A method of positioning and sealing a bag in a vacuum chamber , the method comprising: loading the bag by placing a product in the bag to produce a loaded bag , the bag including an upstream end including a bag mouth , the bag further including a patch adhered thereto, the patch including a fluorescent material, the bag and the patch both being transparent to infrared radiation; placing the loaded bag on an infeed conveyor ; advancing the loaded bag , on the infeed conveyor , to an infrared sensing apparatus including an infrared detector and a first radiation source , the infrared detector being disposed on an opposite side of the infeed conveyor from the first radiation source ; detecting a trailing edge of the product inside the loaded bag by interrogating, through the loaded bag, infrared radiation emitted from the first radiation source , using the infrared sensing apparatus; advancing the loaded bag to a fluorescence sensing apparatus including a fluorescence detector and a second radiation source ; detecting a trailing edge of the patch by interrogating fluorescence emitted by the patch using the fluorescence sensing apparatus, wherein radiation emitted by the second radiation source excites the fluorescent material; acquiring information from detecting the trailing edge of the product and detecting the trailing edge of the patch , and transmitting the information to a controller; controlling a distance of advancement of the loaded bag to a sealing position in the vacuum chamber including a heat seal assembly , using the controller, based on the information acquired from detecting the trailing edge of the product and detecting the trailing edge of the patch ; and closing the loaded bag by heat sealing the loaded bag, using the heat seal assembly , so that a heat seal is applied between the trailing edge of the product and the bag mouth and between the trailing edge of the patch and the bag mouth . loading the bag by placing a product in the bag to produce a loaded bag , the bag including an upstream end including a bag mouth , the bag further including a patch adhered thereto, the patch including a fluorescent material, the bag and the patch both being transparent to infrared radiation; placing the loaded bag on an infeed conveyor ; advancing the loaded bag , on the infeed conveyor , to an infrared sensing apparatus including an infrared detector and a first radiation source , the infrared detector being disposed on an opposite side of the infeed conveyor from the first radiation source ; detecting a trailing edge of the product inside the loaded bag by interrogating, through the loaded bag, infrared radiation emitted from the first radiation source , using the infrared sensing apparatus; advancing the loaded bag to a fluorescence sensing apparatus including a fluorescence detector and a second radiation source ; detecting a trailing edge of the patch by interrogating fluorescence emitted by the patch using the fluorescence sensing apparatus, wherein radiation emitted by the second radiation source excites the fluorescent material; acquiring information from detecting the trailing edge of the product and detecting the trailing edge of the patch , and transmitting the information to a controller; controlling a distance of advancement of the loaded bag to a sealing position in the vacuum chamber including a heat seal assembly , using the controller, based on the information acquired from detecting the trailing edge of the product and detecting the trailing edge of the patch ; and closing the loaded bag by heat sealing the loaded bag, using the heat seal assembly , so that a heat seal is applied between the trailing edge of the product and the bag mouth and between the trailing edge of the patch and the bag mouth . <EOS>