An assay instrument comprising: a first fluorometer comprising a first detection channel having a first light source and a first sensor , the first detection channel being configured to emit and focus light generated by the first light source at a first detection zone, and to receive and focus light on the first sensor ; a carrier comprising a first non-fluorescent surface portion and a void, and configured to support a first receptacle , wherein the carrier and the first fluorometer are movable relative to each other among at least (i) a first position at which a portion of the first receptacle is in the first detection zone, (ii) a second position at which the first non-fluorescent surface portion of the carrier is in the first detection zone, and (iii) a third position at which the void is in the first detection zone; and a controller operatively coupled to the first fluorometer and configured to: determine a characteristic of a sample contained within the first receptacle based on a first measured intensity of light focused on the first sensor while the carrier is at the first position, and determine an operational performance status of the first fluorometer based on at least a second measured intensity of light focused on the first sensor while the carrier is at the third position by determining whether the second measured intensity is within a first predetermined void intensity range. a first fluorometer comprising a first detection channel having a first light source and a first sensor , the first detection channel being configured to emit and focus light generated by the first light source at a first detection zone, and to receive and focus light on the first sensor ; a carrier comprising a first non-fluorescent surface portion and a void, and configured to support a first receptacle , wherein the carrier and the first fluorometer are movable relative to each other among at least (i) a first position at which a portion of the first receptacle is in the first detection zone, (ii) a second position at which the first non-fluorescent surface portion of the carrier is in the first detection zone, and (iii) a third position at which the void is in the first detection zone; and a controller operatively coupled to the first fluorometer and configured to: determine a characteristic of a sample contained within the first receptacle based on a first measured intensity of light focused on the first sensor while the carrier is at the first position, and determine an operational performance status of the first fluorometer based on at least a second measured intensity of light focused on the first sensor while the carrier is at the third position by determining whether the second measured intensity is within a first predetermined void intensity range. determine a characteristic of a sample contained within the first receptacle based on a first measured intensity of light focused on the first sensor while the carrier is at the first position, and determine an operational performance status of the first fluorometer based on at least a second measured intensity of light focused on the first sensor while the carrier is at the third position by determining whether the second measured intensity is within a first predetermined void intensity range. <EOS>