A luminescent medium comprising: a substrate ; and a luminescent material provided on the substrate , wherein the luminescent material comprises a first luminescent material which, when irradiated with visible light, infrared light or ultraviolet light, emits first infrared light (a) having a first wavelength range (W1), and a second luminescent material which, when irradiated with visible light, infrared light or ultraviolet light, emits second infrared light (b) having a second wavelength range (W2), wherein the first wavelength range (W1) of the first infrared light (a) from the first luminescent material differs from the second wavelength range (W2) of the second infrared light (b) from the second luminescent material , and the first luminescent material is disposed in a first planar area on the substrate , and the second luminescent material is disposed in a second planar area on the substrate , and wherein the first planar area and the second planar area overlap each other in which the luminescent material including the first luminescent material and the second luminescent material is disposed in a concentration gradation manner, forming an overlapping planar area of the luminescent material , and the concentration of the first luminescent material in the luminescent material of the overlapping planar area gradually decreases in a direction from the first planar area toward the second planar area , while the concentration of the second luminescent material in the luminescent material of the overlapping planar area gradually decreases in a direction from the second planar area toward the first planar area . a substrate ; and a luminescent material provided on the substrate , wherein the luminescent material comprises a first luminescent material which, when irradiated with visible light, infrared light or ultraviolet light, emits first infrared light (a) having a first wavelength range (W1), and a second luminescent material which, when irradiated with visible light, infrared light or ultraviolet light, emits second infrared light (b) having a second wavelength range (W2), wherein the first wavelength range (W1) of the first infrared light (a) from the first luminescent material differs from the second wavelength range (W2) of the second infrared light (b) from the second luminescent material , and the first luminescent material is disposed in a first planar area on the substrate , and the second luminescent material is disposed in a second planar area on the substrate , and wherein the first planar area and the second planar area overlap each other in which the luminescent material including the first luminescent material and the second luminescent material is disposed in a concentration gradation manner, forming an overlapping planar area of the luminescent material , and the concentration of the first luminescent material in the luminescent material of the overlapping planar area gradually decreases in a direction from the first planar area toward the second planar area , while the concentration of the second luminescent material in the luminescent material of the overlapping planar area gradually decreases in a direction from the second planar area toward the first planar area . <EOS>