Arrangement for and method of detecting an object in an area subject to environmental variations

An arrangement for detecting an object in an entranceway subject to environmental variations and bounded by upright walls, comprising:

• a) a first transmitter means for transmitting a first light beam along a first optical path extending between the upright walls across the entranceway;
• b) a second transmitter means for transmitting a second light beam along a second optical path remote from the entranceway;
• c) a common receiver means subject to the environmental variations and situated in both the first and second optical paths for receiving the first and second light beams, and for respectively generating first and second digital signals that are both changeable by exposure to the environmental variations;
• and
• d) control processor means for establishing a reference value for generating an environmental variation-resistant, differential measurement value corresponding to a difference between the first and second digital signals, and for generating a control signal indicative of the status of the object in the entranceway when the differential measurement value differs from the reference value by a margin value.

A method of detecting an object in an area subject to environmental variations, comprising the steps of:

• a) transmitting a first light beam along a first optical path extending across the area;
• b) transmitting a second light beam along a second optical path remote from the area;
• c) receiving the first and second light beams, and respectively generating first and second digital signals that are both changeable by exposure to the environmental variations;
• d) establishing a reference value;
• e) generating an environmental variation-resistant, differential measurement value corresponding to a difference between the first and second digital signals;
• and
• f) generating a control signal indicative of the status of the object in the area when the differential measurement value differs from the reference value by a margin value wherein the transmitting steps a) and b) are alternately performed and pulsed to transmit first and second light beam pulses, respectively;
• and wherein step c) is performed by receiving the first and second light beam pulses and respectively generating first and second pulsed signals having first and second pulse widths;
• and wherein step e) is performed by determining the difference between the first and second pulse widths.

An arrangement for detecting an object in an area subject to environmental variations, comprising:

• a) transmitter means for transmitting a light beam along an optical path extending across the area during a first time period;
• b) receiver means for detecting the light beam during the first time period and responsively generating a first digital signal having a first pulse width indicative of the detected light beam and the environmental variation, and for detecting the environmental variation during a second time period, subsequent to said first time period, and responsively generating a second digital signal having a second pulse width indicative of the environmental variation;
• and
• c) control processor means for establishing a reference value, for generating an environmental variation-resistant, differential measurement value corresponding to a difference between the first and second pulse widths of the first and second digital signals, and for generating a control signal indicative of the status of the object in the area when the differential measurement value differs form the reference value by a margin value.