Rules
Black walls sit on a white grid. Some walls show a number from 0 to 4. A wall with no number still blocks light.
The bulb lights its own cell and the cells in line with it, including the dotted one. The beam stops at the wall. The cell past the wall is a different room, and it stays dark.
- Light. A bulb lights its own cell and every white cell straight across or down until a wall.
- Sight. Two bulbs must not see each other along such a line.
- Numbers. A number is how many bulbs share an edge with that wall. A corner does not count. A wall with no number still blocks light.
- Every cell. Every white cell ends lit, by its own bulb or by a bulb looking at it.
- Dots. A dot is your note that a cell stays empty. It does not light a cell, and it does not stop a beam.
The corner above is all three rules at once. The 2 has a bulb on each open side. Those bulbs do not see each other, because the wall sits between them. The last white cell is lit by both, so it stays empty. Every puzzle you can play here has one solution, and you can finish it by logic.
- Wall
- Bulb
- Lit
- Dot
- Clash
Numbers
Start with a 0 and with a number that has no spare side. Then mark the sides that are left, and dot a side once the number is met.
A 0 Easy
A 0 has no bulb beside it. Dot every white cell that shares an edge with it. A corner 0 has two such cells. A 0 in the open has up to four. A diagonal neighbour is not beside it, so a 0 does not dot the diagonals.
Both white neighbours stay empty. The far cell is still blank. Its bulb comes later, once nothing else can light it.
Hint says: “This 0 leaves no room for a bulb, so the white cells beside it stay empty.”
The four side cells stay empty. The four corners are not beside the 0, so this step leaves them blank. The corners can be lit either on one diagonal or on the other. This window is only the rule. The puzzles you play have one solution.
Every open side Easy
Count the white cells that share an edge with the number. When that count is the number, every one of them is a bulb. A 4 in the open has four sides. A cell on the edge of the grid has three sides, so a 3 there fills all three. A corner has two sides, so a 2 there fills both. A 1 with only one open side puts the bulb there. The wall sits between its own left and right neighbours, so those two bulbs do not see each other.
Do these, and the 0s, before anything else.
A 4 Easy
The 4 places a bulb on every side. The corners are not sides, so they stay blank here. Their dots come when the bulbs clear their lines.
Hint says: “This 4 needs a bulb on every side.”
A 3 on an edge Easy
The 3 on the top edge has three open sides, so all three are bulbs. The bulbs on its left and its right do not see each other. The 3 is the wall between them. Leave them both.
Hint says: “Every white cell beside this number has to be a bulb.”
A 2 in a corner Easy
A corner has two open sides, so both are bulbs. The last cell is still blank. The bulbs will empty it on the next pass, because each of them can see it.
Both bulbs see the last white cell, so it stays empty. The number, the sight line, and the lit cell are all done.
A 1 with one side Easy
The 1 has a single white neighbour, so that cell is a bulb. The wall further along the row is a different room. The cell past it is still blank. It gets its own bulb later.
The sides that are left Easy
Once a bulb or a dot is already beside a number, count again. If the white cells still open equal the bulbs that number still needs, those cells are bulbs. This is the same idea as the last section, after one side has been used.
The new bulb is the bottom-right cell. The 2 on the right is ringed, and so is the bulb already on its left. That 2 needs one more, and only one side is still open.
- The 0 in the top right has one white neighbour, the cell on its left, so that cell is empty.
- The 2 on the left has two open sides, above it and to its right, so both are bulbs.
- The bulb in the centre empties the cell under it.
You may also notice that the bottom-right cell can light only itself once the cell above it is empty. Hint gives the number as the reason. Both readings land on the same cell.
The number is met Easy
When the bulbs beside a number already equal it, dot every other white cell beside it. You write the dot. Nothing fills it in for you. This is not the ray in the next section. The ray is every cell the bulb can see. This dot is a neighbour the number does not have room for, even when the bulb cannot see that neighbour.
The 0’s only white neighbour is already dotted, and that left the bottom-left cell as the only light for its room. The 1 now has its bulb, so the cell above it stays empty. The ring sits on the 1 and on the bulb it already has.
Hint says: “This number already has its bulbs, so the other cells beside it stay empty.” The bulb still missing from this window is a medium step. It is in the next section but one, and it is not drawn here. This mark itself is the easy step. The window as a whole is not an easy grid.
Lines
A bulb empties every cell it sees. A white cell that only a bulb in that same cell can light takes that bulb.
A bulb clears its line Easy
The moment a bulb is placed, no other bulb can sit in its row or its column until a wall. Dot those cells. A dot already in the line does not end it. A wall does. The cells in the last section were about the number. These cells are about the beam.
The bulb beside the 1 empties the next cell. The wall stops the beam, so the cell past the wall is not part of this dot. Hint says: “A bulb lights this line up to the next wall, so these cells stay empty.”
Each bulb of the 4 empties the corner it can see. The first cells written are the two top corners, the ones the top bulb can see. The side bulbs clear the bottom corners the same way. Those corners were never neighbours of the 4.
Only this cell can light it Easy
List every cell that could still hold a bulb and light the one you are looking at. Walls end the list. Dots and bulbs already placed come off the list. If the only cell left is the one you are looking at, it is a bulb. A one-cell room is the obvious case. The same thing happens when dots have removed every other way to light a cell.
Nothing else can see the middle cell, so the bulb goes there. Hint says: “The only cell that can still light this one is itself, so it is a bulb.”
The wall rings the end of the first room. The cell past it is its own room, one cell long, so it is a bulb.
