hop to playPlay Light Up

A little guide to a bright idea

How to play
Light Up (Akari)

Light every square.
Keep the bulbs out of each other’s way.

A beautiful logic puzzle, explained by the light itself. Follow a beam, try a move, and find your first solution.

Let there be a little light
One bulb lights its row and column. The black wall stops the beam before the white square at row 2, column 6.6 by 6 board. 6 of 30 white squares are lit. Bulbs at row 2, column 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. ?
What will light the square
behind the wall?

01Light

One bulb, four directions.

A bulb lights its own square, then sends light up, down, left and right.

Follow the four paths. Light travels along straight rows and columns, all the way to the edge of the board or the first black wall. White squares let it keep going.

No diagonals. No turning corners.

The bulb at r2c2 lights its own square, then the white squares directly above, below, left and right. Light follows straight grid lines. It never turns a corner.6 by 6 board. 6 of 30 white squares are lit. Bulbs at row 2, column 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

02Walls

A wall changes everything.

Follow the beam to the right. Here, it stops.

The black square is a wall. The white square beyond it stays dark, even though it is in the same row as the bulb. It will need light from another direction, or a bulb of its own.

Numbered and unnumbered walls both stop light.

Follow the beam to the right. White squares let it through; the black wall at r2c5 stops it. The white square beyond, r2c6, stays dark and needs another light source.6 by 6 board. 6 of 30 white squares are lit. Bulbs at row 2, column 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. ?

03The goal

Find the last little piece of darkness.

The goal is to illuminate every white square.

This board has just one dark square left: row 6, column 5. The outlined squares are the only places with a straight, unblocked path to it. But two already receive light from other bulbs. A new bulb there would see an existing one.

That leaves the dark square itself. It needs its own bulb.

The same board, now with five bulbs: 29 of its 30 white squares are lit. Only r6c5 is dark. Its possible sources are r5c5, r6c5 and r6c6. The other two already see bulbs, so the last bulb must go in r6c5 itself.6 by 6 board. 29 of 30 white squares are lit. Bulbs at row 1, column 1; row 2, column 3; row 3, column 6; row 4, column 4; row 5, column 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

04Bulbs

Bulbs need a little space.

Deliberately incorrect

Two bulbs must never shine directly on each other.

These bulbs share an open row. There is no wall between them, so each bulb illuminates the other. The line between them and the exclamation marks identify the conflict.

Being farther apart does not help. Only a wall or a different line can separate them.

Deliberately incorrect: the bulbs at r2c2 and r2c4 face each other along an unobstructed row. Each illuminates the other. The same restriction applies to a column.6 by 6 board. 10 of 30 white squares are lit. Bulbs at row 2, column 2; row 2, column 4. Conflict: bulbs shine on each other. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

Now there is a wall between them.

Moving the second bulb beyond the wall removes their line of sight. Both bulbs can stay. Notice that their rows still match: the wall is what makes this arrangement legal.

This is a valid placement example, not yet a solved board.

Move the right-hand bulb to r2c6. The wall at r2c5 interrupts their line of sight. Two bulbs can share a row when a black wall separates them.6 by 6 board. 12 of 30 white squares are lit. Bulbs at row 2, column 2; row 2, column 6. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

05Crossing

Light can meet light.

Can light beams cross in Akari?

Look at the outlined square. One beam travels across it; another travels down through it. Neither bulb lies in the other bulb’s row or column. Would you allow this?

Yes. An empty square may receive light from more than one bulb. Crossing beams are allowed.

And if we move a bulb?
Yes. The horizontal beam from r2c2 and the vertical beam from r5c4 cross at the empty square r2c4. That square receives light twice. Neither bulb illuminates the other, so there is no conflict.6 by 6 board. 15 of 30 white squares are lit. Bulbs at row 2, column 2; row 5, column 4. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. ?

Deliberately incorrect

Now put the bulb on the crossing.

The lower bulb moves up into the other bulb’s row. What was an empty meeting point now contains a bulb — and that bulb is being illuminated by the first one.

Light meeting light is fine. A bulb lighting another bulb is the conflict.

Deliberately incorrect: move the lower bulb onto r2c4, the crossing square. It now sits directly in the other bulb’s beam. Overlapping light is allowed; a bulb lighting another bulb is not.6 by 6 board. 10 of 30 white squares are lit. Bulbs at row 2, column 2; row 2, column 4. Conflict: bulbs shine on each other. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

06Numbers

The number belongs to the wall.

New example · numbered walls

A 0 means no bulbs immediately beside this wall.

Only the four side-sharing squares matter: above, below, left and right. The × marks are notes reminding us not to place bulbs there. Those white squares still need light.

A × is a note, not a wall. Light passes straight through it.

