Apparatus with an optical system for monitoring a protected field has at least two opposing optical units arranged so that the protected field is between them. Each optical unit includes a picture-taking unit and a processing and switching unit that is operatively connected with the picture-taking unit for generating a danger signal. A marking is applied to each optical unit, and each picture-taking unit is arranged so that the marking on an opposite side of the protected field can be viewed by the first-named picture-taking unit. A processing and switching unit includes a comparison unit for comparing a picture with a reference picture. Each optical unit further has at least one lighting unit for illuminating the oppositely positioned marking on the oppositely positioned optical unit. The picture-taking unit, the markings and the lighting units are arranged in mutual alignment and on a common housing.