What does a color measuring device see?

Color measuring devices called colorimeters or spectrocolorimeters work as follows:

The device illuminates the product and analyzes the scattered or transmitted light.

From the spectrum obtained (light intensity as a function of wavelength), the device calculates the chromatic coordinates of the color in the colorimetric space.

We thus obtain coordinates L * a * b where L * represents the lightness (black-white), a * the green-red axis, b * the blue-yellow axis.

We can thus measure a color thanks to its coordinates but also measure color differences (delta E).