Luxmeters: thoughts, findings, comparison

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djozz
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Luxmeters: thoughts, findings, comparison

Going into details here :bigsmile: This post may look a bit technical for some but fear not, it mostly contains things that I picked up about the subject over the last couple of weeks, I am not at all a physicist specialising in optics (I am a biologist). Disclaimer: having said that, I feel free to make errors in what I write down here Tongue Out .

 

Measuring light with a luxmeter is a tricky business, as I have become to understand and as many on this forum have recognised as well. The main focus of discussion recently on BLF is having the meter correctly calibrated, which is hard enough, but there can be more wrong with a luxmeter than just the calibration, to name a few: the temperature error can be high, the reading can be non-lineair, the spectral sensitivity may not be corrected well for the luminosity curve (also called V-lambda curve, that is the curve of the (phototrophic) human eye wavelength sensitivity, used in the definition of the lux). A Wikipedia quote from the lemna 'optical power meter' (a luxmeter is a type of optical power meter):

wikipedia wrote:

Optical Power Meter calibration and accuracy is a contentious issue. The accuracy of most primary reference standards (e.g. Weight, Time, Length, Volt etc.) is known to a high accuracy, typically of the order of 1 part in a billion. However the optical power standards maintained by NIST, are only defined to about one part in a thousand. By the time this accuracy has been further degraded through successive links, instrument calibration accuracy is usually only a few %. The most accurate field optical power meters claim 1% calibration accuracy. Comparatively, this is orders of magnitude less accurate than a typical electrical voltmeter.

 

Further, the in-use accuracy achieved is usually significantly lower than the claimed calibration accuracy, by the time additional factors are taken into account. In typical field applications, factors may include: ambient temperature, optical connector type, wavelength variations, linearity variations, beam geometry variations, detector saturation.

 

Therefore, achieving a good level of practical instrument accuracy and linearity is something that requires considerable design skill, and care in manufacturing.

For professional use there is the (german originating) DIN norm for luxmeters: DIN 5032 part7. (I am not aware of any other standards for luxmeters, does anyone know if there is an ANSI-standard as well?) According to this DIN 5032-7 there are three classes of luxmeters, classA, classB, and classC. From an indirect source I found a couple of the criteria (for direct insight into the DIN texts I had to pay dear money):

 

criteriumnamemaximum deviation (%) conform DIN 5032-7
  class Aclass Bclass C
V (lambda) correctionf1369
UV-sensitivityu124
IR-sensitivityr124
Cosinus-errorf21.536
liniarity errorf3125
temperature coefficient 0.2%/K1%/K2%/K
frequency dependance of light fluctuationf70.20.51
maximum measuring errorft51020

 

I understand most of this, but not all. What I found pretty striking is that for a class C luxmeter (which goes for over $300 minimum!) the maximum allowed measurement error is 20% ! And all the above mentioned errors will not be corrected by calibration of the luxmeter!

Our chinese luxmeters do not even come with a classification, but that does not neccesarily mean that none of the above criteria are met of course. This all made me wonder how wrong these cheap luxmeters actually are? Are they very bad, or are they quite ok?

 

I recently bought secondhand a top of the range (in the handheld category) luxmeter of which the sensor meets the DIN 5032 part 7 class A criteria, the rests of the meter meets class B. It is a Mobilux A luxmeter. The manufacturer of the sensor is Czibula&Grundmann, a small optical instruments company from Berlin, the sensor is connected to a Mavolux 5032B class B meter made by Gossen.

The overall accuracy of this luxmeter is guaranteed within 5%, so that aspect meets the class A criteria. The meter was officially calibrated in 2009, the reading might be off after 5 years although I do not expect that to be much. (I don't know why I don't expect that, at least the former owner told me that the meter was hardly used, for what that matters Undecided).

The calibration being off or not, at least this meter gave me the opportunity to do a few comparisons with the two chinese luxmeters I had already: a Tondaj LX-1010B , and the Ceto CT1330B. The last one is permanently build into my integrating sphere, and lost its original calibration, so it was only partly used in this comparison.

What I am able to do in my home and in my budget is very limited of course, and light measuring professionals probably can't suppress a mild smile when they read how I do the tests, but thanks to my distant past one thing I could do was have a look at the wavelength response of the luxmeters. This error is one of the biggest errors allowed in the DIN 5032-7 criteria, so apparently this criterium is a difficult one to meet. The wavelength response error is caused by the optical filter in front of the sensor, it is supposed to correct the sensor's wavelength sensitivity for the official V(lamba) curve. 

Meet a piece of history that I inherited from a job long ago:

It is the lamp house and prism-monochromator from a Zeiss Spectrofotometer, which was the standard spectrofotometer for three full decades from the early fifties to well in the seventies. Eventually it as replaced by easier machines, but certainly not more accurate!

