Commonly employed security systems, such as used in museums, art galleries, shops, safe deposit vaults, banks or the like, do not protect each of the objects of value therein as such objects are generally too numerous or too small or cannot be altered to provide the necessary supports for the attachment of security devices. The usual security system solves this problem by concentrating a protection on the case in which the object is displayed, the room in which objects are stored, or the room or building in which the objects are contained. In addition, common security devices are far too expensive for the protection of individual objects, except in instances where only a small number of objects are being protected, or in instances wherein the objects are extremely valuable.
The commonly known type of security system has certain disadvantages. To begin with, the object is specifically what should be protected from thieves or vandals and not the case, room or building in which the object is accommodated. Moreover, false alarms arise too frequently due to occurrences which are not attempts to steal or damage the protected objects, but which are instead accidents, negligence or other occurrences involving the case, room or building accommodating the object. Such occurrences include accidental breaking of windows and the like that should, of course, be investigated by security personnel, but which do not constitute a true alarm situation or alert because of the lack of threat to the object being protected.
Another disadvantage of commonly used security systems is their failure to protect objects which are not located within cases and are accessible to the public during some part of the day. To distinguish between touching and stealing is almost impossible, or in most cases, prohibitively expensive. Touching, while not allowed, is a commonplace occurrence but does not constitute an act which requires setting off an alarm. On the other hand, stealing a painting hung on a wall requires the removal of the same and frame or the cutting of the painting out of the frame leaving the frame mounted on its supporting wall as before. Known systems cannot protect such an object while allowing access to the object for reasons of viewing and appreciation.
U.S. Pat. No. 666,737 shows a burglar alarm system in which is employed the combination of a vault or other like structure, the walls of which are impervious to waves of radiant energy, there being a sensitive electrical device arranged within the vault or other such structure, and adapted to operate upon the admission of such radiant energy through an opening or entrance in such walls, and an electrical signal appliance controlled by such sensitive device. Herein, as distinguished from the invention to be disclosed hereinafter, the protection while ultimately afforded to the object or objects themselves is predicated upon the interference with radiant energy of the walls of the structure surrounding the object to be protected.
U.S. Pat. No. 3,886,351 provides a photo-responsive means in an interface circuit, which photo-responsive means is positioned to receive light signals for developing electrical output signals as a function of the received light energy. A first differential amplifier is connected to receive the electrical output signals as a first input, and a known fixed reference as a second input. The first differential amplifier generates first and second output signals having a differential therebetween as a function of the differential between its received inputs. First and second unidirectional current paths connect the first and second output signals of the first differential amplifier as inputs to a second differential amplifier. Although the circuit disclosed employs a comparison with a fixed reference, this is unlike the comparison employed in accordance with various embodiments of the invention as will become apparent hereinafter.
U.S. Pat. No. 3,813,540 relates to a circuit for optically sensing coded data on a record medium and including a photosensitive transducing means. In order to render the circuit independent of background brightness variations, provision is made of load impedance, particularly arranged so that the voltage drop is proportional to the natural logarithm of current flowing through the transducing element. The voltage difference resulting from sensing contrasting marks on the record medium depends only on the contrast in reflected light and not on the absolute value of current on the transducing element. This particular disclosure relates generalized features which may be employed in accordance with the invention, but does not use these features for security systems as will be discussed hereinafter.