How to verify Lux meter value?

Recently I bought a Lutron LX-101 Lux Meter from Taiwan.

Somehow I have a feeling that the value displayed may not be much accurate even though it is claimed to be calibrated.

How can I verify if the value displayed is close enough to the real value without using another lux meter?

Thanks in advance.

-- dj

First thing to check, does it read '0' in the dark?

Anyone knowwhat the light output of the Sun is? I would think that a noontime measurment of the noontime sun might be useful. Using the sun as a secondary standard might be possible.

Isn't there a formula for converting lux to camera
meter readings? It so, and you had a camera meter
you trusted, you could check your new meter.

Found it. See the "Photometry with a Photographic Light Meter" section.

Greg

You can use regular household light bulbs and a tape measure to check your Lux meter. On the bulb pack it should list the specs...like 600 Lumens on a 60 watt I'm looking at. Since one Lumen is one Lux measured at a distance of one meter, you would have 600 Lux at one meter away from the bulb. I think that's the conversion.

dj,

Can I ask you why you are thinking your meter is not accurate?

[ QUOTE ]
Roy said:
Anyone knowwhat the light output of the Sun is? I would think that a noontime measurment of the noontime sun might be useful. Using the sun as a secondary standard might be possible.

[/ QUOTE ]

It varies quite significantly, in fact, Forrest M. Mims III, has a project thats been going on for years to track this, and pollution levels in cities cause major differences in the numbers, as well as time of year, and haze developing in the northern hemisphere.

I think there were thousands of folks making the readings???

http://www.concord.org/haze/about.html
http://www.concord.org/haze/corner.html

rdshores, hmm are you sure one lumen = one lux at one meter?
Those 2 units should not have anything to do with each other.
You can have 2 lights, both with the same lumens, but one can mave much more lux, if its beam it tighter.

clicking on "Acronyms" and Craig tells me this:

lux = lm/m^2
1 lux = 1 cd @ 1 m
fc = lm/ft^2
1 fc = 1 cd @ 1 ft
1 cd = 1,000 mcd

/ubbthreads/images/graemlins/grin.gif
bernhard

[ QUOTE ]
rdshores said:
one Lumen is one Lux measured at a distance of one meter

[/ QUOTE ]

Lux is a measurement of brightness in the hotspot of a light source, and lumen is a measurement of total light output. For this reason, I think your statement is incorrect. There is no way to convert lux to lumens or lumens to lux. Sorry. /ubbthreads/images/graemlins/crazy.gif

They're related, but only holds if the light source is omnidirectional, radiating out in all directions, not for flashlights.

Yes, Illuminat.. and Wingerr, you are both correct.
So, that previous statement, might be correct as far as an omnidirectional light, but, none really are.

I have conversion programs, that will convert lux to lumens at meter^2 ... but, as said above, that would only apply to a totally omnidirectional light. And even then,,,, that would be a measure of the total amount of lumens hitting the sensor, not the TOTAL lumen output of the light.

I was assuming that if I hung a household bulb down from the ceiling that it would be omni-directional enough to point a lux meter at to get a ballpark reading. No mater where I would go in the room, I would get the same reading, at one meter away. I wasn't planning on launching any rockets. :O)

[ QUOTE ]
djpark said:
Recently I bought a Lutron LX-101 Lux Meter from Taiwan.

Somehow I have a feeling that the value displayed may not be much accurate even though it is claimed to be calibrated.

How can I verify if the value displayed is close enough to the real value without using another lux meter?

Thanks in advance.

-- dj

[/ QUOTE ]

I've heard that incandescent bulbs are fairly consistant given a specific voltage and amps. You should be able to find one that

According to Webster's dictionary, a footcandle is:
"a unit of illuminance on a surface that is everywhere one foot from a uniform point source of light of one candle and equal to one lumen per square foot."

So you should be able to do a ball park reading with 1 candle at 1 foot. A Lux is the same reading at 1 meter.

Another way would be to run a 10,000 mcd 5mm nichia at it's rated current at 1 foot to see if you get about 10 fc. Thsi should work with a 5,000mcd too.

Daniel

If you have 3-4 flashlights that have the lux specs listed on a testing site....compare your readings with the specs.
As has been said, luxeons will vary, that is why you need more than one light to compare with the specs.

Glenn

Sorry for not coming back to the post earlier. I thank all for many ideas.

wholeflaffer, it does it read '0' in the dark.

greg_in_canada, the whole chapter is very technical and I couldn't catch up much on the lens. But the last section of "Photometry with a Photographic Light Meter" is very informative, yet I can't get the exact way to do. I don't have a camera light meter, but can I use a digital camera which gives exposure and shutter speed reading when aiming at the light?

kj, I look at the lux reading in either manufacturer's web site or some modder's report or review, but somehow I don't get that much. This sentiment is also reflected by jtice in his thread.

For an example, Arc-LSH-F gives 745 lux at 3ft. My Arc-LSH-P modded with Lux3 @ Fraen at 611mA gave me less than that.

I fully understand the variation of the light output from different leds, but it is rather disturbing to ponder if my light meter is giving lower value or I am having a lower output light than what it is supposed to give.

And we techies have a problem of being a paranoid and never believe what a manufacturer claim about their light.

Even then, perhaps, I will try some method suggest here with a household bulb just to satisfy my ego.

-- dj

Edit:

I left my Arc-LSH-P/F on a while and measured the light output again and I got about 950 lux at 3 ft compared to Peter's 745 lux. At least now it makes a little sense though not up to the expectation.

From a simple experimental logging earlier, I found that the light output of Arc-LSH from a cold light is about 60% of full brightness and it goes to 95% brightness in a about 30 seconds, I was just impatient to measure the light the moment I switch on.

I guess, I have to accept my light meter reading.