However much I would have liked to measure the absolute V(lambda) curve for the different luxmeters with it, this thing will not give that because the optical power is not defined for each wavelength (that was not relevant for the original use of the spectrofotometer): the light source is a 2700K tungsten lamp that emits a spectrum that increases almost 8 times in optical power going from 400 to 800nm. Although the Tungsten lamp curve is simple, more or less a straight line, I do not dare to correct the measured graph for that spectrum because I also do not know exactly what happens inside the monochromator, are there light losses that are wavelength dependant?, and also my guess is that the used slit in the monochromator will let a wider wavelength range through at longer wavelengths than at shorter wavelengths (I do not know enough about this to know this for sure, and I have no documentation with the monochromator).

This type of monochromator does not filter out a single wavelength, but a wavelength range, this range can be adjusted by altering a slit opening. A wide slit gives more light and a wider wavelength range, a narrower slit gives less light and a narrower wavelength range. I used a slit of 1mm because that let through a minimum amount of light that still was detected well by the luxmeters. I have no absolute idea (could not find it on the internet) what the wavelength range is that is coupled to a 1mm slit size, and that range may very well vary with wavelength. So you have to bare that in mind when reading the graph below.

The luxmeter testing with the monochromator were unavoidably at the low end of these luxmeter's measuring range. For the responses to be correct, this assumes that the luxmeters are lineair over the complete light measuring range, also at the low end, and at all wavelengths. I tried to check that using another oldie, my Leitz Focomat 3C photo enlarger. I recorded the readings of the Mobilux and Tondaj meter for the different diaphragm stops, varying the enlargement (which varies the illuminance range) and applying a few colour filters (red, green and blue). The maximum readings were still a bit on the low end of the range of the luxmeters but I did not detect any significant a-lineairities (data not shown Tongue Out)

So what I do is show the optical-power-uncorrected graph, this does not give the absolute V(lambda) values, but does show the relative differences between the wavelength response of the three meters. BTW, the three luxmeters gave values in a different range because the the detector areas were of very different size, while the projected lightspot was only small (it just filled the Mobilux detector area). (for the Tondaj and Ceto meters not the whole detector area was filled by the spot, so the amount of light on the limited area is interpreted by the meter as coming from the total detector area causing a significant lower reading).

 

So I thought it was handy to correct the three curves so that the maxima were the same: the relative readings between the luxmeters were not relevant anyway because of the different detection areas, and this compares the curves much better. Aligning the maxima is perhaps not the best way to make the three curves comparable (equalling the surface area under the curves would be a bit better I think), but whatever is the most fair way to compare them, aligning the maxima was easy and gives a good idea of the differences.

So what, after all the restrictions caused by the method, is leftover to conclude from this graph? For the following observations I assume that the wavelength response of the Mobilux meter is a good representation of the official V(lambda) curve. This will of course not be entirely true but it is the best I have.

1) the MobiluxA maximum sensitivity is not at 555nm, as it officially should be, but at 545nm. Even if one would correct for the non-flat spectral output curve of the tungsten lamp, that maximum will not shift as far as I can see. So either my monochromator is a bit off, or the classA optical filter of the Mobilux is off (uncertain is my middle name Smile ) I may check the monochromator with a green laser some day. EDIT: I checked the monochromator and it reads 14nm low (see post #5, and I remeasured it with the 532nm green laser). So the three curves will shift 14nm to the right. I will update the graph at a later point. The consequence is that the Mobilux meter's sensitivity peaks out at about 555nm, as it should be. EDIT2: I updated the graph with the nm values shifted 14nm. In the blue and red region it will still be a few (~3) nm off because the monochromator nm-scale is slightly logaritmic and the shift is a constant value. I think such a small deviation is acceptable.

2) the Tondaj meter's sensitivity is seriously off (much too high) in the blue and red region, so much that I expect this to see reflected in measuring cool white and neutral/warm white leds. Especially the blue error is serious because of the extreme blue peak that the cool white Cree leds have at 450nm.

3)the Ceto meter is very ok in the blue region but off in the red region. For measuring cool white leds I think it does better than the Tondaj.

4) measuring colour leds with the Tondaj, as I did before for BLF, (especially red and blue) gives large errors in luxreadings, e.g. measuring a blue led (450nm) with the Tondaj gives a reading that is at least 4 times too high. I am glad that when posting my colour led test results in the past, I always mentioned the uncertainty of the absolute lux-values Undecided .

 

To be continued...

This is it for now, all quite theoretical sofar, but the real question is of course: do these differences matter in 'real life' Wink flashlight hobby use?. When I have time I will  compare the Tondaj meter and the MobiluxA meter by measuring throw of a few flashlights with different colour temperature leds, to see how the measurement errors of the cheap Tondaj luxmeter is affected by the colour temperature of the led. I will update this post when that is done.

