Photochromic materials undergo changes of color and spectral properties upon light exposure due to changes in their chemical composition. The light source may be natural daylight or a more intense artificial light source. UV light will show the strongest effect, although visible light will show an effect, too.
Thus, you have to irradiate your material and analyze the changes of color and spectral properties by UV-vis spectroscopy.
It entirely depends on your material and your focus of study. Photochromism is generally referred as the change of colour upon irradiation of photons. Photons can be of any wavelengths or energy, and UV is only part of it.