I have been wondering what practical ways of doing this. Practical means to me, a small cheap, durable, lightweight device that can be thrown into a book bag and toted to jobs. . Maybe a prism held in front of smart phone camera, manual white balance snapshot the rainbow and compare to ideal light of same cct.
No luck for last two days googling. White swirl prism photography, cereal box, and 700 dollar spectrometry.