At some point I will be curious about the wavelength response performance of the Extech luxmeter that some people at BLF and CPF use for their measurements, that meter is a class more expensive than the 20 dollar chinese meters but still a bit affordable. But I don't have one, so that has to wait.

thanks for reading!

 

 

Update Sept. 6  More familiar data (for a flashoholic) from now on: I measured the throw at 5meter of a bunch of flashlights that I have, not so much for the actual throw numbers but to see how the difference is between the Tondaj LX-1010B and the MobiluxA luxmeters over a wide range of colour temperatures and CRI.

To do that I attached the two sensors right next to each other on a cupboard, measured exactly 5 meters from the surface of the sensors with a tape measure, and clamped the flashlights in a laboratory stand. I blocked light reflected via the floor with a black coat halfway. Other indoor reflections were negligable. I noted the maximum lux values in the hotspot of the flashlight beams, I wiggled the flashlight around a bit to find the maximum reading for each luxmeter. I found it quite an effort to find the maximum values I must say: hotspots from flashlights with a smooth reflector have a significant variance in luxvalues, flashlights with an OP-reflector gave values with less variation over the (center of the) hotspot. My guess is that a 1% measuring error could have occurred due to this difficulty (but not larger).

So here are the results, for every flashlight the host is mentioned (although that is not very relevant for these measurements), the led type, lux@5meter according to the Tondaj meter, lux@5meter according to the MobiluxA meter, the percentage that the Tondaj was off compared to the Mobilux reading. The order is the order that they were measured.

 

HostLedTondaj LX-1010B readingMobilux A readingTondaj, percentage off
small Ultrafire 602H 16340 hostXP-E2 green, 535 nm6972-4%
small 16340 hostXP-E2 blue, 470 nm13636+277%
small 14500 zoomieXP-E2 red, 625 nm4436+22%
small 16340 hostPhilips Luxeon Z cyan, 505 nm13887+59%
Sunwayman D40A, stockXM-L2 'cool white'880837+5%
Maratac AA copperOsram Oslon Square 3500K 80CRI7678-2.5%
Xeno 03XP-G2 3C140143-2%
EagleTac D25C Ti 2013XM-L2 6A1 80CRI6972-4%
Olight S20 Baton, stockXM-L2 'cool white" (more towards neutral, bit green  Sick    )140143-2%
Fandyfire A10B (constant output reference light)XM-L2 4C119124-4%
Convoy S5, tripleNichia 219B 5000K 92CRI131128+2.5%
Convoy S5, tripleNichia 219A 4500K 92CRI9490+4.5%
small 14500 zoomieXP-E2 7A, dedomed650700-7%
Jinheng 12AXM-L 3000K476511-7%
Ultrafire LZZ F15XM-L2 5D2 80CRI595601-1%
No-brand 26650 12dollar dive light, but with magnetic switch Coolangry cool white XM-L (7000K?)700600+16%
Ultrafire K10 (constant output reference light)XM-L2 0D4240+5%
Uniquefire UF-T20XP-G2 3C, dedomed41004330-5.5%
Convoy M2 (constant output reference light)XM-L2 5A1 80CRI243248-2%

 

So here's reflected in practice what the graph already showed. Although I would have liked that, the single colour results do not follow the graph literally unfortunately (there are apparently some inaccuracies in my method, who would have thought that Wink ) Let's discuss the table:

*First, if you compare the Tondaj and Mobilux meters using a wide range of neutral white leds, they agree within a few percent. This tells me two things: 1) the calibration of the Tondaj (luck or not) is not bad at all, 2) when measuring neutral white leds, you get away with a significant deviation from the V(lambda) curve.

*The single colour leds as expected show the worst problems: although green reads almost the the same between the two luxmeters, blue reads 4 times too high (!!!), cyan 1.6 times, red 1.2 times. That is pretty bad. I will update my XP-E2 colour and XM-L colour tests with these new insights (at the time I already warned for big errors in the actual lux-values), those tests were done with the Tondaj meter Undecided (but mind that only the absolute values of those tests are affected by this, things like curent at maximum output, and output curve shape are not affected).

*Going cooler than neutral white generally the Tondaj reads too high, as much as 16% for the typical cool white XM-L found in cheap flashlights. Going warmer than neutral white the Tondaj reads too low, but I did not find more than 7% too low for very warm white leds.

*The 92CRI Nichia's behave different from the other neutral white leds: they read too high on the Tondaj meter. I guess this has to do with the relative large amount of blue and far red that they have compared to the 'green' region of the spectrum.

 

Conclusions.

