The invention relates to a method for illuminating a wood fibre web for deviation detection. The method comprises turning on LEDs of an overhead sidelight that are side-directed towards a first edge of the web for illuminating a first half of width of the web, capturing an image of the first half, turning off the LEDs, turning on LEDs of the overhead sidelight that are side-directed towards a second edge of the web for illuminating a second half of width of the web, capturing an image of the second half, and turning off the LEDs. The invention also relates to an overhead sidelight, a machine vision system, and a lighting system.
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