Abstract: A photographic film blackened area measuring device, having two rod shaped light conducting members, each of which is formed as an elongated piece of transparent material with part of its surface, parallel to its axis, being roughened so as to diffuse light, while light is totally internally reflected along the length of the rod shaped member from the remainder of its smooth surface. These rod shaped light conducting members are arranged one on each side of a piece of developed photographic film in a automatic film developing machine, and the film passes between them. A light emitting device is provided at one end of one of the rod shaped light conducting members, and a photo sensitive device is provided at the end of the other rod shaped light conducting member which is remote from the light emitting device.