The player places light bulbs in white cells such that no two bulbs shine on each other, until the entire grid is lit up.
A bulb sends rays of light horizontally and vertically, illuminating its entire row and column unless its light is blocked by a black cell.
A black cell may have a number on it from 0 to 4, indicating how many bulbs must be placed adjacent to its four sides; for example, a cell with a 4 must have four bulbs around it, one on each side, and a cell with a 0 cannot have a bulb next to any of its sides.
An unnumbered black cell may have any number of light bulbs adjacent to it, or none. Bulbs placed diagonally adjacent to a numbered cell do not contribute to the bulb count.
If a cell contains a diagonal wall ("mirror"), it should be lit.
A cell with a mirror cannot contain a bulb.
A black cell may have a number on it from 0 to 4, indicating how many bulbs must be placed adjacent to its four sides; for example, a cell with a 4 must have four bulbs around it, one on each side, and a cell with a 0 cannot have a bulb next to any of its sides.