Anyone use an incident light meter?

[bald1]

I have an old Sekonic Studio Delux with a set of direct reading slides, lumidisk, and lumisphere. The manual includes conversion data for taking the meter's foot candle reading and converting it to Lux using weighted factors if necessary (eg high slide requires 32x the reading to get the fc right before converting to lux.

If someone has used one to measure their flashlights using an 18% card for reflected readings or using an incident meter in direct mode, I'd appreciate some comments about your experiences and the accuracy you got.

This would be a lot cheaper than buying a new digital meter like the LM631 /ubbthreads/images/graemlins/smile.gif

Trying it with a torch rated at 25 Lumens, I got 130842 Lux at 1 foot. Does this pass the sanity check?

--Bob

 

[bald1]

[ QUOTE ]
Trying it with a torch rated at 25 Lumens, I got 130842 Lux at 1 foot. Does this pass the sanity check?

[/ QUOTE ]

More details: the meter read 380 foot candles with the lumisphere and high slide. 32x for the high slide then x10.76 for Lux.

But my error was using the lumisphere.... supposed to use the lumidisk /ubbthreads/images/graemlins/frown.gif That gave 425 fc on the meter x 32, x 10.76 = 146,336 lux.

Again, that pass the sanity check for a 25 lumen torch at 1 foot?

--Bob

 

[Doug Owen]

You won't be able to do what you want I'm afraid.

Lumens are a measure of total light output, Fc or Lux (Lux is typically taken at one meter, BTW, Fc at one foot) are level per unit area measures. So what 25 Lumens means at the surface depends on the optics (beam angle).

Incident light meters are made to *estimate* the light that will be reflected back into the camera from an average subject with that level of light falling on it. You point it at the sun to estimate the correct exposure for say friends in a canoe on a lake (where you can't just measure the actual light from them to the camera). I'm not sure it's much use in flashlights. A direct reading head might be another matter again.

Doug Owen

 

[bald1]

Doug,

I was afraid of that /ubbthreads/images/graemlins/frown.gif But I do indeed appreciate the sanity check.

Best!

--Bob

 

[Roy]

if you divide by 10.76 you get a more realistic number...1,264. Are you sure you're supposed to multiply by 10.76?

remember that 1Fc @ 1FT = 1LUX @ 1M

 

[bald1]

Roy,

I rechecked the manual: footcandle readings x 10.76 = lux when using the lumidisk. If the high slide is also employed the readings must be multipled by 32 and to get footcandles, then by 10.76 to get lux.

So my reading of 425 with the lumidisk and high slide equaled 13600 fc x 10.76 = 146,336 lux

The manual was written in the early 70s and may be in error. There is no other passage addressing the same issue.

--Bob

 

[Doug Owen]

[ QUOTE ]
bald1 said:
So my reading of 425 with the lumidisk and high slide equaled 13600 fc x 10.76 = 146,336 lux



[/ QUOTE ]

But that's unrealistically big, by several orders of magnitude (not just a little bit).

"It can't be right".

Doug Owen

 

[SilverFox]

Hello Bob,

For a quick check, go to Doug's (Quickbeam) throw vs output charts. Pick some lights that you have and do a comparison. You will have to take Doug's throw numbers and square them to get lux.

The standard lux measurements are taken at 1 meter, so that may have to factor into your experiment.

Tom

 

[Wingerr]

The luxmeter is essentially an incident light meter but calibrated for units of lux and footcandles, so if you get some calibration points for readings that you make, it should give you a good idea of the illuminance. Main disadvantage may be that the resolution of readings are lower, and having to calculate the equivalent values, but it should work well enough.
I have an old incident light meter that I could try to get some comparison values against a luxmeter, but its calibration is suspect, because the diffuser dome seems rather yellowed from age, so it might be too far off.
Multiplying fc by the conversion factor to get lux should be correct, so your manual looks okay.

 

[bald1]

Tom & Wingerr,

Appreciate the additional sanity checks. All I'm using it for now is comparative readings. The unit dates from the mid 70s so the calibration is suspect although I am able to zero the needle.

It was just fun to dig out of my camera bag given I haven't touched my rather extensive 35mm setup in years having become a digital convert /ubbthreads/images/graemlins/smile.gif

Best!

--Bob