A wearable sensor system (101, 200) for use in conjunction with an article (150, 250), the wearable sensor system comprising: a plurality of light sources; a photodetector; a non-transitory computer-readable medium storing computer-executable instructions; and a processing device communicatively coupled to the non-transitory computer-readable medium for executing the computer-executable instructions, wherein executing the computer-executable instructions configures the processing device to perform operations comprising: obtaining a plurality of measurements of light by, for each measurement: causing a first light source of the plurality of light sources to transmit a pulse of light through the article to an area of skin, wherein the first light source comprises a first geometry; and determining, via the photodetector, a measurement of light returned from the article; determining, from the plurality of measurements of light, a periodic change in an amplitude of the returned light; and identifying the periodic change in amplitude as a heart rate having an identical periodicity, characterised in that: the identifying comprises: determining an energy level of the plurality of measurements of light; and responsive to determining that the energy level is lower than a threshold, causing a second light source of the plurality of light sources to transmit an additional pulse of light through the article to the area of skin, wherein the second light source comprises a second geometry that is different from the first geometry. a plurality of light sources; a photodetector; a non-transitory computer-readable medium storing computer-executable instructions; and a processing device communicatively coupled to the non-transitory computer-readable medium for executing the computer-executable instructions, wherein executing the computer-executable instructions configures the processing device to perform operations comprising: obtaining a plurality of measurements of light by, for each measurement: causing a first light source of the plurality of light sources to transmit a pulse of light through the article to an area of skin, wherein the first light source comprises a first geometry; and determining, via the photodetector, a measurement of light returned from the article; determining, from the plurality of measurements of light, a periodic change in an amplitude of the returned light; and identifying the periodic change in amplitude as a heart rate having an identical periodicity, obtaining a plurality of measurements of light by, for each measurement: causing a first light source of the plurality of light sources to transmit a pulse of light through the article to an area of skin, wherein the first light source comprises a first geometry; and determining, via the photodetector, a measurement of light returned from the article; causing a first light source of the plurality of light sources to transmit a pulse of light through the article to an area of skin, wherein the first light source comprises a first geometry; and determining, via the photodetector, a measurement of light returned from the article; determining, from the plurality of measurements of light, a periodic change in an amplitude of the returned light; and identifying the periodic change in amplitude as a heart rate having an identical periodicity, characterised in that: the identifying comprises: determining an energy level of the plurality of measurements of light; and responsive to determining that the energy level is lower than a threshold, causing a second light source of the plurality of light sources to transmit an additional pulse of light through the article to the area of skin, wherein the second light source comprises a second geometry that is different from the first geometry. determining an energy level of the plurality of measurements of light; and responsive to determining that the energy level is lower than a threshold, causing a second light source of the plurality of light sources to transmit an additional pulse of light through the article to the area of skin, wherein the second light source comprises a second geometry that is different from the first geometry.