Here is a DIY CD Spectroscope that can be potentially used for testing darkroom safelights:
Text: https://www.exploratorium.edu/snacks/cd-spectroscope?media=7439
Video: https://www.exploratorium.edu/video/cd-spectroscope-introduction
Text: https://www.exploratorium.edu/snacks/cd-spectroscope?media=7439
Video: https://www.exploratorium.edu/video/cd-spectroscope-introduction