Under certain wavelengths, diamonds can fluoresce to varying degrees. The fluorescence of a diamond can vary in intensity, from none to very strong, and in colour, from blue to yellow. Diamonds are measured in laboratories for their reaction under longwave ultraviolet light, and this scale can also affect the appearance of the diamond in daylight. Diamonds with a strong fluorescence can appear milky in daylight. However, diamonds that fall below I colour with a strong blue fluorescence can appear to have a better colour in daylight, as the blue tones can counteract yellow when the diamond is viewed face up.