A type of distance metric showing the distance between two colours. The official CIE standard, this metric is of high quality, but computationally intensive. Values of less than 0.5 signify differences not visible to the human eye, values between 0.5 and 1.0 signify colour differences visible under close scrutiny. Values between 1.0 and 50 indicate the colours are more similar than different, while values higher than 50 mean that the colours are more different than similar. The highest possible value is 100.