I did a search on "ultraviolet sensors" in this forum, and came up with only three results, so, it appears, that this has not been asked before.

I have a hand-held ultraviolet light sensor from Extech (Extech UV505) to measure power density from a 300 nm UV LED.


The UV LED is collimated with a lens at NA=0.9, and produces a good spot about 10 mm in diam. The output of the LED is listed as 30 mW at 6V, and yet the sensor reads 3 mW. The sensor is bigger than the spot. For another LED emitting at 365 nm, the sensor shows the expected value (or ballpark). Even though I mostly use the sensor for relative values, I need to have an idea of the power density to compare to published values. E.g. in the past people used UV lamps, and I am trying to replace the expensive 100W $5K lamp with a $200 LED.

Even though the specs of the sensitivity state "290-400nm", I suspect that the response is very much non-flat, and that the sensitivity in the 300/10 nm range is much smaller than at 365. The datasheet does say "peak at 365", that's as much of the spectrum as is available from the manual. I asked for the response curve from Extech, and their answer was "we do not have it", literally (facepalm).

What type of a sensor might this be, so I can look up the curve online? This must be an old technology, since the meter shows a similar number at 365 as an old UVP meter I borrowed from a colleague (the meter says 1991 on it). Interestingly, the UVP unit is still sold, and the design and the pricing both look as if it was still 1991:


Another puzzling feature about the Extech unit is that the sensor looks covered with a translucent plastic window which does not seem to be removable.

  • \$\begingroup\$ Have you looked at curves of sensors? Such as Figure 1 in optoelectronics.liteon.com/upload/download/DS86-2013-0004/… \$\endgroup\$
    – scorpdaddy
    Nov 9, 2018 at 17:46
  • \$\begingroup\$ You say the Extech is "to measure power density" by you are quoting your measurement in mW which is power (without the density). Can you clarify? What is the diameter of the sensor? \$\endgroup\$
    – Transistor
    Nov 9, 2018 at 17:57
  • \$\begingroup\$ You can't use a "wet-finger-grade" Extech photometer to measure an arbitrary 300 nm LED (what is its spectral bandwidth BTW?). You need a full-blown NIST-calibrated UV spectrophotometer, Perkin-Elmer, Cole-Parmer, Agilent, etc. UV-range instruments are not that expensive, $5K - $10K \$\endgroup\$ Nov 9, 2018 at 18:22
  • \$\begingroup\$ I believe this type of meter is calibrated on the assumption that the sensor is uniformly illuminated. If your spot of light is significantly smaller than the sensor area, it violates this assumption. \$\endgroup\$
    – Dave Tweed
    Nov 9, 2018 at 18:29
  • \$\begingroup\$ The range of the meter is 290 nm to 390 nm. You tried it with light at the edge of this range i.e. 300 nm then you tried it with another source at 360 nm. That second source is much more centrally placed in the spectrum and is much more likely to produce a bigger reading. \$\endgroup\$
    – Andy aka
    Nov 9, 2018 at 18:44

1 Answer 1


After much research, I purchased a calibrated 300 nm-calibrated sensor which did what I wanted. The cost was 3x the cost of the Extech meter (about 800 EUR) which is reasonable. Interestingly, the unit came with a motorola smartphone which communicated with the sensor via an app. My LED is rated at 30 mW, and my power reading was about 12 mW/cm2 from a re-focused spot from the LED which went through two UV lenses. Since the entire spot fit on the sensor (you could even see the blue spot by eye), I presume this means that the output is abt 12 mW.



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