An apparatus comprising: a light source (108, 408, 908, 1008, 1108, 1208, 1308, 1408, 1508, 1608, 1708, 1808, 1908, 2108, 2208, 2308, 2408, 2508, 2608, 2708); one or more discrete, high-aspect-ratio photodetectors, each high-aspect-ratio photodetector having a die with a photosensitive area and configured to provide an output signal that indicates the total amount of light that is incident on the photosensitive area of the die of the high-aspect-ratio photodetector (212,912, 1012, 1112, 1212, 1312, 1412, 1512, 1612, 1712, 1812, 2112, 2212, 2312, 2412, 2512, 2612, 2712), the photosensitive area of each high-aspect-ratio photodetector having: a first length (986, 1090, 1190, 1290), being the distance between two lines parallel to a ray emanating from the center of the light source and that pass through opposing ends of the photosensitive area, and a first width (988, 1092, 1192, 1292) in a direction perpendicular to the first length; and control logic, the control logic communicatively connected with the light source and each high-aspect-ratio photodetector and configured to: cause the light source to emit light, obtain one or more measured light intensity measurements from the one or more high-aspect-ratio photodetectors based on the output signal from each high-aspect-ratio photodetector, and determine a heart rate measurement based, at least in part, on the one or more light intensity measurements, the light source is mounted on a surface of a substrate, the photosensitive area of each high-aspect-ratio photodetector is positioned such that an edge of the photosensitive area of each high-aspect-ratio photodetector closest to the light source is between 1 mm and 4 mm from the center or edge of the light source, and each photosensitive area faces away from the surface of the substrate on which the light source is mounted, characterized in that the ratio of the first length to the first width of the photosensitive area of each high-aspect-ratio photodetector element is between 3:1 and 5:1. a light source (108, 408, 908, 1008, 1108, 1208, 1308, 1408, 1508, 1608, 1708, 1808, 1908, 2108, 2208, 2308, 2408, 2508, 2608, 2708); one or more discrete, high-aspect-ratio photodetectors, each high-aspect-ratio photodetector having a die with a photosensitive area and configured to provide an output signal that indicates the total amount of light that is incident on the photosensitive area of the die of the high-aspect-ratio photodetector (212,912, 1012, 1112, 1212, 1312, 1412, 1512, 1612, 1712, 1812, 2112, 2212, 2312, 2412, 2512, 2612, 2712), the photosensitive area of each high-aspect-ratio photodetector having: a first length (986, 1090, 1190, 1290), being the distance between two lines parallel to a ray emanating from the center of the light source and that pass through opposing ends of the photosensitive area, and a first width (988, 1092, 1192, 1292) in a direction perpendicular to the first length; and a first length (986, 1090, 1190, 1290), being the distance between two lines parallel to a ray emanating from the center of the light source and that pass through opposing ends of the photosensitive area, and a first width (988, 1092, 1192, 1292) in a direction perpendicular to the first length; and control logic, the control logic communicatively connected with the light source and each high-aspect-ratio photodetector and configured to: cause the light source to emit light, obtain one or more measured light intensity measurements from the one or more high-aspect-ratio photodetectors based on the output signal from each high-aspect-ratio photodetector, and determine a heart rate measurement based, at least in part, on the one or more light intensity measurements, cause the light source to emit light, obtain one or more measured light intensity measurements from the one or more high-aspect-ratio photodetectors based on the output signal from each high-aspect-ratio photodetector, and determine a heart rate measurement based, at least in part, on the one or more light intensity measurements, the light source is mounted on a surface of a substrate, the photosensitive area of each high-aspect-ratio photodetector is positioned such that an edge of the photosensitive area of each high-aspect-ratio photodetector closest to the light source is between 1 mm and 4 mm from the center or edge of the light source, and each photosensitive area faces away from the surface of the substrate on which the light source is mounted,