Professional Engineers are qualified to take on more liability than just creating a design. So you don't need an Engineer to design a product, but (depending on jurisdiction) you will need an engineer to certify that your product won't hurt or kill people.
If you do a lot of design work, then you may want to hire a Professional Engineer in-house so that your company has more confidence that they will produce compliant designs with fewer iterations.
Engineers are often the best people to work with if you need to do things that are done infrequently since theoretically, they're trained in the prerequisite first principles so they can make judgments that are rooted in rigorous analysis in addition to their practical experience.
Edit: also, I'm not exactly filled with knowledge on new lamp design/construction, but I have seen startups/kickstarters that make new lamps that seem like redesigns from the ground up.