Fig. 1

Externally applied red light penetrates through the entire rat spinal cord. a Photograph shows the ventral surface of the spinal cord following removal of the T10 vertebral body in a cadaver rat. Topography of the vertebral column is shown centred around the 10th vertebral body under normal light conditions. b The identical region as shown in a, with a 670 nm LED array light source (35 mW/cm²) placed directly on the dorsum of the animal and with ambient lights switched off. Note the visible red light illuminating from the ventral surface of the cord (exposed, arrow) indicating excess penetration through dorsal layers of the hair, skin, muscle, bone and spinal cord. c Intensities measured by a 670 nm power meter are shown for six freshly sacrificed cadaver rats (each dot represents the mean of triplicate readings). Readings shown are taken at the light source (through the Perspex restraining box, intensity at dorsal surface) and at the ventral surface of the spinal cord as shown by the white arrow in b (intensity at ventral surface). Black arrow indicates proportion of light absorbed and/or scattered by intervening tissues. d Spectral analysis of the light source indicating central frequency of 675 nm