• Full number: a 4, or a number on an edge with exactly that many open sides, needs a bulb on every open side.
• Count closed: once a number has its bulbs, the other cells beside it stay empty.
• Ray: a bulb lights its line up to the next wall, so those cells stay empty.
• Dead end: the only cell that can still light a white cell is the cell itself, so it is a bulb.

Medium grids also need a wider look along a line:

• Only light: only one cell can still light this one, so that cell is a bulb.

Hard grids need one more idea, still without guessing:

• Diagonal: a bulb that would see two cells a number still needs cannot be placed.
• One try: assume a bulb or an empty cell, and drop the choice that cannot be finished.

Beginner tips

Start with the 0s and the 4s, then with numbers on the edge. They often place several bulbs or several empty marks at once.

After you place a bulb, clear the rest of its row and column up to the walls. Those cells cannot hold another bulb.

If you are new to Light Up, start with the daily 7×7 easy grid, and move up once zeros, full numbers, and dead ends feel natural.