The 0 has dotted both of its sides. The far cell is the only light left for that cell, so it is a bulb. The dotted cells are lit by this bulb too.
Easy grids on this site finish with the moves up to here. Medium is the next section. Hard is the diagonal, and a trial only when that still stalls.
Only one cell can still light itMedium
Sometimes the cell you are worried about is already a dot, or the one place left for a bulb is a different cell. If only one cell can still light it, that cell is a bulb. Easy grids never need this step. Medium grids do.
The dotted cell on the top right cannot hold the bulb. The wall on its left ends the row. The only cell left in its column is the one under it, so that cell is a bulb. The new bulb also empties the white cell to its left. That dot is the beam from the next section’s idea, not this step. The answer in the picture is the bulb alone.
None of the earlier moves place this bulb. Hint says: “Only one cell can still light this one, so that cell is a bulb.”
One short Hard
When a number still needs one fewer bulb than it has open sides, exactly one of those sides stays empty. A bulb that could see two of those sides would put both of them out, and the number would come up short. Dot that cell. A 1 with two open sides, a 2 with three, and a 3 with four all use this. The picture here is the 1, which is small enough to see in one look.
Use this only when the earlier sections have stopped. It is a hard step. Some hard grids finish on this step alone.
The 1’s open sides are the cell on its right and the cell under it. It needs one of them. The dotted cell sees the one on the right down the column and the one under the 1 along the row. A bulb there would rule out both sides.
Hint says: “A bulb here would see two cells a number still needs, so this cell stays empty.”
After the dot, the lines finish the window. There is no second picture.
- The cell to the right of the 1 is now a bulb. Its column partner is the new dot, and the wall closes its row.
- The 1 has its bulb, so the cell under the 1 stays empty.
- That empty cell can only be lit from the cell on its left, so that cell is a bulb.
- The new bulb empties the cell under it.
What if Hard
Use this only when every number, every ray, every dead end, every single light, and every diagonal has stopped. Pick one unknown white cell. Try a bulb, or try a dot. Follow the lines. If a number ends up with too many bulbs, or a cell is left that nothing can light, that try is wrong, so write the other mark. Then go back to the lines. Each try is one cell. You do not try a second cell inside the first try.
Easy and medium grids never need this. A hard grid may finish on the diagonal and never try a cell. When a hard grid does try one, it may need more than one cell before the lines finish, one at a time. It is not a guess you leave on the board.
- Suppose the white cell between the left-hand 1 and the 2 is empty. Walls close its row on both sides, and the wall under it closes the column. The only cell that can light it is the one above, so that cell is a bulb.
- That bulb empties the rest of the top row.
- The 1 on the left has lost the cell above it and the cell you tried, so its last side, the cell under it, is a bulb.
- The 2 to the right of the tried cell has lost the cell above it and the tried cell, so both cells it has left are bulbs: the one on its right, and the one under it.
- The 1 at the right of the third row now has bulbs above it and on its left. It needed one. The empty try dies.
- Suppose the top-left cell is a bulb. It empties the cell on its right. The 1 stops the row.
- The 1 under that bulb is finished, so the cell on its right and the cell under it stay empty.
- The bottom-left cell is walled above that empty cell and walled on the right. It is a bulb.
- That finishes the 1 on the bottom row, so the cell above the 1 and the cell on its right stay empty.
- The cell above the bottom 1 can only be lit from the cell on its right, so that cell is a bulb, and it empties the cell above it.
- The 1 on the top row has only the cell on its right left, so that cell is a bulb, and the cell under it stays empty.
- The 1 on the right of the third row is finished by the bulb on its left, so the bottom-right cell stays empty. The wall on its left closes the row, and the wall above closes the column. Nothing can light it. The bulb try dies.
In the first window the red dot is the try that fails, the 1 that ends up with two bulbs is ringed, and the two bulbs the chain places are drawn. The kept mark is a bulb in that cell. In the second window the red bulb is the try that fails, the bottom-right cell is the one that goes dark, and the 1 the bulb finishes too early is ringed. The kept mark is a dot.
Common mistakes
These are the marks that feel like progress and break a rule, or that were never forced.
What to play next
Each setting adds a kind of step. Start where the grid still finishes from the numbers.
- EasyNumbers, rays, and a cell aloneStart with the 0s and with numbers that have no spare side: a 4, a 3 on an edge, a 2 in a corner, a 1 with one open cell. Empty a line when you place a bulb. If a cell can only be lit by putting a bulb in that same cell, place it.Play Easy →
- MediumOne cell can still light itThe numbers and the dead ends have stopped. Find a white cell, often one you have already dotted, that only one cell can still light. That cell is a bulb.Play Medium →
- HardOne short, then one cellLook for a number that is one bulb short of its open sides, and dot a cell that would see two of those sides. If the grid still stalls, try one unknown cell, keep the mark that does not break, and go back to the lines.Play Hard →
The same steps cover 7×7, 8×8 and 10×10. Each of those sizes has easy, medium, and hard. When 7×7 feels quick, try 8×8 easy, then 10×10 easy. Print packs also offer 14×14, easy and medium only, with no hard: print a Light Up pack. The daily puzzle is a 7×7 easy grid. Open today’s daily or the 7×7 easy board.
- GuideNonogramsShade from the clues along each line.
- GuideStar BattlePlace stars so they never touch, not even on a corner.
- GuideSudokuRows, columns and boxes, from singles to triples.
- GuideHashiJoin islands with bridges until every number is met.
- GuideBinairoFill each line with zeros and ones, and keep the pairs apart.
- GuideSlitherlinkDraw one loop. Start from the 0s and the 3s.