In one aspect, an elongated lighting module includes plural colors of LEDs that flash in a cyclical sequence to provide multi-spectral illumination. Several such modules can be arranged end-to-end to span a conveyor system that transports a stream of plastic waste items in a recycling center, enabling capture of differently-illuminated image frames depicting the items. Each module may include N LEDs of a first color (e.g., red) and M LEDs of a second color (e.g., blue), where N and M are different. Drive circuitry can be simplified by configuring the LEDs in strings of common colors, but with different counts. Electrical noise due to switching transients can be reduced by operating the LEDs at a low current when not being flashed. A great variety of other features and arrangements are also detailed.