For my experiment I need to take a 0.8 mm HeNe beam to 20 mm beam. Since we just want illumination, it appears there is not need for beam expander or a setup of special lens (there are a few configurations online). It was suggest to use a Plano-convex cylindrical lens. I talked to application engineers from Edumnds and Newport, they said that the thin-lens approximations are sufficient if finding the correct focal length. However, I am still struggling. It's a beam of light. I have been saying that the object high is the radius of the beam, but what do I put for the object distance. I want to say it is infinity, but it appears it could be a little arbitrary. I am pretty stuck on this.