The present invention discloses a protective device for safeguarding a hazardous area and a method for checking the functional reliability of such a device. The protective device has an image recording unit with an image sensor which includes a large number of light-sensitive pixels. During operation, the image recording unit records an object image. The image sensor is also supplied with a defined test image in order to check the functional reliability, wherein the test image recorded by the image sensor is compared with a defined expectation. According to one aspect of the invention, the object image is specifically made dynamic by means of a testing device and the modified object image is used as the test image.