This image was acquired from
wikimedia. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Keywords from Image Description:
SpectroscopyResearch . After this he would estimate the intensities of the spectral lines of the element under observation relative to those of pure copper National Institute of Standards and Technology National Institute of Standards and Technology PDUSGovNIST PDUSGov National Bureau of Standards National Institute of Standards