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I'm looking for the best way to detect a weak bluish light (440nm) in a dark environment - with "best" I mean with highest sensitivity possible. I've noticed that most LDRs and photodiodes have their peak wavelength around 550~600 nm, and phototransistors at that range also include UV, driving up their prices.

So what would be the most efficient way? Using a LED to filter to the blue bandwidth, buy a more expensive UV+Blue phototransistor, rely on a IC like a light-to-frequency converter, or does it all depend on the measuring circuit?

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Probably a PMT (Photo Multiplier Tube), if you really need "the highest sensitivity possible".

Here's the response of one commercial module.

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Somewhat less effective, would be a blue-enhanced avalanche photodiode.

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It should be notes that "highest sensitivity" in this context means single photon detection. Photomultipliers can resolve single photons. I'd strongly suggest that they're massively overkill for what the OP wants, but considering the poor specificity of the OP's question, it's a completely valid answer. –  Connor Wolf Feb 11 at 2:38
    
It's not an insane solution if the OP needs just one- they're available on eBay for tens of dollars, probably a similar amount to a specialized blue-sensitive photodiode. –  Spehro Pefhany Feb 11 at 5:01
    
They're not so simple to use. You need a high-voltage supply to make them work at all, generally in the range of several kilovolts. They're not easy to power. –  Connor Wolf Feb 11 at 5:03
    
@ConnorWolf True, but that's just another interesting electronics project, and you can get the dynode power supplies surplus too. –  Spehro Pefhany Feb 11 at 5:08
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