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Candela/Lux/Footcandle

Candela/Lux/Footcandle

Candela/Lux/Footcandle

(OP)
Using the correct terminology can help so much in reducing the confusion level.

I have this nifty Federal requirement that states I need a 20 candela red beacon.  That's pretty straight forward, as candela is a unit of luminous intensity.  I'm also suppose to measure this intensity at some distance, and still measure 20 candela as the requirement is written.

I guess the spec writer never heard of the Inverse Square Law, or Lux (Footcandle).

So now I have to go through formal channels, write for clairification, wait for a response stating that there is a problem, give a solution, wait for approval.  Cripes.

"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Candela/Lux/Footcandle

"Cripes" I haven't heard that expression for ... yonks!


Making the best use of this Forum.  FAQ559-716
How to get answers to your SW questions.  FAQ559-1091
Helpful SW websites every user should be aware of.  FAQ559-520

RE: Candela/Lux/Footcandle

Limey, you obviously need a subscription to the Beano.

MadMango, Luminous intensity is not the same as other kinds of intensity such as acoustic intensity.

Acoustic intensity is acoustic power per UNIT AREA (and hence has an inverse square law behaviour for an omnidirectional source)

Luminous intensity is luminous flux (unit lumens) per SOLID ANGLE. So the steradian bit takes care of the inverse square law bit.

The "light" equivalent of the acoustic intensity is illuminance (measured in lux which is lumens per square metre).

M

--
Dr Michael F Platten

RE: Candela/Lux/Footcandle

Dr. Mike, can the spec can stand as written, or does it need to be changed?  Is there no unit for flux per unit area?

William

RE: Candela/Lux/Footcandle

Yes, flux per unit area is called "illuminance" and the SI unit is lumens. I have never (knowingly) measured a candela, lux or lumen. Light is not my thing. I simply looked-up the definitions of the various SI units on Google!

M

--
Dr Michael F Platten

RE: Candela/Lux/Footcandle

Correction: "illuminance" is measured in "lux" which is lumens per square metre (or in base units candela steradians per square metre).

M

--
Dr Michael F Platten

RE: Candela/Lux/Footcandle

I have read the definitions for each one a million times, but have remained somewhat confused.  I thought perhaps you might finally be the one to..to...uh, shed some light on it for me, to coin a phrase.

William

RE: Candela/Lux/Footcandle

(OP)
Candela refers to the output of the light.  Lux (or the older term footcandle) is a measurement of that output at a specific distance, over a given area.

The way the spec is written, they want me to measure at a distance and achieve 20 candela.  Is sort of like asking for a garden hose to have a flow rate of 50gpm and to measure 15 gallons.  It just doesn't make sense.

"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Candela/Lux/Footcandle

Perhaps I didn't make myself clear. It often happens

I think your standard DOES make sense.

The standard (as you quoted it) says that you need a 20 cd beacon. Let's assume that it is effectively an omnidirectional point source. Say I have a light detector which measures the total luminous flux falling on the area of the sensor.

I stand at a distance L and point it at my beacon and it tells me there are x lumens.

So I can calculate the illuminance (in lux) as x divided by the area of the sensor.

I can also calculate the luminous intensity (in candela) as x divided by the solid angle subtended from the beacon to the area of my detector. Hopefully the answer is 20 cd.

Now I reposition my sensor so that I am a distance 2 L away from it.

My sensor now says there are x/4 lumens. This is precisely what we expect from the inverse square law.

Again I calculate the illuminance (in lux) by dividing the luminous flux by the area of the sensor. Of course it is 1/4 of what it was before.

Finally, when I calculate the lumious intenstity, I divide the measured luminous flux by the solid angle subtended from the beacon to the area of my detector and I find that the answer is STILL 20 cd. This is because the subtended solid angle has decreased by a factor L squared.

The amount of light falling on the detector has gone down due to inverse square law BUT SO HAS THE SOLID ANGLE.

I guess what your standard is trying to check is that (a) the beacon is omnidirectional and (b) there is no significant scattering of the light by whatever is between you and the beacon.

M

--
Dr Michael F Platten

RE: Candela/Lux/Footcandle

(OP)
I'm not doubting your math, and you are correct.  But I still feel the spec is written incorrectly.

In this regard, the beacon placement is key.  If it is only 2ft away from the measurement source, I need a less powerful beacon, than say if it was 8ft away.  That would be a factor of 64.  That would equate to around 215cp light, not 20cp.  People standing only 1ft away from the light would need sunblock and opaque sunglasses.  That would be a bright light inside a vehicle.

"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Candela/Lux/Footcandle

You have lost me now. What do you think is wrong with the standard? How do you think it should be written?

M

--
Dr Michael F Platten

RE: Candela/Lux/Footcandle

(OP)
They shouldn't state the required candela of the beacon in my mind.  They should be asking for min/max X Lumens or X Footcandle at point Y.

"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Candela/Lux/Footcandle

Seems straight forward to me, they want 20 candela at a specific distance. Are they saying that you can't go over 20 at a closer than specified distance? It seems like its more of wanting a flow from a pipe of 50gpm and pressure of 10psi at say 10 feet from the source. If you measure closer to the nozzle you could get a 50psi reading at 50gpm, measure at 100 meters and you have zero psi and still have a flow of 50gpm.  Just my 0.02

RE: Candela/Lux/Footcandle

Most often spec writers are staff members that have been out of touch with the day-to-day nuts and bolts of engineering.  We usually take exception to certain requirements in a compliance matrix and show this will be met by some other means like analytically verses actual testing.

Their is nothing I hate more then reading a spec that's full of fluff it makes the day go by so slooooow.  But I guess they have to justify their being.

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 2.0 & Pro/E 2001
Dell Precision 370
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      o
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"There is no trouble so great or grave that cannot be much diminished by a nice cup of tea"  Bernard-Paul Heroux
 

RE: Candela/Lux/Footcandle

ctmtwilliams,

If the beacon produces 20 cd in all directions, and there is nothing between you and the beacon to scatter any of the light then it doesn't matter if you are 1 metre away or a million miles; the luminous intensity will still be 20 cd.

I think MadMango's point is that the standard takes no account of how far the person is from the beacon. If the purpose of the beacon is to warn someone of some event and that person is going to be 1m away from it, then it's luminous intensity doesn't need to be as great as a beacon that is trying to warn someone who is 100m away from it. His standard apparently thinks otherwise.

Remember that although the luminous intensity doesn't change with distance, the illuminance (what we would perceive as brightness) does.

M

--
Dr Michael F Platten

RE: Candela/Lux/Footcandle

MikeyP,

You are right, I forgot about the difference between luminous intensity and illuminance. Sorry, I'll go back to my dirt now

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