A new clue diagram. The 0 at r2c2 forbids bulbs immediately above, below, left and right. The bulb at r1c1 is diagonal, so it does not count. The × marks are notes: light can pass through them, and their white squares still need illumination.6 by 6 board. 8 of 28 white squares are lit. Bulbs at row 1, column 1. Numbered walls at row 2, column 2: 0; row 4, column 4: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 0 2

A 2 needs exactly two bulbs beside it.

Count the bulbs touching a side of the numbered wall. A diagonal bulb does not count. Neither does a distant bulb in the same row. The number counts neighbouring bulbs, not illuminated squares.

An unnumbered wall blocks light, but asks for no particular number of bulbs.

The 2 at r4c4 counts the bulbs immediately above at r3c4 and left at r4c3. The diagonal bulb at r5c5 does not count. The clue is satisfied: r5c4 cannot hold another bulb, but its × note does not stop the light reaching it.6 by 6 board. 23 of 28 white squares are lit. Bulbs at row 1, column 1; row 3, column 4; row 4, column 3; row 5, column 5. Numbered walls at row 2, column 2: 0; row 4, column 4: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 0 2

07Think

New board · a complete worked puzzle

Don’t guess.
Follow what you know.

Let’s solve one small puzzle together.

Start with the clues that leave the fewest choices. Every move below follows from a wall, a bulb already placed or a square still waiting for light.

Rows go across; columns go down. Row numbers increase from top to bottom, and column numbers from left to right. Use the labels to follow each move.

An original six-by-six puzzle, with no bulbs yet.6 by 6 board. 0 of 25 white squares are lit. No bulbs placed. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

Deduction 1 / 6

First, listen to the zero.

The 0 at r5c5 rules out bulbs at r4c5, r5c4, r5c6 and r6c5. Mark all four. These are still white squares: they will need light from somewhere else.

The 0 at r5c5 rules out bulbs at r4c5, r5c4, r5c6 and r6c5. Mark all four. These are still white squares: they will need light from somewhere else.6 by 6 board. 0 of 25 white squares are lit. No bulbs placed. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

Deduction 2 / 6

Two neighbours. Two bulbs.

The 2 in the top-left corner, r1c1, has only two white neighbours: r1c2 and r2c1. It needs exactly two adjacent bulbs, so both squares must contain one.

The 2 in the top-left corner, r1c1, has only two white neighbours: r1c2 and r2c1. It needs exactly two adjacent bulbs, so both squares must contain one.6 by 6 board. 9 of 25 white squares are lit. Bulbs at row 1, column 2; row 2, column 1. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

Deduction 3 / 6

One available neighbour.

The 1 at r1c6 touches a wall on its left and the board edge above and to its right. Its only white neighbour is r2c6. That square must hold its one bulb.

The 1 at r1c6 touches a wall on its left and the board edge above and to its right. Its only white neighbour is r2c6. That square must hold its one bulb.6 by 6 board. 14 of 25 white squares are lit. Bulbs at row 1, column 2; row 2, column 1; row 2, column 6. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

Deduction 4 / 6

The same idea, at the other edge.

The 2 at r6c2 has a wall on its left and the board edge below. Its only white neighbours are r5c2 and r6c3, so both must contain bulbs.

The 2 at r6c2 has a wall on its left and the board edge below. Its only white neighbours are r5c2 and r6c3, so both must contain bulbs.6 by 6 board. 19 of 25 white squares are lit. Bulbs at row 1, column 2; row 2, column 1; row 2, column 6; row 5, column 2; row 6, column 3. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

Deduction 5 / 6

Three neighbours are already ruled out.

Around the 1 at r3c2, r2c2 already sees the bulb at r1c2; r3c1 sees r2c1; and r4c2 sees r5c2. Another bulb in any of those squares would conflict. Only r3c3 remains, so place the bulb there. The 1 is satisfied; its other neighbours stay marked.

Around the 1 at r3c2, r2c2 already sees the bulb at r1c2; r3c1 sees r2c1; and r4c2 sees r5c2. Another bulb in any of those squares would conflict. Only r3c3 remains, so place the bulb there. The 1 is satisfied; its other neighbours stay marked.6 by 6 board. 22 of 25 white squares are lit. Bulbs at row 1, column 2; row 2, column 1; row 2, column 6; row 3, column 3; row 5, column 2; row 6, column 3. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

Deduction 6 / 6

Follow the darkness to its source.

Before this move, the marked square r5c4 was still dark. Walls block it on both sides, and the 0 prevents a bulb there. The possible sources at r1c4, r2c4, r3c4 and r6c4 already saw bulbs, leaving only r4c4. The bulb shown at r4c4 now lights r5c4: all white squares are lit, every numbered clue is exact, and no bulbs see each other.

Every white square is now lit. Every number has its exact count. No bulbs see one another.

