Readers' contributions and Updates to the paper "How to find hidden cameras" by Marc Roessler marc@tentacle.franken.de The latest version of this file can be found at http://www.franken.de/users/tentacle/papers/hiddencam-contrib.txt The paper "How to find hidden cameras" can be found at http://www.franken.de/users/tentacle/papers/hiddencams.pdf 26 Jul 2002 It seems clear by now that the SpyFinder(tm) works based on the effect of retro-reflection. Experiments with a red laser pointer showed very visible reflections from the optics of a standard sized camcorder. Even the lenses of subminiature pinhole cameras were distinguishable suprisingly well. In order to achieve acceptable results the sensor (in the most simple case: your eye) should be as close to the axis of the emitter (e.g. a laser pointer) as possible. The emitter must be as punctual as possible, a laser meets this criteria. Obviously this works best at low ambient light levels -- while this all works pretty well at room light level a sunny day will probably make things more difficult. Because a laser pointer only produces a small red dot by means of which the whole environment needs to be swept it is advisable to construct an optical scanner. Mirror motors salvaged from defective laser printers are a good start. This only projects a thin red line, so the room still needs to be swept carefully. The scanner could be made still more comfortable by using a laser module with integrated optics that project a thin line. Combined with the spinning mirror this will generate a two dimensional projection by means of which the environment can be scanned easily. A drawback of the retro-reflection scan method is that curved objects made of metal or glass (e.g. bottles) will cause very visible reflections as well, so extra care needs to be taken to rule out the false positives. 07 May 2002 An anonymous reader not affiliated in any way with SEA Inc. suggested that the laser based camera detector described in section 4.8 may work based on retro-reflection. Retro-reflection means the reflection of light right back to its source, independent of the direction the light comes from [1]. This effect is often noticed on photographs, where the photo flash can make the subjects' eyes appear to glow red [2,3], caused by the retina at the back of the eye reflecting the light. Commercial applications of retro-reflection include traffic signs and reflective clothes commonly worn by road construction workers. Another application is pupil/eye detection and tracking [4,5]. The CCD/CMOS chip usually mounted at the focal plane of the video camera's lens is a good reflector and thus can be expected to cause visible reflections. [1] 3M webpage: What is Retroreflectivity http://international.3m.com/intl/AU/reflective/retro.html [2] Cliffshade Digital Photography page: Red-eye control http://www.cliffshade.com/dpfwiw/red-eye.htm [3] Photographytips.com: Red eye http://www.photographytips.com/page.cfm/434 [4] Antonio Haro, Irfan A. Essa, Myron Flickner: Eye Detection and Tracking http://www.cc.gatech.edu/cpl/projects/pupil/ [5] Scott Fritzinger, Dr. George Bebis, Dr. Dwight Egbert, Dr. William Wells: Infrared Eye Detection http://www.cs.unr.edu/CRCD/Summer01/scottf/ 25 Mar 2002 Steven Mann pointed me to two of his patents, canadian patent No. 2280420 ("Selective vitronic viewing concealed by material such as polymer diffuser") and canadian patent No. 2354113 ("Intelligent bathroom fixtures and systems"). Both patents are available from the Canadian Patents Database at http://patents1.ic.gc.ca/. The first patent suggests to use PDLC (Polymer Dispersed Liquid Crystal) technology for hiding cameras. PDLC layers are transparent when unpowered, but become intransparent (frosted look) when AC voltage is applied to them. The patent suggests to integrate PDLC technology into transparent wall tiles, for example. Pulsed AC currents are then applied to the assemblys. As long as the tile is transparent only for very short intervals (this is when the video camera shutter opens), it seems to be opaque to the human eye and can not be distinguished from regular wall tiles. The patent suggests further to integrate cameras concealed by means of this technology into toilet bowls. This is to detect the presence of humans sitting on the toilet as well as the presence of waste in the toilet bowl in order to trigger the flushing mechanism automatically. (Big Brother in the toilet bowl?) The second mentioned patent suggests the use of several image sensors (that is: cameras) for controlling showers, faucets, urinals or water closets automatically. The cameras are to be integrated into wall tiles, toilet bowls, the nozzles of showers and similar. In addition the camera images can be provided to monitoring stations, for security reasons. It is suggested to "[install this] bathroom control system in a variety of bathrooms in a bathroom network", providing investigatory access to security officers or "other officials who may help ensure bathroom security". Transparent bath tubs are suggested in order to ease monitoring. Mr. Mann considers the proposed system to "enhance the the privacy of users" by ensuring that officers and security guards need not enter the showers. In addition the system provides the benefit of distinguishing "a first class users" from "a second class of users", providing "a more pleasant shower experience to a first class of users, and a discomfort mode for providing a less pleasant shower experience to a second class of users". See section 3.9, "Taking over pre-installed cameras", of my paper "How to find hidden cameras". --- end of document ---