During the process of manufacturing of home-brew PCBs you need to expose the photosensitive film with ultraviolet light. A cheap and low power solution can be found in UV led strips.
UV LED Exposure Box : 26 Steps (with Pictures) - Instructables
GitHub - chris-zen/pcb-insulator: Design to build a PCB insulator using UV LEDS
LED UV exposure box part 1, the box
UV LED PCB Exposure panel
UV Exposure Box
UV exposure box - Jonatan Martinez
SpierceTech PCB UV Exposure Box Kit
UV-Exposure box V3 – ElektronicaStynus
UV-Exposure box V3 – ElektronicaStynus