Setup to Measure Nav/Strobe Intensity (candela)

[Dan Langhout]

OK, I know there are some lighting experts out there who can give me an informed opinion on this.

I am trying to get a ballpark measurement of intensity (in candela) of an LED Position/Strobe light. Based on my understanding of the photometric units of measure (which is tenuous at best) it would seem that you would measure the illuminance (lux or foot-candles) with a light meter and then multiply by the square of the distance from the sensor to the light (meters^2 or ft^2). to get the intensity value of the source (candela)?

Given that, here is a photo of my admittedly crude measuring set up:



and the light meter I'm using:




Note that the distance between the light meter sensor and the light is 1 foot, therefore my measurements in foot-candles should also equal the candela value (1^2=1) - right?

I take the measurements in the dark even though the meter has a relative setting to null out ambient light. I use the normal setting to measure the position light and the "peak" function to measure the strobe.

OK - Please - tell me what, if anything, is wrong with my assumptions or measuring setup. I'm not getting results even close to what I expect.

[az_gila]

...of the nav light OK and the strobe reading "funny"?

I don't think that meter can do the averaging for a pulsating strobe source.

Just to check that the sensor is not saturating, move to 5 ft away and check the reading drops by 1/25.

[Dan Langhout]

Originally Posted by az_gila

...of the nav light OK and the strobe reading "funny"?

I don't think that meter can do the averaging for a pulsating strobe source.

Just to check that the sensor is not saturating, move to 5 ft away and check the reading drops by 1/25.

Both the position light and strobe read much lower than I would expect. I have played with the sensor distance to see if the effect on the reading was "logical" (i.e. the reading increases or decreases ~ by the square of the distance). According to the manual for the meter, the "Peak" function enables the meter to capture pulses as short as 100 microseconds. Typically, I think LED strobe pulses are more like 100 milliseconds so I would expect for the meter to be able to capture the peak OK.

Obviously, what I'm trying to do here is to see if this light is in the neighborhood of meeting the FAR for a position and anti collision light (it's supposed to). Calculating the "effective" candela for the strobe is a little more complicated than just measuring the peak brightness (see FAR 23.1401) but the "peak" has to at least meet the minimum (400 cd) to even be within shouting distance of being compliant. The readings I'm getting are low by greater than a factor of 2 for the position and 8 for the strobe . I'm figuring that I must be doing something wrong here .

[az_gila]

...the manufacturer of your lights. A simple measurement test like your setup should get reasonable readings, and the shipped device should be designed with some margin over the FAA mandated minimums.

In previous VAF postings about LED lights, I've have asked if anyone has run actual tests, and I never really got a positive answer.

It all seems to be calculations based on the LED makers data sheets, which are based on a measured average, and don't often give the actual production variation range.

This paper gives a good description of the "effective" candela rating for the flashing bit.

http://www.periheliondesign.com/down...using_leds.pdf

.....and you haven't even got to the directional coverage measurements yet...

Did you try running on 14.4 volts input?

[Dan Langhout]

Originally Posted by az_gila

<snip>
In previous VAF postings about LED lights, I've have asked if anyone has run actual tests, and I never really got a positive answer.
</snip>

Well . . . that's exactly what I'm trying to do here ("trust but verify" ) - I'm just not sure my measuring methodology or equipment is correct.

<snip>
This paper gives a good description of the "effective" candela rating for the flashing bit.

http://www.periheliondesign.com/down...using_leds.pdf
</snip>

I have read that paper and it gives a really good explanation. An important take-away from the paper and the FAA equation is that the actual intensity value has to be HIGHER than the minimum in order to meet the EFFECTIVE intensity value described by the equation

<snip>
.....and you haven't even got to the directional coverage measurements yet...
</snip>

Absolutely right!

<snip>
Did you try running on 14.4 volts input?
</snip>

I did - the lights internal power supply does an excellent job of keeping the power constant (the input current goes down as the voltage goes up).