For a general idea of the output/throw of most common white led flashlights, the Tondaj LX-1010B luxmeter is adequate: if the calibration is good, the reading is for most leds within 10% of the reading of a very good luxmeter that performs according to the best standards.

I assume (also because I measured the wavelength response of a third luxmeter, the Ceto CT1330B, which was different but did not seem to behave worse than the Tondaj) that the various other cheap chinese luxmeters that are commonly used, -with variations- perform similar to my Tondaj meter, so I carefully expand the above conclusion to those meters as well. If I can lay my hands on another one or two different ones, I can be more sure about that.

For measuring the true lux-values of colour leds with any accuracy, these chinese meters are completely crap. They can be used effectively, but only for relative measurements within light of the same wavelength. Even the Mobilux meter probably will not behave to specs when measuring colour leds.

If you calibrate a cheap chinese meter (or have it calibrated), the calibration only counts for leds very similar in colour temperature (and to lesser extend: CRI) to the light source that the calibration was done with. If not, despite the calibration the reading can be pretty much off.

I am planning to send around a constant output flashlight that I have made, to compare 'djozz-lumens' to what other people measure (it has a 4C tinted XM-L2 by the way), but with this knowledge of the colour temperature variance in the luxreadings of cheap meters, the usefulness of this is limited of course (but it is still a good idea I think, it will reduce some of the variation among us all)

I had some testing fun again Smile

 

Further tests?

I am curious about the wavelength response of some mid-range luxmeters, will those curves be mid-range as well?? As mentioned above, I am especially curious about the Extech meter that is used by more than a few flashlight 'guru's'. But to be honest, I don't have one of those and I have enough other little flashlight projects ahead of me to keep me busy in my (scarse) spare time.

Thanks for reading again!

Edited by: djozz on 09/05/2014 - 21:05
LSX
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So for someone who has never used a luxmeter before. What would people recommend I try up to say $100?

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LSX wrote:
So for someone who has never used a luxmeter before. What would people recommend I try up to say $100?

:bigsmile:  this is almost real science: we have very interesting results but hardly a clue how this translates to real life use. 

Wait for the flashlight throw number comparison for an answer to your question Smile

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djozz wrote:

LSX wrote:
So for someone who has never used a luxmeter before. What would people recommend I try up to say $100?

:bigsmile:  this is almost real science: we have very interesting results but hardly a clue how this translates to real life use. 

Wait for the flashlight throw number comparison for an answer to your question Smile

Sounds great, I look forward to your results then. I just took a look at those Extech ones, there was one for about $145 which didn’t look too bad. I wonder how they compare…

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LSX wrote:
djozz wrote:

LSX wrote:
So for someone who has never used a luxmeter before. What would people recommend I try up to say $100?

:bigsmile:  this is almost real science: we have very interesting results but hardly a clue how this translates to real life use. 

Wait for the flashlight throw number comparison for an answer to your question Smile

Sounds great, I look forward to your results then. I just took a look at those Extech ones, there was one for about $145 which didn’t look too bad. I wonder how they compare…

I owned an extech meter, I bought it online from a tool pawn (for $100 BNIB) and had it shipped directly to the factory for certified calibration (another $99). I got it about a month after getting my HS2010 and was pretty under impressed with its performance, my aunt (professional photographer) offered me my money back for it and I jumped all over that cause I didn’t really notice any more accuracy then my china one provides. Note for $145 new the meter will NOT be certified, buying new or used doesn’t matter, that’s the same $99 (plus shipping both ways) if you want that.

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I could not help checking the monochromator with a green and a red laser. The green one reads 519nm (=in reality 532nm), the red one reads 640nm (=in reality 650nm). so the monochromator reads indeed about 10nm too low. This will affect the graphs in that they will shift 10nm to the right. Relative to each other nothing happens of course. It does mean that the sensitivity peak of the Mobilux meter is at the correct wavelength: 555nm.

I will update the graph.

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Cereal_killer wrote:
I owned an extech meter, I bought it online from a tool pawn (for $100 BNIB) and had it shipped directly to the factory for certified calibration (another $99). I got it about a month after getting my HS2010 and was pretty under impressed with its performance, my aunt (professional photographer) offered me my money back for it and I jumped all over that cause I didn’t really notice any more accuracy then my china one provides. Note for $145 new the meter will NOT be certified, buying new or used doesn’t matter, that’s the same $99 (plus shipping both ways) if you want that.

Thanks for the info! So do you still use the HS2010 now?

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I thought that the luxmeter test measurements in the OP that I wrote last week were quite revealing, but there's not much response in this thread. I guess that the subject is a bit specific, and that most forum members like reading about practical stuff more than about theoretical stuff Smile

So I updated the OP with more practical stuff: a comparison between the cheap and very good luxmeter using throw measurements of many of my lights. It shows in practice what the graph already more or less predicted: luxreadings of my cheap chinese meter vary with colour/colour-temperature of the led.

