This is specifically mentioned in the article: "Due to the expense and tuning problems with a “real” theremins a number of “light” theremins have been produced

This is specifically mentioned in the article: "Due to the expense and tuning problems with a “real” theremins a number of “light” theremins have been produced. These all use some form of light sensor to detect the position of the players hands, but all suffer the same problem they are sensitive to changing light conditions and have very restricted range of detection."