A method of arranging an optical device into a predefined position with respect to a cartridge , using an assembly which comprises, a cartridge comprising a channel along which a sample fluid can flow, one or more elements which are configured to manipulate light in a predefined manner, and wherein said one or more elements are located in a predefined position relative to the channel ; an optical device which is configured such that it can be selectively operated to emit light which is incident on the cartridge ; a means for moving the optical device with respect to the cartridge , or a means for moving said cartridge with respect to the optical device ; and a sensor which can receive light from said cartridge , said light having been emitted by said optical device , and which can output a signal which is representative of the light which it receives, the method comprising the steps of, operating the optical device to emit light which is incident on the cartridge ; moving the optical device , from a starting position, over the cartridge and over said one or more elements on the cartridge ; as the optical device is moved from said starting position, over the cartridge and over said one or more elements on the cartridge , recording the position of the optical device relative to the starting position; at each position for the optical device , detecting light using said sensor and outputting from the sensor a signal which represents light detected by the sensor , so as to provide an output signal which represents light detected by the sensor at each position of the optical device ; determining, from said output signal, the position at which the optical device was aligned over said one or more elements ; characterized in that the method further comprises the steps of, using said determined position and the predefined position of the one or more elements relative to the channel , to determine how to move the optical device so that it is aligned above the channel ; moving the optical device so that it is aligned above the channel . operating the optical device to emit light which is incident on the cartridge ; moving the optical device , from a starting position, over the cartridge and over said one or more elements on the cartridge ; as the optical device is moved from said starting position, over the cartridge and over said one or more elements on the cartridge , recording the position of the optical device relative to the starting position; at each position for the optical device , detecting light using said sensor and outputting from the sensor a signal which represents light detected by the sensor , so as to provide an output signal which represents light detected by the sensor at each position of the optical device ; determining, from said output signal, the position at which the optical device was aligned over said one or more elements ; characterized in that the method further comprises the steps of, using said determined position and the predefined position of the one or more elements relative to the channel , to determine how to move the optical device so that it is aligned above the channel ; moving the optical device so that it is aligned above the channel . <EOS>