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really appreciate the effort. Seems that we have a fair to middlin chance of the Extech being within 30% or better. Silly Depending on the light, the day, the time of day, and whether anyone in the room has had a bowel movement lately. Good to know we’re close anyway. Wink

I think the idea of sending a “calibration light” around would be an excellent idea. Even if 3 or 4 folks got it then it’d give a good sampling of what is going on with our numbers and perhaps allow us to sleep at night.

The quest, part and parcel of the flashaholic creed.

Thanks again for your time and effort, and indeed monies spent! Great work and greatly appreciated.

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DBCstm wrote:
really appreciate the effort. Seems that we have a fair to middlin chance of the Extech being within 30% or better. Silly Depending on the light, the day, the time of day, and whether anyone in the room has had a bowel movement lately. Good to know we're close anyway. Wink
He's talking about Light meters, not Gas sniffers.Tongue Out

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djozz wrote:

LSX wrote:
So for someone who has never used a luxmeter before. What would people recommend I try up to say $100?

:bigsmile:  this is almost real science: we have very interesting results but hardly a clue how this translates to real life use. 

Wait for the flashlight throw number comparison for an answer to your question Smile

So I did the throw measurents, and to answer your question: IMO if you would like to have a clue about light output a bit more objective than your own eyes (eyes are horrible at that) you can buy one of those 20-30 dollar luxmeters, do a rough calibration of the output with one or two flashlights that come with ANSI-standard measured specs, and be done with it. You will be within 20 or so percent (perhaps better if you are lucky) which is way better and consistent than your eyes do that.

If you want to be more accurate than that: trouble starts! And big spending.

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Thank you for your efforts. I guess light is a hard impossible thing to measure accurately. Here’s a thought: Everything else that we measure comes through a conduit, and can be measured accurately because there is no spill whatsoever. Light can’t be contained so adequately. It’s like we’re trying to measure water volume and pressure from 5ft past the end of a garden hose!

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Nice work djozz, once again!

I suppose we are assuming the Class A/B Mobilux A meter is pretty much the gold standard. It's revealing and somewhat disconcerting to see the variations, and also disconcerting to hear that maybe the NIST calibrated Extech meter in the $100-$500 price range so praised about in our community, doesn't even meet a class C rating of errors up to 20%. This could explain why some have seen very little difference in the LX1330B and an Extech.

By comparing the meters side by side as you did, eliminates a lot of variables - temp, pressure, variations in battery effects over multiple trials, etc., so the results seem very real and should be reproducible. Of course I would like to see more common lights/LED's in the test, but obviously you have limitations.

Don't feel the data, testing and info you provided here is under appreciated from the lack of much response. It's going deeper than many want to go. I subscribed to this thread from the first day but held off posting.

Also, all those discussions going on about universal standardizing our meter calibrations seems out the window now, or radically changes it - we'd have to take tint/emitters into account in calibrating our readings, and that's not always well known.

Personally, I was looking at a data logging model of an Extech to buy, but now, I'm not sure it's worth it, accept for the better features, the precision may not be much improvement at all.

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I read the post on Monday but was too busy scratching my head to post replies Silly .I’m sure I’m not the only one.

After a second reading today,comprehension is dawning.I’ve got a HS1010A from Gameboy sailor which,I presume,

is the same as the Tondaj(I can send it to you for “calibration“if you like Wink )

Would it be feasable to add weak red and blue filters in front of the Tondaj to bring it in line with the lambda curve or

doesn’t it work like that.

Spotted this thread earlier this week,might be worth a $10 experiment
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Thanks Jos for the tests!

That must have taken up a lot of your free time...

My conclusion from all those tests is..... what the heck, I will just keep using my cheapo meter and be happy with it.
Seems like you just never know if the results are100% correct.. and in most cases its only a small % difference.

There are many variables... and even the more expensive meter could over/under measure in those different circumstances, who knows which one was actually more correct?

It`s great to have a reference though!

I`m happy with the lux meter I have, as I mostly just use it to compare numbers between the lights I have, and it`s good to be able to see if there is actually an output drop....

When I measure the "throw" and it`s in the same ballpark as the specs tell... I`m confident the meter did its job Wink

but that kind of rules out when you start to mod that light Sad

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Thanks for your effort Djozz.. Thought provoking!

A question……I use a Ceto CT1330B and it appears to have been quite reasonable in findings until now. Its results have been about average for a range of lights tested on BLF eg A60, STL-V2 etc. But in testing some lights recently it maintains the same readings as in the past for lights in the 100+k lux range (eg Maxtoch 2X), but strangely now varies 18-20% low in the 30-80k lux range.

