Fig. 1: Monte Carlo simulation.

a Schematic diagram of the distribution of the signals in reflective and transmissive modes, respectively. The green arrows represent the attenuation of the light source when penetrating the tissue (set as 520 μm, divided into 7 layers). The red layers and arrows represent the signals from the interested layers. The blue layers and arrows represent the signals from other layers. The thickness of the arrow represents the intensity of signal. b, c The normalized luminous flux over depth for quantitative analysis of attenuation of light in the simulated tissue for reflected-detected imaging (b) and transmissive-detected imaging (c). The white arrows indicate the direction of light propagation. The zoom-in regions of (b) and (c) represent the light path from the certain depth to the surface of the tissue, where the color represents the light flux intensity distribution in the light path, and the data were normalized to the light flux when the illumination light first accessed the tissue, where the light first accessed to the surface for reflective-detected mode and first accessed to the bottom of the tissue for transmissive-detected mode. d SBR of two modes when the depth of signal layer was set as 0, 100, 200, 300, 400, 500, 520 μm, respectively. e, f SBR with the increased thickness from top layer (e) and bottom layer (f) while the depth of signal layer was fixed as shown in (a). g SBR of two modes when the depth of signal layer was set as 0, 100, 200, 300, 400, 500, and 520 μm of the simulated dermis, respectively, while 80 μm of epidermis and 20 μm of stratum corneum were successively above the dermis. h, i SBR with the increased thickness from top layer (e) and bottom layer (f) of dermis while the depth of signal layer is fixed as 100 μm to the surface of dermis in (g). Here, SBR was defined as the proportion of light emanating from the target layer to the total light emanating from each layer