Lux meter

I am considering buying a lux meter but have no idea about such things.
I had a look at one and in the specs it mentioned it was calibrated to 2850deg K.
Are all Lux meters calibrated to this?
How likely is it that the measurements I make with this meter will match up with any other Lux meter?

question 1: nope.. some have photopic filters,
some don't,
some pretend to have photopic filters, but don't.
.. not 100% sure what it means by being calibrated to 2850K..
q2
matching up? I'd give at least a +/- 10% margin I think...

Just get the AEMC CA813. It has all the filters and consistant: Always:twothumbs

I have the cheap DX meter (The one that could read to 200K lux, people have reported that it read low by 15-20% vs. other higher quality meters.

But consistent it is. for relative bench marks of different lights, it's excellent.

Even with high quality meters, the consensus is that the read outs don't seem to match between brands.

There was a GOOD thread a few years back here on CPF on folk testing a variety of luxmeters with a set of 'standard' pass-around lights.

The key is that luxmeters (or lightmeters) respond differently with certain light sources. White LEDs in particular due to the phosphors and underlying LED source produce what appears to our eyes as white, but in reality have very large peaks/troughs at various frequencies and these will cause large discrepancies between readings from various brands of meters. Different white LEDs will also have different peaks/troughs so even with the same light meter, what it reports as 'brighter/dimmer' relative to another LED isn't necessarily accurate.

For relative measurements to adjust your optics etc, they will work 'ok', but one should never take the actual lumen reported as accurate.

Basically, any light meter that doesn't measure energy at many individual frequencies will never report accurately for light sources like white LEDs. Also an integrating sphere would be needed.

cheers,
george.

Are you referring to this thread?
https://www.candlepowerforums.com/threads/94232

Packhorse said:

I am considering buying a lux meter but have no idea about such things.
I had a look at one and in the specs it mentioned it was calibrated to 2850deg K.
Are all Lux meters calibrated to this?
How likely is it that the measurements I make with this meter will match up with any other Lux meter?

Click to expand...

This is an interesting subject! I downloaded the manual for bigchelis' AEMC CA813 light meter. The manual says "Accuracy for 2856K Light Source of ±3%" in the specs column. So a Google of 2856K gives this NIST link and this Wiki link. Apparently 2856° Kelvin represents the 1968 upgrade to the CIE standard "Illuminant A" or the common incandescent light bulb (non halogen). In 1931 the Illuminant A standard used to be 2848°K. Global warming?

The manual goes on to say it has an accuracy of 18% for "common light sources". There is a row in the specs chart called spectral response and says "CIE Photopic curve". There is a little graph on the corner of the page called CIE Photopic Curve with a double Y axis of Luminous Efficiency and Lumens per Watt as a ratio to Wavelength in nm on the X axis. So the meter's sensor has been normalized to the spectral sensitivity of the human eye within 18% accuracy. A reasonable inference since the lumen by definition has been normalized to the CIE Photopic Curve which represents the spectral sensitivity of human vision.

But as ma_sha1 pointed out, their real value in our price range is whether or not they give consistent results as comparators. Hmm, I wonder if that $28 DX lux meter has a set of dichroic filters in it? I've been meaning to pick one of those up. If you're just fine tuning the driver or reflector/optic of a light, it's not going to matter how accurate it is.

Michael also use the cheap DX meter for the DEFT & up adjust with a correction factor. That made me decide to get the DX metter, as I am benchmarking DEFT numbers for my Aspheric mods.
https://www.candlepowerforums.com/posts/3348264&postcount=12

saabluster said:

I have a cheap DX meter so have had to improvise. When I take 5 meter measurements of my Raidfire Spear and calculate back I get a little bit more than 16K lux. From everything I have seen here that is absurdly low and there are other reports of the DX meter reading excessively low. The lux reports I have seen seem to be in agreement that it puts out around 24K lux. I even modded mine with an R2 WH so it should be ever so slightly brighter. So my rather unscientific way of getting measurements is to use my Spear as a "calibrator". I set up the meter and then backed off until I got a reading of 22lux(about 26'). I did 22 instead of 24 as I wanted a little conservative budge. So I test all my lights at the exact distance I got 22 from the Spear. The first DEFT-HO of this run gave me a reading of 180 using this method.

Click to expand...

My understanding is that Michael moves his light closer until the Spear flashlight readout out goes from 16K lux to 22K lux, that's an increase of 37.5%. Unless my interpretation is in correct, I think that might be a bit over adjusting. From various other posts I read, most people compared DX meter to good meters reported 15% to 20% difference.

I am using the more modest adjustments, for example on my MagDragon Jr., I got 139,000 lux @ 1 meter real measurements (Measure 10 meter lux & convert to 1 meter), then I up adjust slightly & estimate my lux to be around ~150,000 (~8% up adjusted). I want to be conservative, so my numbers can back me up in case one day the MagDragon Jr. run into a DEFT FTP & duke it out
On my MagDEFT, I measured 100Klux & estimated real lux to be >115K lux, that was a 15% up adjustment.

Overall, I think get a DX cheap meter & do 10-15% up adjustment is a pretty good alternative vs. buying an expensive meter.

.
.

ma_sha1 said:

Are you referring to this thread?
https://www.candlepowerforums.com/threads/94232

Click to expand...

Yes, that's the thread. I'd HIGHLY recommend reading through that LOOONG thread since pretty well all the answers regarding accuracy, relative measurements etc etc have already been covered in excruciating detail

cheers,
george.

I ended up buying a Digitech Lux meter from Jaycar ( QM1587).
I have no idea how accurate or precise it is but I did a few measurements in a rather unscientific way.
Batteries were is various states of charge. This may effect some lights while not others depending on driver/ LED's.
Results are as follows.

W200 P7 2.8amp Kai driver 3 mode 1300,400,100
W200 MCE 2.8amp AMC driver 4800 lumen ( to my eye the P7 and MCE are equal in terms of brightness but the MCE is much warmer)
W200 XR-E with aspheric slightly out of focus 1050ma driver 10200
W200 aspheric XR-E 1000ma 10200
SST-90 @6a W300 13100
MC-E W300 @ 3.15a 11300
Ring of light 16x XP-G no optics @ 700ma per LED 1300
WF501 XR-E aspheric 2 mode (+strobe) amc 1050ma 14600 680

So interesting results that I dont think much can be determined from.

I guess this meter is probably good to compare changes in Lux of a given LED with different optics and current levels but maybe not so good for comparing one LED to another when they are of different tints.
An example of this is the P7 W200 vs the MCE W200. Also the Ring of light which matched the P7 in terms of measured lux but does not throw nearly as far.

Next I will try my 2 aspheric based SAR Spot lights.