For example – Two 2Xs just tested both read 112kcd at 6m, fairly consistent with Maxtoch’s factory read of 120k at 10m. I tested an example of the same model light 3-4 months ago and it also read exactly 112kcd at 6m. That’s consistency! But in testing some new Maxtoch M24s they dropped from 83k lux on my meter 3-4 months ago (Maxtoch’s recent factory test 83.5) to just 65-72kcd now on my meter. Odd. I then retested my Jacob A60 which has always read about 52-53kcd, only to find it now reads 43kcd – all tests with fresh pana 3100s. Cleaned A60s threads/contacts and no difference – 18% loss in metered lux reading.

Question – can a light meter go bad in part of its reading curve – lose 18-20% in part of curve but remain ok in another? Seems to be the case. Batt in meter fairly new.

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mudgripz wrote:
Thanks for your effort Djozz.. Thought provoking! A question......I use a Ceto CT1330B and it appears to have been quite reasonable in findings until now. Its results have been about average for a range of lights tested on BLF eg A60, STL-V2 etc. But in testing some lights recently it maintains the same readings as in the past for lights in the 100+k lux range (eg Maxtoch 2X), but strangely now varies 18-20% low in the 30-80k lux range. For example - Two 2Xs just tested both read 112kcd at 6m, fairly consistent with Maxtoch's factory read of 120k at 10m. I tested an example of the same model light 3-4 months ago and it also read exactly 112kcd at 6m. That's consistency! But in testing some new Maxtoch M24s they dropped from 83k lux on my meter 3-4 months ago (Maxtoch's recent factory test 83.5) to just 65-72kcd now on my meter. Odd. I then retested my Jacob A60 which has always read about 52-53kcd, only to find it now reads 43kcd - all tests with fresh pana 3100s. Cleaned A60s threads/contacts and no difference - 18% loss in metered lux reading. Question - can a light meter go bad in part of its reading curve - lose 18-20% in part of curve but remain ok in another? Seems to be the case. Batt in meter fairly new.

Oh my, that is exactly the kind of headaches I get when measuring light, and why I started the process of getting more grip on light measuring (which caused more headaches).

But, assuming a consistent luxreading set-up, it sounds more like a flashlight thing than a luxmeter thing. My experience with cheap luxmeters (and your Ceto, which I use as well, in my integrating sphere) is that with all their flaws of which I tried to adress one in the OP, at least they are very consistent over time: if I take a reading and do the same a month later with the same flashlight, it reads exactly the same, and for most of the drainage independant of battery level. I have no idea what is going on electronically in those things but in the sensor's point of view I can not think of a reason why it would be off just in a certain range.

EDIT: I did not test it (yet), but luxmeters apparently can have a very bad temperature consistency. As can be read in the first table of the OP, a class C luxmeter is allowed to have an error of 2%/K, and who knows about our chinese meters? I guess that 2%/K means that if the ambiant temperature is 10 degrees (Kelvin/Celsius) different (which can easily happen when measuring outside), the reading can be 20% off Surprised (my measurements were always indoors, with temperature not varying more than 2 degC, but still 4% error is big!). It may be not that bad, but we just don't know until it is tested: higher end meters come with spec.sheets with the characteristics, the chinese are not telling anything.

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Interesting comparison djozz. Sorry for replying so late. Im more interested in the practical stuff, but I appreciate the effort on the theoretical part as well.  Beer

Ill share some thoughts/questions regarding the typical cheap budget meters (may not be wise to assume they are all fairly similar). What you think about them?

-Trying to find out if certain emitters are the bin they state are hard. Example. XM-L2 U2 1A vs XM-L2 U2 2C If you compare several of each emitter, and find out that 2C does read a bit lower, it does not necessarily have to not be U2 bin. It may just be the (cheap) lux meter showing lower number due to the wavelength. 

-When comparing domed and de-domed emitters, the loss may not be as large as measured by typical cheap lux meters. As an example, my latest de-dome job (3x XM-L2) showed 10% loss, but it may have been less if measured with a better lux meter. (Im mostly using my LX-1010B, which I assume is quite similar or the same sensor as your LX-1010B)

-Your measurements on de-domed XP-L showed basically zero loss right? Could this be due to having less of a "strange change" in tint compared to XM-L2? Im not much familiar with de-doming those emitters yet, so feel free to fill me in. Just wondering if some of the before/after changes that we expect when de-doming are more exaggerated due to cheap lux meters.

-Difference when measuring the color emitters were quite disturbing.

-Getting people`s calibration closer to each other others are IMO easily doable if people (who buys a lot of lights) are willing to put in a little bit of effort and calibrate their numbers a bit closer to various other peoples calibrations.

