# Light Meters



## tommygun (Oct 22, 2005)

Does anyone know of a quality illuminometer, capable of measuring to at least .01 foot-candles, with a backlit display?

I've been trying to measure light in environments that are so dark that I cannot read the display on the two Gossen light meters that I've purchased. I need something with a backlit display, but haven't seen such a product available.

Can anyone "enlighten" me on this?

Thanks in advance!


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## jtr1962 (Oct 22, 2005)

I can't enlighten you as to specific meters but I can tell that a meter with a backlit display will either be a battery killer or will require an AC adaptor. Incidentally, I have the same problem as you, and I'm also interested in 0.01 lux/fc resolution instead of the 0.1 lux resolution that my present meter offers. Since a backlit meter will run down batteries quite often, if there is a choice, I'd say purchase a meter which uses AAAs or AAs so at least you can use inexpensive NiMH.


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## Sigman (Oct 22, 2005)

Wavetek Meterman LM631

At a decent price here and don't forget to check out their "Promotional Special" that will allow you to get a free DMM, 10 assorted plier/cutter set, or a pen DMM here. (Read the fine print and enter the correct promo code in the correct space when ordering.)


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## vcal (Oct 22, 2005)

Much agreed on the LM631 Meterman. BTW-the LED backlight on mine has had almost no effect on the batteries (new alkaline-11 months ago, and still very good).

I was able to get my Meterman from eBay for about $51. in a legit auction. IMO, the Meterman is still a good value even at 90-$100. :thumbsup:


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## dwminer (Oct 22, 2005)

Thanks Sigman, I ordered one. Now they only have 3 in stock.
Dave

I also selected the freebie "Promo Code:: PR-3211". Thanks for the pointers.


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## Sigman (Oct 22, 2005)

Did you select a "freebie" Dave? 

I've ordered several times from Circuit Specialists...never a problem!


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## The_LED_Museum (Oct 22, 2005)

I'd also recommend the Wavetek Meterman LM631.
I've been through several sets of batteries in mine since I've had it, but I use it frequently and almost always with the backlight on.


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## McGizmo (Oct 23, 2005)

You might want to take a look at the specs and consider the resolution offered against the accuracy of the device. I have still not been able to fathom some of the accuracy measures offered but it seems that some of these give nice small numbers and resolution but can be off by more that the reading itself!?!?! :shrug:

EDIT: Oh yeah, I like the extech EZ view 30 best of the three meters I have and it does have back lighting as well as the desired resolution.


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## tommygun (Oct 26, 2005)

Thanks to Sigman and others for their assistance.

I did take your advice and order the LM631 through Circuit Specialists. So, I also have to thank you for providing the link for purchase.

One concern I have is that the device be suitable for forensic applications. I saw nothing about the LM631 being NIST certifiable, so I'll likely use it for firearms range use only. I still need something NIST compliant for measuring light at the scene of low light police shootings.

Thanks again to all. I'm new to CPF and have already found this place to be a great find.
Tom


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## NextLight (Oct 26, 2005)

tommygun said:


> ...  I still need something NIST compliant for measuring light at the scene of low light police shootings.
> 
> Tom



Need NIST certs, buy it _with_ NIST certs.

http://www.professionalequipment.com/xq/ASP/lightmeters/ID.13/qx/default.htm


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## NextLight (Oct 26, 2005)

McGizmo said:


> You might want to take a look at the specs and consider the resolution offered against the accuracy of the device. I have still not been able to fathom some of the accuracy measures offered but it seems that some of these give nice small numbers and resolution but can be off by more that the reading itself!?!?! :shrug:



A common spec is "±3%rdg + 0.5%FS" for levels below 4000 lux.
For a 3.5 digit meter, the best circumstance woud be measuring 199.9 lux on the 200 lux range. Total uncertainty is ~+/- 3.5 %, because the reading and full scale are almost the same. 

The worst common circumstance would be measuring 21 lux on the 200 lux range (just above the level where we could downrange) Uncertainty is 3% of reading - ok 0.63 lux, PLUS .5% of full scale, or 1 lux. There is an additional resolution uncertainty of +/- 1 count, here .1 lux, so the total potential error is +/- 1.73 lux or >+/- 8% of the reading. Well over twice the uncertainty in the first example.

At lower levels we run into instrument noise and sensitivity limits, at very high levels there are often sensor non-linearity issues, and no electro- optical sensor is level all across the color spectrum, whether the manufacturers like to talk about it or not.

This is all from HP (now Agilent) Test & Measurement Fundamentals Training; It is not the whole story, but applies to electronic measurement in general. I don't have lab experience with the particular meters discussed here, so I can't reccomend one over the other.


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## tommygun (Oct 26, 2005)

NextLight:

Thanks for the input, but none of the samples that you linked me too met my other criteria: that the device have a backlit display capability.

I already have two $1,000 Gossen models that I can't read at very low light levels.


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## cy (Oct 26, 2005)

tommygun, if you are interested in accuracy of light meters. there's a current thread silverfox and McGizmo is putting together of sending out calibrated light sources for CPF members to test on their light meters. 

the most common referred light meter on cpf is the meterman you just purchased. you may consider participating on pass-around.


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