This answer to Pre-mixing cryogenic fuels and using only one fuel tank quotes John D. Clark's Ignition! Chapter 11: The Hopeful Monoprops, and the quote includes the line:
How he avoided suicide (the first rule in handling liquid oxygen is that you never, never let it come in contact with a potential fuel) is an interesting question, particularly as JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it.
It is also quoted in this answer to Why did it take so long for methane to be used as a rocket propellant?
Question: How did JPL detonate a liquid oxygen methane mixture by shining light on it? Is there any information on the procedure and/or the intensity of the light?
CO2 lasers come in kilowatt+ varieties for example, and were around since about 1967. They can burn through just about anything, so just saying that they used light does not necessarily mean that they used "a light touch".
Was it a purely bulk thermal effect, or was this really a photo-initiated chemical reaction, i.e. individual photons stimulated chemical reactions directly?