Carpentry is another profession learned through apprenticeship. An apprenticeship is a system of training a new generation of practitioners of a trade or profession with on-the-job training and often some accompanying study (classroom work and reading). Apprenticeship also enables practitioners to gain a license to practice in a regulated profession.
A black light is a source of electromagnetic radiation that emits near-ultraviolet light, usually in the 350-375 nanometer wavelength range. This range is just slightly below the 400nm that human eyes can detect. When this light is reflected off certain materials, it looses energy and its wavelength expands to over 400nm. The reflected light is then visible to human eyes, and the illuminated objects appear to glow.
One type of lamp has a violet filter material, either on the bulb or in a separate glass filter in the lamp housing, which blocks most visible light and allows through UV, so the lamp has a dim violet glow when operating. Blacklight lamps which have this filter have a lighting industry designation that includes the letters BLB.