Sending out "calibration lights" would require several lights with different beams and tints in order to better get some accuracy. For some it may not be relevant to use 4C as a calibration reference, since some people mostly stick to emitters in the 4800-7000K range. On top of that, if there is only a single light, and that light is measured with a lux meter with an "old calibration" that may or many not be that accurate, what would be the purpose then? We know different meters are likely to have different readings with different tints. And we are not sure that a meter that was calibrated a long time ago me be perfectly accurate today. Basically, the calibration may not become more accurate, and you would have to get "everybody" in on the same calibration. And that is the main issue, and not very likely to happen.  Also, IMO if some project with sending out calibration lights at some point did happen, they would also have to be reference lights for lumen in order to be worth the effort and to attract more people.

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Maybe it’d be easier to pass around one meter, everyone takes readings with the same device we’d all be on the same page. In theory.

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RaceR, you are right that it is debatable if a 4C tinted light would be the best light to send out, it depends on what you want to check. It is a tint that measured close between the Mobilux and the Tondaj, being less sensitive for errors in the blue region it may be a good tint for calibration comparisons. To reveal wavelength response errors a cooler led could be more useful. I am thinking now of sending two lights, one is the 4C-tint light and another could be 1C or so. But I may be building a new integrating sphere this fall/winter, one that incorporates the Mobilux meter and is a bit bigger so that calibration of the conversion factor is less important with every light measured. Then measure the lumen and throw of the two lights before sending them. The relative differences between the two lights should be correct then.

The XP-L was tested in my integrating sphere, so that was not the Tondaj meter but the Ceto meter. The Ceto corresponds better with the Mobilux in the blue region, so I would expect the effect of a tint-shift on the luxreading smaller than with the Tondaj meter.

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The colour filters of the Tondaj and Ceto meter are clearly different, these pictures (that I posted before) show left the Tondaj sensor, right the Ceto. The colours of the filters show quite correct on the photo, the tondaj green/blue-ish, the Ceto very green:

 

(I am not going to open up the Mobilux sensor!)

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Sorry for not reading it right sooner. I skimmed it earlier but had nothing to add of value. Not sure I do now. The inherent issues of measuring lux or even lumens are daunting to say the least. 

@ mudgrips: Try changing the battery. My meter underreads when it has anything but a fresh-ish battery.

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[quote=djozz]

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Yep will do ledsmoke. Bought a new battery for the meter yesterday to try as its the only variable I can really check.

Will test it tonight.

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Well … did retest last night on 2 lights with new battery in lux meter – but no difference in readings.

Meter remains accurate over 100kcd but still reads 18-20% low in that 30-80kcd range – have no idea why.

Dam! – meter must be b*ggered!

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djozz wrote:

 I am thinking now of sending two lights, one is the 4C-tint light and another could be 1C or so. But I may be building a new integrating sphere this fall/winter, one that incorporates the Mobilux meter and is a bit bigger so that calibration of the conversion factor is less important with every light measured. Then measure the lumen and throw of the two lights before sending them. The relative differences between the two lights should be correct then.


How will make make sure that your lumen calibration is more precise compared to others?

How will you make as many people as possible join in on using your lights/numbers as reference?

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RaceR86 wrote:

djozz wrote:

 I am thinking now of sending two lights, one is the 4C-tint light and another could be 1C or so. But I may be building a new integrating sphere this fall/winter, one that incorporates the Mobilux meter and is a bit bigger so that calibration of the conversion factor is less important with every light measured. Then measure the lumen and throw of the two lights before sending them. The relative differences between the two lights should be correct then.


How will make make sure that your lumen calibration is more precise compared to others?

How will you make as many people as possible join in on using your lights/numbers as reference?

The goal is not to dictate my calibration to other people, I have no calibrated light source either (I call my calibration 'djozzlumens' for a reason), but to get a pretty exact idea what differences there are, so that you can perhaps define a standard 'BLF-lumen' Wink . I must admit that the incorrect wavelength sensitivities of probably many luxmeters very much complicates the idea of a standard calibration.

I just hoped that people find it a neat idea and want to participate. The costs are no more than sending a small package to the next in line. 

BTW, my lumen-numbers are uncertain, but I think that my lux-numbers, coming from this very good once calibrated meter, are indeed more precise than most other lux-numbers around.

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Maybe you could talk with say 2-3 meter suppliers as we do with torch manufacturers, ask for some sample product, and test those particular lux meters for best accuracy. Then its a matter or referring blf people to those meters/suppliers. Could be basis for group buy and would also standardise readings for alot of us who use meters.

Would be matter of thoroughly testing 2-3 meters rather than sending ‘calibrated’ torches around alot of people. I’d certainly buy a tested and recommended meter – especially if it has been referenced against good expensive meter equipment. You might end up with two recommended meters in different price categories eg $0-50, and $50-150?