Let me try
Before this move, the marked square r5c4 was still dark. Walls block it on both sides, and the 0 prevents a bulb there. The possible sources at r1c4, r2c4, r3c4 and r6c4 already saw bulbs, leaving only r4c4. The bulb shown at r4c4 now lights r5c4: all white squares are lit, every numbered clue is exact, and no bulbs see each other.6 by 6 board. 25 of 25 white squares are lit. Bulbs at row 1, column 2; row 2, column 1; row 2, column 6; row 3, column 3; row 4, column 4; row 5, column 2; row 6, column 3. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2

08Your turn

You’ve seen the light.
Now trust your eye.

Three small decisions. Take your time; a wrong answer is a useful thing to understand.

Is this crossing allowed?6 by 6 board. 15 of 30 white squares are lit. Bulbs at row 2, column 2; row 5, column 4. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

A small decision / 01

Two beams. One empty square.

Is this crossing allowed?

Choose an answer to see why it works.

Read the explanation

Yes. The beams meet on an empty square. Neither bulb illuminates the other, so there is no bulb conflict.

Does the diagonal bulb count towards the 2?6 by 6 board. 23 of 28 white squares are lit. Bulbs at row 1, column 1; row 3, column 4; row 4, column 3; row 5, column 5. Numbered walls at row 2, column 2: 0; row 4, column 4: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 0 2

A small decision / 02

Count the neighbours.

Does the diagonal bulb count towards the 2?

Choose an answer to see why it works.

Read the explanation

Exactly. Only bulbs immediately above, below, left or right count. The diagonal bulb does not share a side with the wall.

Does a × mark stop a beam?6 by 6 board. 6 of 30 white squares are lit. Bulbs at row 2, column 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes.

A small decision / 03

A note is still a white square.

Does a × mark stop a beam?

Choose an answer to see why it works.

Read the explanation

Right. The × means “do not put a bulb here”. The square stays white, lets light through and must still be illuminated.

An optional little puzzle

A room of your own.

Try the worked puzzle from a blank board. Light every white square and match the numbers. The lesson stays above if you need it.

Your moves stay here while this page is open.

YOUR LIGHT LAB0 squares lit
Row 1, column 1: wall needs 2 bulbsRow 1, column 5: unnumbered wallRow 1, column 6: wall needs 1 bulbsRow 2, column 5: unnumbered wallRow 3, column 2: wall needs 1 bulbsRow 3, column 5: unnumbered wallRow 5, column 1: unnumbered wallRow 5, column 3: unnumbered wallRow 5, column 5: wall needs 0 bulbsRow 6, column 1: unnumbered wallRow 6, column 2: wall needs 2 bulbs
LitUnlitNo-bulb note

Start with the 0: its neighbours cannot contain bulbs. Choose Bulb or Note, then tap a white square.

A little nudge?

Ask for a reason, then make the move yourself. You can undo any move, including a revealed solution.

Keyboard & controls

Tab into the board. Arrow keys move between white squares. Enter or Space places or removes the selected Bulb or Note. X toggles a note. Undo restores the previous move. Marks never block light.

Skip the mini puzzle
Your practice board, currently unstarted6 by 6 board. 0 of 25 white squares are lit. No bulbs placed. Numbered walls at row 1, column 1: 2; row 1, column 6: 1; row 3, column 2: 1; row 5, column 5: 0; row 6, column 2: 2. Empty lit squares have corner ray marks; unlit squares have a small dot. Cross marks are no-bulb notes. 2 1 1 0 2
Your practice board · not started

09Keep the light going

A bright idea
to take with you.

Follow the light. Respect the walls. Find the square that needs you. Your next little revelation is one puzzle away.

Play Light Up Or step into The Last Lighthouse

Free to play. No account needed.

Keep these three things

A pocketful of rules.

  1. Light every white square. A bulb lights its own square and straight along its row and column, stopping at walls or the board’s edge.
  2. Keep bulbs out of each other’s light. Beams may cross and overlap on empty squares.
  3. Match the numbered walls. A number counts only the bulbs in immediately adjacent squares: above, below, left and right.

Little questions.
Clear answers.

Can light beams cross in Akari?

Yes. An empty square can be lit by two or more bulbs. A bulb cannot shine on another bulb. The crossing diagram above shows the difference.

Does light travel diagonally?

No. Light follows rows and columns only. Diagonal bulbs also do not count towards a numbered wall.

What does a black square without a number mean?

It blocks light just like a numbered wall. It has no required adjacent bulb count.

Can a no-bulb mark be illuminated?

Yes — it must be. A × is only a solving note. It does not block light or turn a white square into a wall.

Do I have to use the fewest possible bulbs?

No. To finish, light all white squares, satisfy every numbered wall and prevent bulb conflicts. There is no separate rule asking you to minimise the bulb count.

Where should I start a new puzzle?

Look for a 0, a wall with just enough available neighbours for its number, or a dark square with only one possible source of light. After each placement, check the newly lit rows and columns.