A light sensing system comprising: a substrate; a plurality of light-emitting devices (220, 221, 225, 226) disposed on the substrate, the plurality of light-emitting devices having a first optical axis; a plurality of light-receiving devices (320, 321, 325, 326) disposed on the substrate, the plurality of light-receiving devices having a second optical axis that is substantially parallel and substantially coaxial to the first optical axis; wherein the plurality of light-emitting devices and the plurality of light-receiving devices have a monolithically integrated structure; and a plurality of light receiving-emitting unit devices arranged repeatedly, each of the plurality of light receiving-emitting unit devices comprising at least one of the plurality of light-emitting devices (220, 221, 225, 226) and at least one of the plurality of light-receiving devices (320, 321, 325, 326); wherein either: a light-emitting material layer is formed monolithically on the substrate and comprises, for each of the light receiving-emitting unit devices, a light-emitting region comprising the at least one light-emitting device (220, 225) and a non-emitting region, wherein the non-emitting region comprises a same material as a material of the at least one light-emitting device; the at least one light-receiving device (320, 325) being formed monolithically on the non-emitting region; or: a light-receiving material layer is formed monolithically on the substrate and comprises, for each of the light receiving-emitting unit devices, a light-receiving region comprising the at least one light-receiving device (321, 326) and a non-receiving region, wherein the non-receiving region comprises a same material as a material of the at least one light-receiving device; the at least one light-emitting device (221, 226) being formed monolithically on the non-receiving region. a substrate; a plurality of light-emitting devices (220, 221, 225, 226) disposed on the substrate, the plurality of light-emitting devices having a first optical axis; a plurality of light-receiving devices (320, 321, 325, 326) disposed on the substrate, the plurality of light-receiving devices having a second optical axis that is substantially parallel and substantially coaxial to the first optical axis; wherein the plurality of light-emitting devices and the plurality of light-receiving devices have a monolithically integrated structure; and a plurality of light receiving-emitting unit devices arranged repeatedly, each of the plurality of light receiving-emitting unit devices comprising at least one of the plurality of light-emitting devices (220, 221, 225, 226) and at least one of the plurality of light-receiving devices (320, 321, 325, 326); wherein either: a light-emitting material layer is formed monolithically on the substrate and comprises, for each of the light receiving-emitting unit devices, a light-emitting region comprising the at least one light-emitting device (220, 225) and a non-emitting region, wherein the non-emitting region comprises a same material as a material of the at least one light-emitting device; the at least one light-receiving device (320, 325) being formed monolithically on the non-emitting region; or: a light-receiving material layer is formed monolithically on the substrate and comprises, for each of the light receiving-emitting unit devices, a light-receiving region comprising the at least one light-receiving device (321, 326) and a non-receiving region, wherein the non-receiving region comprises a same material as a material of the at least one light-receiving device; the at least one light-emitting device (221, 226) being formed monolithically on the non-receiving region. a light-emitting material layer is formed monolithically on the substrate and comprises, for each of the light receiving-emitting unit devices, a light-emitting region comprising the at least one light-emitting device (220, 225) and a non-emitting region, wherein the non-emitting region comprises a same material as a material of the at least one light-emitting device; the at least one light-receiving device (320, 325) being formed monolithically on the non-emitting region; or: a light-receiving material layer is formed monolithically on the substrate and comprises, for each of the light receiving-emitting unit devices, a light-receiving region comprising the at least one light-receiving device (321, 326) and a non-receiving region, wherein the non-receiving region comprises a same material as a material of the at least one light-receiving device; the at least one light-emitting device (221, 226) being formed monolithically on the non-receiving region.