I’ve got to buy a new lux meter and would be keen to buy a make/model that someone has carefully calibration tested. Could help bring alot of blf readings/reviews into line.

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Hoping this is an all right place to say this.....

It seems that in order to get a good idea of what a flashlight will do in terms of light output (quantity vs beam pattern) a better way to show this would be to use a 3D graph such as like the following describes:

This kind of graph showing beam spread and the colours representing intensities as from a flashlight shining upwards the highest portion being an arbitrary fixed distance from the emitter

 

Example 1

added to this graph showing the beam intensities as on a flat wall for example at a certain distance

Graph B

resulting in something along the lines of this graph (but other side up and much better and clearer)

Graph C

These things can be plotted by any amateur physics hobbyist who can do simple trigonometry and calculus. Additionally specifying the light in the area under the cone would result in the quantity of light within the beam at a specified angle and distance.

This is just a thought as the light meters mentioned above are being used to obtain a smaller portion of what is mentioned here. If we can create these kinds of graphs then only the emitter colour temperature measurement will be left to solve.

Any ideas? If there is a better place to post this please advise. This was posted as a thought having read about the light meters above and additional thoughts were solicited.

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Photons Away wrote:

Hoping this is an all right place to say this.....

It seems that in order to get a good idea of what a flashlight will do in terms of light output (quantity vs beam pattern) a better way to show this would be to use a 3D graph such as like the following describes:

This kind of graph showing beam spread and the colours representing intensities as from a flashlight shining upwards the highest portion being an arbitrary fixed distance from the emitter

 

Example 1

added to this graph showing the beam intensities as on a flat wall for example at a certain distance

Graph B

resulting in something along the lines of this graph (but other side up and much better and clearer)

Graph C

These things can be plotted by any amateur physics hobbyist who can do simple trigonometry and calculus. Additionally specifying the light in the area under the cone would result in the quantity of light within the beam at a specified angle and distance.

This is just a thought as the light meters mentioned above are being used to obtain a smaller portion of what is mentioned here. If we can create these kinds of graphs then only the emitter colour temperature measurement will be left to solve.

Any ideas? If there is a better place to post this please advise. This was posted as a thought having read about the light meters above and additional thoughts were solicited.

There's not many rules at BLF, usually you can post things where you like and this thread is quite appropriate I think.

I'm not sure exactly what you mean but it looks an awful lot like the first flashlight test that I ever did a few years ago and that I posted on CPF   :

http://www.candlepowerforums.com/vb/showthread.php?326918-DImyself-measu...

I did use the geometry of the flashlight beam but did not do any computer work, just simple calculations, to measure the lumen-output of a light directly with a luxmeter. I now think it was a bit of luck but the actual output of that light as I later measured was within 10% of that measurement Undecided

That post was also a disappointment about CPF, the test was a bit of work, I had not read of anyone else doing it that way on CPF, but it attracted only one reply. It was a kind compliment from HKJ by the way Smile

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Because it was a quite easy test, I did a rough test on temperature effects on the reading of the Tondaj and Mobilux luxmeters. Method: I put the sensors (not the body) in the fridge, next to each other together with a thermometer. After 3.5 hours (the thermometer read 12degC) I took them out, attached them to a cupboard and measured the lux of the hotspot of my k10 constant output reference light at an arbritrary distance (1.2 meter or so) clamped in a stand. Then I took the readings of the meters the next 40 minutes with the sensors warming up to -hopefully- room temperature (in this case 24degC).

time after taking sensors out of fridge

Tondaj readingMobilux reading
0201201
30s201201
1minute201201
2200201
3200200
5199200
7199199
10199199
15199199
20199199
25199199
30200199
35201201
408585

Now that was a lot less exciting than I expected. Both meters did not care less about the temperature difference. I can think of two reasons:

1)The reading of both luxmeters do not suffer significant from a 12degC temperature difference. This is good news!

2)Or, the actual sensor of the Tondaj and/or Mobilux luxmeter is so well thermally insulated from the surroundings that they were not enough cooled down in the fridge/warmed up during the measurements to show the errors that would arise from a temperature difference. This scenario is also good news, it means that if you start a measuring session with the luxmeters at -say- room temp, it does not matter much if the temperature fluctuates, the insulated sensors will not notice that.

Conclusion: Warned by the DIN-norm I expected a significant worse temperature performance from the Tondaj compared to the Mobilux, but I do not see it in this test.

(And I never did a runtime test on my -two7135,XM-L2,0D- K10 before, 40 minutes runtime on an Efest 14500 of which 35 minutes flat regulated: you gotta love those 7135 chips for their constant performance Smile)

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Thanks for the continued testing djozz.

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