If you have ever worked in the Photographic Darkroom, then you know how GREAT it is that there is a thing called the Safe Light!
A Safe Light allows the Darkroom Technician the ability to do some operations of the printing process, in some amount of light. If you have done work in the darkroom, you may not have fully appreciated this invention, but perhaps we should! I now appreciate it more probably, then I did when I was a Photographic Darkroom Technician, and Photographer doing work in the darkroom. At the time, I just took the technology for granted. Now, I know there is a long history to this single, useful, darkroom aid.
“In 1800, Dr. Herschel’s “Memoirs on the Heating Power of the Solar Spectrum” were published, and out of his observations on the various effects of differently coloured darkening glasses arose the idea that the chemical properties of the prismatic colours, and coloured glass, might be as different as those which related to heat and light. His suspicions were ultimately verified, and hence the use of yellow or ruby glass in the windows of the “darkroom,” as either of those coloured glasses admit the luminous ray and restrain the violet or active photographic ray, and allow all the operations that would otherwise have to be performed in the dark, to be seen and done in comfort, and without injury to the sensitive film.”
(Source: The Evolution of Photography, 1854-1890, by John Werge, Published in 1890.)