Luminance Ratio

a dimensionless quantity used in illuminating engineering to express quantitatively the difference between visual (light) sensations evoked by two adjacent surfaces of the same color. If L₁ and L₂ are the luminances of the surfaces being compared and L₁ = L₂ + nΔL, where ΔL is the luminance threshold, or minimum difference of luminance that can be discerned by eye, the luminance ratio of the first surface to the second will be equal to n.