What is a cost-effective way (say roughly $15) to shine a wide but thin beam of light, directly away from the source, so that it would project a line onto a nearby surface?
My particular beam would need to be about 4' wide and maybe 1mm-4mm thick. The light would not necessarily need to be parallel like a laser, but needs to be pretty focused (1-4mm) for the full range between 1" and 4" from the source, so that if an object is moved within that range, the beam projected on it is always thin, and fairly parallel so that ~0.25" bumps on the object would not block parts of the line.
I considered using a strip of SMD LEDs behind an acrylic lense that is concave along the thin axis, and plano (straight) along the long 4' axis, at a distance such that the light focuses to a line in that 1"-4" range...but that would probably be a long focal length, which would require more space than I might have. Also the LEDs would need to be spaced such that the overlap of each LED's contribution makes the line fairly even in intensity. But before going that direction, I wonder if there is a better solution.