How do I clean the fresnel lenses? Carefully. The best practice is to handle the lenses carefully from the day you get them so that they don't get dirty in the first place. Having failed that... Alcohol will degrade the fresnels. They're made of optical acryllic. I personally have found that using a mild solution of dish soap and a soft brush works well on the grooved side. The smooth side can be cleaned with a soft cloth. Rinse the fresnel with distilled water (Available cheap in the pharmacy aisle at your local grocery store) to avoid any spots showing up on it. Dry with a clean soft cloth, and possibly another soft brush on the grooved side. Be very careful of the grooves, since the ridges are (and need to remain) quite sharp. DO NOT RUSH IT.