flame photometry
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Flame photometry is the simplest, oldest member of the atomic-emission family — essentially measuring the colour a flame turns when you feed it a sample. Spray a salt solution into a gas flame, the flame glows with the colour of whatever metal is present, and a simple light meter reads how bright that colour is.
More precisely, the sample is sprayed as a fine mist into a flame; the heat frees the metal atoms and excites a fraction of them to emit light. A coloured filter picks out one element's characteristic colour, and a photodetector measures its brightness, which rises with the element's concentration. It is essentially a stripped-down emission instrument with a filter instead of a full wavelength selector.
It matters because for easily excited metals — sodium, potassium, lithium, calcium — flame photometry is fast, cheap, and good enough; clinical labs long used it for blood sodium and potassium. Its limit is reach: a flame is too cool to excite most metals well, so it handles only a handful of the easiest elements.
A clinical flame photometer aspirates a diluted serum sample; a red filter passes lithium light and a yellow filter passes sodium light, so the meter reports both blood sodium and a lithium internal standard in one quick run.
A cool flame plus a colour filter: enough to measure easily excited metals like Na and K.
Flame photometry is atomic emission, not atomic absorption — the flame itself produces the light, and no lamp shines through it. The same flame and nebulizer hardware, fitted with a lamp, becomes a flame AAS instrument instead.