# Talk:Foot-candle

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## candle-power to watts

how much light in terms of watts is 1 candle-power or how can i relate to a candle-power —Preceding unsigned comment added by 24.208.2.154 (talkcontribs) 17:39, 28 December 2004

-- watts is a measure of power, candles are a measure of intensity (power/ unit area)

The intensity will diminish as you move away from the light source . The rule is

Intensity = Power/area

and, since area of a sphere is 4 pi R^2, whre R is the distanee from the source

Intensity = Power/(4 pi R^2)

—Preceding unsigned comment added by 129.86.230.254 (talkcontribs) 12:44, 17 October 2005

It is incorrect to say that lux and EV measure the same thing. Lux is a measurement of luminous flux hitting an area, and as such has a time component. EVs are a measure of the luminous flux hitting an area per second. The two measurements are not equivilent and therefore it is not possible to convert between then.—Preceding unsigned comment added by 84.9.50.201 (talkcontribs) 09:42, 3 September 2006

I deleted that from the article. At the least, a relation between illuminance and EV will be valid for only one particular film speed. From the EV article, EV seems more directly related to the luminance received at the camera than to the illuminance of the scene. These quantities are of course related, but are not equivalent.--Srleffler (talk) 02:52, 14 September 2008 (UTC)

## convert footcandles and lux to candelas

"Candelas are equal to the square of the distance multiplied by the number of footcandles. For example, if your meter is ten feet away from the light source and your meter reading is 10 footcandles, the equivalent candelas equals 10 feet squared (e.g. 100) times 10 -- which equals 1,000 candelas.

The conversion of footcandles and lux to candelas is most accurately achieved in the context of a single point source of light measured in the dark. If the light source is diffused, you should take several readings at different angles in order to calculate an "average" candela measurement." [1] -69.87.200.181 16:13, 24 May 2007 (UTC)

It's very misleading to talk about "converting" one quantity to a quantity that has a different dimensionality, and I think we should stay well away from it. It's sort of like saying you can convert gallons to dollars by multiplying by three, but that the conversion is most accurately achieved if the liquid involved is gasoline, the year is 2007, and the place is the United States. And that you should take readings at several different gas stations and average the results.
"Converting" candelas to footlamberts or apostilbs or whatever reminds me of some TV drama I saw eons ago. In it, a washed-up prizefighter gets drunk and stumbles out into a freight yard where he prepares to take a swing at an onrushing freight train. He explains to friends who are trying to stop him that the speed of the freight train can't be more than forty miles an hour, and "my punch travels ninety miles an hour. if something travelling ninety miles an hour hits something travelling forty miles an hour, which do you think is going to get hurt?" Dpbsmith (talk) 17:31, 25 May 2007 (UTC)

I am trying to understand your objection... You seem to be objecting to the use of the term "convert" or "conversion", and I can see your point, technically. The issue here is not "conversion", but how to do calculations properly. Given a point source, radiating uniformly over a certain beam angle, illuminating the interior of a sphere of a certain radius, we need to know how to calculate fc, lumens, lux, steradians, etc. We need to know the proper formulas. We need to make these formulas readily available and understood, with examples. What words should we be using? (And we really need to give very clear examples of this to underscore the varying dimensionalities.)

I don't think your example is fair; I think most people are casual about the meaning of the word "convert". People need to know that if gas is two dollars a gallon, and one buys three gallons, multiply 2 by 3 to get the total cost. If I loosely called this process "converting" gas price *and* quantity to cost, most people would understand that these units have different "dimensionality", and not care... But I am happy to try to word things in the best way possible, as long as we don't use the search for proper wording as an excuse for not conveying the information. -69.87.204.207 14:11, 7 June 2007 (UTC)

Many seekers of this sort of information are likely to search for "convert" or "conversion", as the most obvious general-purpose word in English for changing something into something else. The proper response to "How to convert gallons to dollars?" is not to quibble about the verb, but to simply reply "Multiply the gallons by the price in dollars per gallon."

The proper response to "How to convert Lux to Candela?" is, "Multiply the Lux reading by the square of the distance in metres, assuming a point source radiating evenly in that direction." The proper response to "How to convert Candela to Lumens? is, "Model with a point source radiating evenly within a beam angle. Multiply the Candelas by the Steradians of the beam; in the case of full illumination from a point source within a sphere, evenly in all directions, multiply by 12.566, which is the number of Steradians covering the inside of a sphere." This sort of information should be readily available, but is not currently. (It is easy to stumble upon folk arguing about the 12.566 conversion factor, without actually explaining the issue. Apparently, the people arguing don't actually understand what they are arguing about. Many people seeking this information are not finding it.) -69.87.199.228 20:47, 7 June 2007 (UTC)

I wish I had seen this conversation when it was fresh. The problem with "convert" is that most of the time when people ask how to convert lux into candelas, what they want is a general formula that applies regardless of the type of source. This formula does not exist; such a conversion is impossible. Not all sources are point sources. The proper response to "how to convert lux to candela?" is "What does your source look like?" The relationship between lux and candela for a point source is important and should be discussed, but the discussion must make it clear that it is a special case that applies only to a point source, not a general "conversion" from one unit to another.
Technically, one cannot "convert" lux to candelas, since they are measures of different physical quantities. For a particular source, such as a point source, one can determine from the source's luminous emittance (in lux) what its luminous intensity (in candelas) must be. This is deduction, not a unit conversion.--Srleffler (talk) 02:33, 14 September 2008 (UTC)

## calculating foot-candles with a camera

At the bottom of the introductory section of this article, it says:

Each f-stop has an approximate corresponding foot-candle reading (see the table below).

I don't see any such table, though. Did this accidentally get deleted? Was it ever there? Is it in a referenced article? Other?

I don't know where this table might be sourced, so I'm asking for someone to fill it in. Thanks!

Lindes (talk) 21:52, 27 December 2009 (UTC)

I took it out; it's obvious enough that a light meter can be used to measure illuminance, but beyond that it was uninformative; with a source, perhaps it could be turned into something useful. Dicklyon (talk) 22:06, 27 December 2009 (UTC)

## Incorrect title?

"A footcandle (sometimes incorrectly listed as 'foot-candle' or 'foot candle'; abbreviated fc, lm/ft², or sometimes ft-c) [...]"

OK, I'll bite. If it's incorrect, why's the article at foot-candle, and not footcandle? 84.203.35.116 (talk) 07:05, 12 August 2012 (UTC)

## Inappropriate image

General Electric's "foot candle" advertising novelty

This image is placed in the article such that it looks like it should be relevant to the definition of foot-candle. I think a diagram would be best, but even an image of a typical candle would be better because it wouldn't undermine the actual notion this article is about. Jbeyerl (talk) 20:52, 11 April 2016 (UTC)

But it's relevant – as a play on words by a company in the business. Dicklyon (talk) 04:21, 12 April 2016 (UTC)
I agree with both of you- it's relevant because it's a play on works from a company that has been in the business- however I have thought this image was weird for as many times as I've looked at this article too, and I'm thinking that it shouldn't be the key/main/first image of the article. --Chonolith (talk) 03:00, 4 November 2016 (UTC)