Provided is a substrate whereby diffusion of light toward a backlight can be suppressed and luminance of an image display device can be enhanced. A substrate having, on a transparent substrate, (a) a color conversion light-emitting layer and (b) a light diffusion prevention layer in this order from the transparent substrate side, the refractive index of the light diffusion prevention layer with respect to a wavelength of 550 nm being 1.20-1.35.