The disclosure provides a display backlight unit and its light down conversion film. The light down conversion film may include a quantum-dot layer sandwiched between input substrate and an exit substrate. First and second refractive asymmetric micro-prisms are disposed on two opposite input and first exit surfaces of the input substrate, respectively. On the input surface of the input substrate, multiple arrays of the asymmetric refractive asymmetric micro-prisms preserve the large off-axis angle of incident light with first wavelength. On the first exit surface of the input substrate, multiple arrays of the refractive asymmetric micro-prisms increase the reflectance of the large incident angle light. A second exit surface of the exit substrate includes refractive symmetric micro-prisms. The refractive asymmetric micro-prisms of the input substrate and the refractive asymmetric micro-prisms of the exit substrate have rounded tips and valleys for enhancing refraction of the first light and the second light.