I wired up what I believe to be the world's first solid state spectrograph in order to monitor the progress of the extraction of tin from my arc electrolysis process. I used a medium quartz spectrograph, replacing the photographic plate with one coated in fluorescein in gelatine. This converted ultra violet light to green light which could be measured by silicon photodiodes. By measuring the ratio between a diode monitoring a tin line and another monitoring a reference line, it was possible to estimate the tin remaining in the melt.NRDC thought it was not worth patenting it so I published the idea in 1973

Here is the spectrum, with a short visible spectrum on the right and some of the fluorescent light from the ultra violet light next to it

Diagram of circuit. Despite its crudeness, it worked well. A Japanese product based on the same fluoescein fluorescence technique was, I understand, on sale in about a year. A British product taking advantage of advances in electronics with linear CCD detectors and extensive number crunching took more than a decade to reach market.

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