# Lux and lightmeters?

Discussion in 'Lighting Equipment' started by garry edwards, Oct 5, 2003.

1. ### garry edwards

1. Is it possible to measure light output of up to 100,000 lux with a
photographic light meter?<br>2. How can I 'convert' the lightmeter
reading into lux?<p>I have searched the archives and found several
references, but none seem to answer my questions.<br>Can anyone help

2. ### jimstrutz

I'll quote what I read on the net. Can't remember who wrote it though.
Under restrictions of measurement-method and sensor-sensitivity, for 100 ASA and a direct (non-reflective) light reading:
Light intensity (Lux) = 2.5 x 2^EV - Or vice versa: EV = log (Lux/2,5) / log 2
Example: 10000 Lux => EV = 12, which equals 1/125s @ f5.6
For those wanting to avoid EV, and get straight aperture/time combinations: Light intensity [Lux] = 2,5 * aperture^2/time
Enter a preferred aperture or time, and with a given Lux you can calculate time or aperture.

I also found this explaination...
A foot-candle is equivalent to one lumen per square foot. A wax candle flame has a luminous intensity of approximately one candela. If you hold the candle one foot away from a surface, the illuminance of the surface at this distance due to the light from the candle will be approximately one foot-candle. It will be 1/4 fc at two feet, 1/9 fc at three feet, and so on in accordance with the inverse square law for point light sources.
The manual for a Sekonic L-408 lists the following:
Luminance (cd/m^2) = .125 x 2^EV"
After messing with this for a bit I discoverd that the "carrot" symbol is used to designate "raised to the power of." (Math is not my long suit.) Also these calculations are set for 100 ISO.
Most light meters will give you an EV reading, so it's not too hard to figure it out from there. I just created a little EV to Lux chart that I need when checking lighting levels in offices. That was a lot cheaper than buying a dedicated Lux meter.
I believe that 100,000 lux is a little over EV 15 at ISO 100. Well within the parameters of most light meters.

3. ### garry edwards

Jim - many thanks