Yes a quality meter whose resolution is in mV, maybe $80 or so. The sensor isn't cheap btw. You'll be hacking into $160, just the cable though. It's basically a solar cell/resistor that emits a small amount of voltage when light from a certain spectrum hits it.
If you can afford it get a Parmeter. They'll calibrate it for you. You can't really do that with a voltmeter.
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