Lightmeters:another unsung hero?

[Mags]

Sorry for stealing the name of the thread from the other one about the G2. It seems lightmeters dont get any attention at all around CPF. I mean, the only time it is used is when "And the lightmeter says ****** lux, yes that beats the ***** by more than twice as much...." I mean, it might be a whole undiscovered world! like the different brands of lightmeters, different statistics on using them, different types like certain kinds for LED, or incan maybe, who knows? Why arent lightmeters given much, if any attention?

 

[Mags]

See how little respect and attention lightmeters are given?

 

[Roy]

I use the Extech 401021 Light Meter Adapter plugged into my Radio Shack 22-805 DDM. The adapter is calibrated such that 1 mvDc on the display of the DDM = 1 Footcandle. I measure everything at one foot so that my results equal footcandles at one foot. That way, for example, 120 Fc@1Ft = 120 LUX@1M.

 

[rdshores]

The light that is being reflected off the table in the above photo will make to reading highter than if there was no reflection. You can see the shadow line of the table on the wall. This type of thing is why everyones LUX reading vary. I was using a similar setup when I got 900 LUX on a XM-3. When I changed the setup so that the light was not reflecting off my table, I got 710 LUX.

 

[Mags]

[ QUOTE ]
rdshores said:
I was using a similar setup when I got 900 LUX on a XM-3. When I changed the setup so that the light was not reflecting off my table, I got 710 LUX.

[/ QUOTE ]

Wow, what substance do you use to polish your tables? /ubbthreads/images/graemlins/grin.gif

From what Roy said, it seems there is more to measuring lux than I thought.

 

[Roy]

Since I'm measuring the light in the brightest part of the beam, I don't think that any light reflecting off the table is going to make that much difference in the measurment.

I'd be really supprised if my measurments closely matched anyone elses numbers! So many variables to be considered! Also bear in mind that any light measurments I claim are the by-products of measuring the run time of a set of batteries in a flashlight.

 

[SilverFox]

Another thing to remember that the light bouncing off of Roy's table will be reflected to the ceiling and with the inverse square law, will have little effect on his readings.

I believe baffles are often used to eliminate reflections as well as painting everything "velvet" black.

Another thing that is often done is to block the light meter and measure the stray light that comes in.

I take measurements in my bedroom. The curtains, rug, and bedding do a great job of eliminating the stray light. My wife sometimes throws things at me, but I consider those the natural hazards of measuring light... /ubbthreads/images/graemlins/smile.gif

Tom

 

[modamag]

[ QUOTE ]
SilverFox wrote:
My wife sometimes throws things at me, but I consider those the natural hazards of measuring light...

[/ QUOTE ]

/ubbthreads/images/graemlins/crackup.gif/ubbthreads/images/graemlins/crackup.gif/ubbthreads/images/graemlins/crackup.gif/ubbthreads/images/graemlins/crackup.gif/ubbthreads/images/graemlins/crackup.gif
Pretty soon someone will offer "flashaholic" insurance for these "natural hazards.

 

[OddOne]

I use an Extech too, only I have the lux model instead of the footcandle one (model 401020). Mine's plugged into a calibrated Fluke 89-IV DMM, so on the lowest lightmeter setting I can read lux directly. (The 89-IV has a full-five-digit, and shows down to 0.1mv - the Extech emits 0.1mv/lux, which makes it stupidly easy to read.

My target distance is one meter, and is spaced away from any other surfaces by about another meter so reflections aren't an issue. I also take a baseline read before testing and subtract it from the actual test reading so as to account for ambient lighting levels.

oO