The cause for this test was a 4-year old luxmeter comparison on TLF from member ' forest' (link) that I had never seen before and that was recently referred to on BLF. In that test, the noname orange-coloured LX1330B luxmeter (many people on BLF have one too) gave surprisingly good results in comparison to a high-end Gossen meter. Since I never had a look at that specific luxmeter, and that I needed a decent but not too expensive luxmeter for a new to be build integrating sphere, I bought one on dx.com, in the hope that it is old stock enough to be of the same build as the one in the german test. At least it looks exactly the same .
But before using it I wanted to know a bit more about it. The german test compared quite a few flashlights, but the variation in tint and CRI of that collection was quite limited: all cool and neutral 70CRI flashlights, and an incandescent light source. But I'm not so much interested in the overall calibration, which when checked is easy to correct with one correction factor, I'm more interested in a correct wavelength response, or in other words: is the same calibration valid for all tints and CRI, or does it need different corrections for every different tint or CRI that is measured ? (I discussed the importance of a good wavelength response in my lengthy first luxmeter thread 4 years ago ( https://budgetlightforum.com/t/-/28689 )
So I made it into a new direct 5 luxmeters shootout. I have done some testing on all of them before, but it is nice to test them all together now :-) I chose 6 different flashlights with different tints and CRI that all have a good constant output setting so that I could conveniently measure the throw at 7 meter distance.
Sofirn SP33 led: Samsung LH351D 4000K 90+ CRI |
Sunwayman D40A led: Cree XM-L2 6500K? 70 CRI |
Kaidomain K2 host led: Cree XM-L2 3000K 90+ CRI |
Supwildfire 'Mitko thrower' led: Cree XP-G2 3D dedomed ±4500K 70 CRI |
Olight M10 'Maverick' led: Nichia E21A 6500K 95+ CRI R9080 |
Ultrafire C10 led: Epileds 5050 powerled 400nm UV |
|
MobiLux Class A (2nd row: calibration corrected by +3.4%) |
171 177 |
281 291 |
133 138 |
1544 1596 |
31.6 32.7 |
2.25 2.33 |
Tondaj LX-1010B | 155 | 263 | 116 | 1273 | 32 | 73 |
UNI-T UT-383 | 162 | 282 | 116 | 1344 | 30 | 104 |
Tasi 632A | 179 | 274 | 142 | 1554 | 31.5 | 0.42 |
noname LX-1330B | 176 -0,6% |
306 +5,2% |
129 -6,5% |
1428 -10,5 |
37.8 +15,6 |
43 |
For discussion of this table I focus on the two main possible causes of a different reading: overall calibration diferences and differences in wavelength response.
You may notice that for all light sources, the LX-1330B and the Tondaj meter differ by the roughly the same factor: the LX-1330B measures between 12% and 16% higher. So they likely have a similar wavelength response but have a 14-ish% different calibration (I hope this does not already disturb people ). If you choose one to be right, you can easily correct the other with one number.
In contrast, if we compare the LX-1330B with the Mobilux meter, the LX-1330B measures the 4000K 90CRI Samsung led quite the same, the dedomed 3D Cree XP-G2 is measured 8% too low, and the 6500K R9080 Nichia is measured 20% too high. So what is going on is not a simple calibration difference but a fairly different wavelength response, almost impossible to correct.
If I have to choose the luxmeter that performs closest to my high end MobiLux meter, that will not be the LX-1330B but the Tasi meter: two readings are within 1% of the Mobilux, and the the most extreme reading deviations are just 9% apart (compared to the LX-113B: 28%, UNI-T 15%). So the Tasi meter will be attached to my new sphere!
As in my first luxmeter thread, I took the time to get my trusty Zeiss monochromator with regulated tungsten lamp out of the cupboard, and recorded the tungsten spectrum response of the 5 meters. For details of the set-up see over there. Here are the results, but below the graph is a needed discussion of what you see.
Discussion of the graph:
*It is tempting to accept the graph as an absolute wavelength response curve for the luxmeters, but for that the radiometric power of all emitted wavelengths of the light source should be equal, instead the tungsten spectrum increases 8 times from 450nm to 750nm. I could correct for that (the increase is pretty lineair in that region) but I have no data on if this type of monochromator has a fixed percentage power loss over the entire spectrum, probably not because that is not a requirement for the spectrometer what this thing was part of (measurements are not absolute but always relative to a blanco). So because I do not know the added error of the monochromator, I will leave the graph as is.
*However, the curves relative to each other are correct. For example, at 450nm the LX-1330B measures 7 times as high as the MobiLux meter, this would be no different if the light source was equally powered over the wavelengths. This makes the graph still highly interpretable, the differences between the wavelength responses are well visible. Do correct in your mind that the further right in the spectrum you go, the lower the real numbers are compared to what you see .
*The readings of the different luxmeters were quite different despite that each time the same light source was measured. This is a results of the small light projection (5x5mm) on the varying diffuser sizes over the detectors (and distance of the detector behind the diffuser). For a fair comparison, the diffusers should be filled out with the light source, which was not possible. So the readings were corrected to each other, the best way I could think of is making the surface area under the graphs equal.
*Necessarily because what comes out of the monochromator is not much light, the responses of the luxmeters were recorded at the very low end of their ranges. So I accept the results to be true assuming that the wavelength responses do not alter at different light intensities. I never noticed such a thing, and have not read anything about it, but I also have not extensively reasearched it.
*The graphs underline that the Tasi meter's wavelength response comes closest to the MobiLux (that in itself may not be perfect but, being class A, should have a response closest to the luminosity curve of all). The LX-1330B has a very similar wavelength response to the Tondaj meter, quite far away from the MobiLux graph and I suspect that both optical filter and detector of the two luxmeters are identical. The UNI-T meter does not do a bad job actually both in calibration and wavelength response, especially considering of the low price shipped (I payed 13 dollar!)