How to play
Light Up is played on a square grid of white and black cells; some black cells show a number from 0 to 4. Place bulbs in white cells so that every white cell is lit. A bulb lights its own cell and shines along its row and column until the light hits a black cell or the edge.
Two more rules make it a puzzle: a bulb may never shine on another bulb, and a number tells exactly how many bulbs touch the four sides of its black cell (diagonals do not count). Black cells without a number can have any count. Pick a size and a level, then press Start; the clock stops while you pause or switch tabs. When every white cell is lit, no two bulbs see each other and every number matches, the puzzle is solved.
Controls
- Mouse: click a white cell to switch it from empty to a bulb, to a dot and back to empty. A dot is your note that a cell holds no bulb. Right-click toggles a dot directly. Drag across cells to dot them all; a drag that starts on a dot removes dots.
- Touch: tap and drag the same way; the page does not scroll while your finger is on the board. Dot mode makes every tap a dot. When cells are small, a Zoom button appears; while zoomed, move the board with two fingers or pinch.
- Keyboard: arrows move a cursor (the first press only shows it). Space or Enter toggles a bulb, X a dot, Backspace clears, Shift+arrow dots the empty cells you pass. Ctrl+Z / Ctrl+Y undo and redo; H hint, C check, D dot mode, P pause, N new puzzle, F full screen.
Lit cells turn yellow, crossed by a band of light from each bulb. A number turns grey when it has the right number of bulbs. A red ring around a number means too many bulbs; a dashed ring means too few free cells are left beside it to reach its number. Two bulbs that see each other get red rings joined by a dashed line.
Check crosses out wrong bulbs and rings wrong dots until your next move. Hint first removes a wrong mark if you have one; otherwise it makes the easiest step it can find, highlights the cells involved and says why, or adds one bulb of the solution when no simple step is left.
How it works
Let lit(c) be the number of bulbs whose light reaches cell c, including a bulb in c itself. A finished grid obeys:
Most progress comes from counting: a 0 rules out its neighbours, a number with exactly as many free sides as it still needs gets bulbs on all of them, and a dark cell that only one cell can still light needs a bulb there. Harder steps combine two numbers that share neighbours, or ask what a bulb would do: if it would starve a number or cut off every source of light for another cell, it cannot be there.
Every puzzle here has exactly one solution. The 1,350 puzzles (150 per size and level) were made by scattering black cells in a pattern that looks the same after a half turn, placing random bulbs until everything was lit, numbering every black cell and wiping numbers one by one while a solver could still finish by reasoning alone. Easy needs only counting; Medium adds the two-number and “what would this bulb do” steps; Hard sometimes needs you to try a cell both ways and follow the forced moves until one side breaks a rule. A separate program counted the solutions of every puzzle in all eight rotations and mirror images the game uses: exactly one each time.
Light Up was invented in 2001 and first appeared in a Japanese puzzle magazine. In general, deciding whether a Light Up grid has a solution is NP-complete: no fast method is known that works for every grid.
Tips and strategy
- Start with every 0 (dot its neighbours) and every 4 (bulbs on all four sides).
- A number with exactly as many white sides as its value (a 3 against the edge, a 2 in a corner) takes bulbs on all of them.
- A white cell walled in by black cells or edges on all four sides must hold its own bulb.
- A 3 diagonal to a 1 shares two neighbours with it. The 1 allows at most one bulb there, so the 3 needs bulbs on both of its other sides.
- On Hard, imagine a bulb in a tricky cell and follow the moves it forces. If a cell ends up dark with nothing left to light it, put a dot there instead.
Frequently asked questions
Is Light Up the same as Akari?
Yes. The puzzle is also known as Akari; the rules are the same under both names.
Why are the cells small on my phone?
Fourteen cells across a phone leave about 25 pixels each. Use the Zoom button, pinch with two fingers, or play 7×7.
Sources
- McPhail B (2005). Light Up is NP-complete. Manuscript, 28 February 2005. The proof, by a reduction from circuit satisfiability, that deciding whether a grid has a solution is NP-complete.
- Puzzles.wiki contributors (accessed 25 October 2026). Akari. Puzzles.wiki. Wiki article on the puzzle genre. The year of invention (2001) and the first publication in a Japanese puzzle magazine.