Spectrophotometers measure transmittance and reflectance of light as a function of the wavelength of light. That is, it measures transmittance and reflectance for all colours of light, and shows how the transmittance/reflectance varies as the colour of the light is changed. Unlike with a colorimeter, the range of wavelengths that can be measured with a spectrophotometer extend beyond the visible range into infrared and ultraviolet regions of the electromagnetic spectrum. Multiple measurements may be taken, to ensure that the true colour is recorded.

The way in which the human eye perceives colour is dependent on the lighting. One might assume that a person’s colour can be measured by simply taking a photograph, however, there are many problems with this approach. The colour of a photograph is highly subjective and very dependent on the ambient lighting conditions. Modern cameras and mobile devices contain sensors that are not the same as the human eye, and often perform post-processing to correct the colour of an image. Unfortunately, these corrections will further distort the colour.

A spectrophotometer measures colour the way that a human eye sees it, by calculating the way in which light is reflected. The measurement is independent of the lighting conditions, ensuring that a formulation always matches the original colour even if the light changes.

The spectrophotometer can detect and quantity the most subtle of differences between different colour shades. This allows a specific colour to be measured, formulated, and then tested to ensure that it is a precise match to the original colour.