[JonJay]

LED manufacturers publish lumen outputs for "ideal" junction temperatures. Reality is that when you put the LED die into anything that makes it a useful fixture, then put it into any reasonable environment, your lumens will be substantially depreciated. I would not be surprised that you are getting half of what you would expect to see since you are expecting to see what the manufacturer publishes.
The IESNA, Illuminating Engineers Society of North America, of which I have been a member of for more than 20 years, is nearly complete with new standards which will require testing of complete systems and fixtures. This is supposed to be out very soon.
All that said, illuminance is really not that important for the application of strobes and nav lights. What is important is surface brightness. LED's appear bright because they generate their light in a very tiny point. It looks bright, but does not really put out much useable light. However, the appearance of brightness is what is important for you to see someone to avoid a collision. Who cares how much light is actually leaving the fixture. If it is bright to the eye, you can see it. There are meters that test for surface brightness. They measure in foot lamberts. Unfortunately, the FAA does not test to that standard because that would be the smart thing to do.
As far as your light meter goes, the lensing system in light meters vary. A good one is set up to follow the spectrum spread that is most visible to the average human eye. LED's have trouble providing light in wide spectrums as they are essentially monochromatic, but then use a phosphor, or multiple color leds, to try to compensate for better color across a wider spectrum. Light meters may or may not represent accurately what you see with your human eye.
LED technology is changing at a very high rate. Two years from now, you will have products at least twice as bright, twice as good, and half the cost.

[rocketbob]

Originally Posted by Dan Langhout

Both the position light and strobe read much lower than I would expect.

I would expect so. The test device is going to do some filtering/averaging to get a stable reading (dv/dt). Just off the top of my head you would need a constant current source, a phototransistor, a digital storage oscilloscope, and come up with a way to calibrate everything. Its a different ballgame to quantify a light source with short duration. You could use the light meter to calibrate your test setup.

[az_gila]

Originally Posted by rocketbob

I would expect so. The test device is going to do some filtering/averaging to get a stable reading (dv/dt). Just off the top of my head you would need a constant current source, a phototransistor, a digital storage oscilloscope, and come up with a way to calibrate everything. Its a different ballgame to quantify a light source with short duration. You could use the light meter to calibrate your test setup.

....but Dan can't get the steady state reading to agree with the advertised (or is it implied?) numbers.

The meter should be reasonably accurate for that measurement mode.

[Brantel]

I see you have that com radio sitting there and I assume your checking for RFI interference???

What is the result on that if you are?

[Dan Langhout]

<snip>
All that said, illuminance is really not that important for the application of strobes and nav lights. What is important is surface brightness. LED's appear bright because they generate their light in a very tiny point. It looks bright, but does not really put out much useable light. However, the appearance of brightness is what is important for you to see someone to avoid a collision. Who cares how much light is actually leaving the fixture. If it is bright to the eye, you can see it. There are meters that test for surface brightness. They measure in foot lamberts. Unfortunately, the FAA does not test to that standard because that would be the smart thing to do.
</snip>

That makes perfect sense! This light certainly looks bright. But I assume from what you said that the FAA testing method would be more along the lines of what I am trying to do?

The light I am testing is commercially available and the manufacturer has stated that it has been tested in compliance with FAA requirements and has been submitted for TSO approval (no specific lumen or candela values are given). I could just take them at their word (and wait for the paperwork) but it seemed to me that I should be able to independently get test results that were at least in the same universe with the specs. Given that my results so far are not even close, I am assuming that there is a problem with my measurements/calculations.

<snip>
As far as your light meter goes, the lensing system in light meters vary. A good one is set up to follow the spectrum spread that is most visible to the average human eye. LED's have trouble providing light in wide spectrums as they are essentially monochromatic, but then use a phosphor, or multiple color leds, to try to compensate for better color across a wider spectrum. Light meters may or may not represent accurately what you see with your human eye.
</snip>

My light meter is just a garden variety unit used to measure light levels in the workplace, etc. The specifications say that it provides "color and cosine corrected measurements". The sensor seems to be covered with a white translucent dome. Perhaps it's not suitable for this sort of measurement?