I feel like my lux meter is reading very low

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-JOE-
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I feel like my lux meter is reading very low

I have been doing some extensive testing the past few days and my numbers don't seem right. I have the DX meter. http://dx.com/p/digital-lux-meter-with-stand-200000lux-5100

I use x100 (200000) range for my  1meter throw readings. I'm not sure if I multiple the numbers by 2 and then by 100? I'm really confused because my numbers seem way off.

Here are a few examples so far:

(1m throw)
BTU Shocker= 550
STL-V6= 410
HD2010= 380
ZY-T08= 290
C8= 240
UF-980L= 230
C108= 220

So can anyone tell me what the readings actually are/should be?

Edited by: -JOE- on 02/10/2013 - 19:09
djozz
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if you multiply the numbers by 100 (the x100 setting like you wrote) you are not that far off the throw numbers suggested by others, or do I read the post wrong??

rdrfronty
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Are you doing the readings at 1m or longer distances? Sounds like numbers for 10-15m range. But the are not consistant – so you have me stumped.
You always multiply this distance times itself and then towards your reading. For example at 15m you do – 15 × 15 = 225, then 225 × 550 (btu number) = 123,750. That would be about correct, though NW is likely lower. But then trying to do the hd2010 like that it reads too high, like 85k.
So you do have me stumped.

M3TAL_L0RD
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rdrfronty wrote:
Are you doing the readings at 1m or longer distances? Sounds like numbers for 10-15m range. But the are not consistant - so you have me stumped. You always multiply this distance times itself and then towards your reading. For example at 15m you do - 15 x 15 = 225, then 225 x 550 (btu number) = 123,750. That would be about correct, though NW is likely lower. But then trying to do the hd2010 like that it reads too high, like 85k. So you do have me stumped.

Looks like measured at 1m, so BTU doesn't manage to focus yet.

You need to just multiply by 100 if measured at 1m.

-JOE-
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All measurements are from 1 meter away. Measured from the meter to the front of the flashlight.

Do you usually do lux readings at 1meter or do you do them from further away and then divide to get 1m?

M3TAL_L0RD
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-JOE- wrote:

All measurements are from 1 meter away. Measured from the meter to the front of the flashlight.

Do you usually do lux readings at 1meter or do you do them from further away and then divide to get 1m?

You get more accurate readings from further distance. And you multiply it by distance x distance (e.g. at 5m you multiply by 25 (5 x 5 = 25)) and then you multiply it again by 100 (if that area was selected from lux meter)

-JOE-
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M3TAL_L0RD wrote:

You get more accurate readings from further distance. And you multiply it by distance x distance (e.g. at 5m you multiply by 25 (5 x 5 = 25)) and then you multiply it again by 100

So the number you get will be a 1m equivalent lux reading?

Everyone seems to post lux readings @1m so I just assumed you literally tested lights at 1m.
Tom E
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1 meter measurements are not that reliable, but the spec calls for quoting at 1 meter. I think NEMA says to measure at least 2 meters, but the longer the better - at 1 meter, you get beam focusing and accuracy issues (try moving the light 1 inch closer or further away, measurable difference). Good source info here.

-JOE-
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I defiantly think 1m is too close as well, I just wanted to get numbers that other people could compare with.

I'm still confused at to why I always see lux readings posted @1m. I also assume when people say that a light has 55k throw they are talking 1m too?

Tom E
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Well 55k is short for 55,000 candela. Here's the key info: "The lux reading is multiplied by the square of the distance in meters to get candela", it's in the wiki reference I included above. So candela is based on 1 meter. ANSI-NEMA came up with this to establish some sort of standard of measurement and comparison.

jmpaul320
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i usually take readings at 13 and 20 feet and then convert to 1m…

but just to give you an idea – my tn31mb still wasnt fully collimated at 20’…. michael (saab) said he takes his thrower measurements at 8meters (26’ 4”) and converts back

Would you mind keeping the wrong flashlight?
Best wish, May
Tmart service team

Soumil wrote:

-JOE-
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OK, thanks a lot guys. Hopefully next weekend I will get the time to retest. I will probably retest at 5meters.

-JOE-
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OK I retested the BTU quick at roughly 5 meters. I got 2,750 lux.

So that means 2750 x 5 x 5 = 68,750 lux @1m, right?

Pulsar
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what im getting is youd want to think of it like this

you take your distance, we will call it D. then take your readings, we will call that R.

youd want to do:
(D*D)*R=Lux@1m

what i got was 68,750 for what your readings was on that last test.

now i may be wrong, i dont have a meter and never used one.

-JOE-
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Pulsar wrote:
what im getting is youd want to think of it like this you take your distance, we will call it D. then take your readings, we will call that R. youd want to do: (D*D)*R=Lux@1m what i got was 68,750 for what your readings was on that last test. now i may be wrong, i dont have a meter and never used one. edit: seems you already corrected your numbers :p

Yup, I noticed I made a mistake.

I ended up doing a more in-depth test with the ZY-T08 this time because it is much easier to handle than the BTU. Here is what I got:

1 meter : 28000 x 1 x 1 = 28000

2 meters : 6750 x 2 x 2 = 27000

3 meters : 3250 x 3 x 3 = 29250

4 meters : 1870 x 4 x 4 = 29920

5 meters : 1160 x 5 x 5= 29000

6 meters : 790 x 6 x 6 = 28440

7 meters : 600 x 7 x 7 = 29400

Slewflash
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JOE wrote:

OK I retested the BTU quick at roughly 5 meters. I got 2,750 lux.

So that means 2750 × 5 × 5 = 68,750 lux @1m, right?

The calculations are right, but your numbers don’t seem to be right. Maybe if you moved the light further back still? I measure my lights at 13 meters and calculate back, and I got ~130k candela for my btu shocker.

Slewflash

Tecmo
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My Shocker does 126kcd. Something ain’t right. Does it look brighter from high to turbo? I think some batteries can’t handle turbo.

Electroman187
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Hmm, I just measured my unmodifed 3A ZY-T08 from 1m to 5m and consistently got ~50kcd.

Slewflash
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Electroman187 wrote:
Hmm, I just measured my unmodifed 3A ZY-T08 from 1m to 5m and consistently got ~50kcd.

A single emitter light would most likely have a much shorter distance before the beam is fully focused. The Shocker has 3 emitters, and as such it gets the roughly the same reading at 1 meter as my Crelant 7G5V2, however at 13 meters, the Shocker is about 3 times the throw.

Slewflash

rdrfronty
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Yeah, I’ve gotten readings anywhere from 126K-130k on my BTU depending on the distance. I have never actually tried it at 1m though. I never test my throwers that close.
The OP’s isn’t CW, so it is likely lacking some throw, but still should be well over 100k.

-JOE-
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I took my time on the ZY-T08 readings. My margin of error was not more than 2", that's for sure.

I was using Trustfire 3000mah batteries both at 4.19v.