Relationship between lux and footcandles

Lumens, Footcandles, Candlepower, Measuring Light Output

relationship between lux and footcandles

Let's split off from this and talk about the difference between RADIANCE and A lux equals one lumen incident per square meter of illuminated surface area. Problems with Foot-candles, Lux and Lumens By Erik Runkle and Bruce Bugbee difference in measuring horticultural lighting in lumens/foot-candles/lux. Simply put: a lux is a very similar measurement to the footcandle, it's just a The angle is measured between two directions for which the light.

We'd have a bit of a light up if we put the book right next to the candle, you know. If that book happens to be one foot by one foot, it's one square foot. OK, got the math done there. Now, all the light falling on that book, one foot away from your candle equals both……. Lumens Ahh, we've confused you.

relationship between lux and footcandles

Again, you measure it at the source. Unless you're talking about measuring the radiance of something intensely hot, like the Sun. Then you might want to measure it at night, when it's off.

You turn your flashlight on in a dark room, and you light something up. It also gives you another problem when you note the burglar is pointing your duck gun at your bellybutton. Illuminance is the intensity or degree to which something is illuminated and is therefore not the amount of light produced by the light source. This is measured in foot-candles again! And when people talk about LUX, it's illuminance measured in metric units rather than English units of measure.

To reinforce that, LUX is the measurement of actual light available at a given distance. A lux equals one lumen incident per square meter of illuminated surface area.


They're measuring the same thing, just using different measurement units. Pretend you're an old photographer, like O. Winston Link, or Ansel Adams. These two gods of black and white photography and a print made by either can fetch quite a hefty sum of money these days used a device called a light meter to help them judge their exposure.

There is another way of judging exposure-that's when someone whispers in our ear at a cocktail party, "You silly twit, your fly's come undone!

How to Communicate Light: Lumens, Lux, and Foot-Candles

These light meters were nifty devices. You could use it to show how much light was falling on an object, light from the sun, and reflected light energy from every thing else.

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Or you could use it to show how much light energy was reflected off the object itself. All this brings back two points. First is if we measure the output of a light at the source that gives us one thing. Second is that we use an entirely different unit of measure if we are measuring the results of that light's output.

Lux to Footcandles | Kyle's Converter

Third is the instructor is right off his trolley, isn't he? Now back to the book at the end of the ruler. We've measured two different things. We have a unit of measure for how much light is produced. We Yankees express that as a foot-candle. Being lazy, we use it all over the place. Candlepower is a way of measuring how much light is produced by a light bulb, LED or by striking an arc in a Carbon-Arc spotlight. Is it a measure of how much light falls upon an object some distance away?

Is it a measure of how well we see an object that is illuminated by that light source? That's something all together different, and we are not going there! Further confusing the matter is beam focus. That's how much candlepower. Obviously, if you project all your light bulbs intensity at a given spot, or towards something, it will be more intense, and the illuminance will be higher.

relationship between lux and footcandles

And here comes the confusion! And a candela is the metric equivalent of the light output of that one candle, based on metric calculations. And since using a candle is rather imprecise, the definition was amended to replace a light source using carbon filaments with a very specific light source, see the following: Candlepower is a measure of light taken at the source-not at the target.

Foot-candles tell us how much of that light is directed at an object we want to illuminate. Now, lets convert the lumens, a metric unit of light measurement, to candlepower. We understand a candle radiates light equally in all directions, its output, in this consideration is not focused by any mechanical means lenses or reflectors. Pretend for a moment that a transparent sphere one meter in radius surrounds your candle. We know that there are Remember your Solid Geometry classes?

The amount of light energy then reflected from that surface is defined thusly: The amount of energy emanating from one square meter of surface is one lumen. And if we decrease the size of the sphere to one foot radius, we increase the reflected energy LUX is an abbreviation for Lumens per square meter.

Foot-candles equal the amount of Lumens per square feet of area. So, that one candlepower equivalent equals Then divide that value by You don't have foot-candles, remember foot-candles are illuminance. And we are measuring radiance.

Lux to footcandles (fc) conversion calculator

Summing it all up: Candlepower is a rating of light output at the source, using English measurements. Foot-candles are a measurement of light at an illuminated object.

Lumens are a metric equivalent to foot-candles in that they are measured at an object you want to illuminate.

relationship between lux and footcandles

Divide the number of lumens you have produced, or are capable of producing, by We've now converted a measurement taken some distance from the illuminated object, converted it from a metric standard to an English unit of measure, and further converted it from a measure of illumination to a measure of radiation!

This has been an ideal proof of the superiority of the metric system. Light meters often measure lux values or footcandles, but these are directly related: Formally, lux is a derived unit from lumen, which is a derived unit from candela.

Yet, the concept of lux is more easily compared to candela than to lumen. The difference between lux and candela is that lux measures the illumination of a surface, instead of that of an angle. The net result is that the distance of that surface from the light source becomes an important factor: In the picture at the right, screen A has the same size as screen B.

relationship between lux and footcandles

One steradian on a sphere with a diameter of one metre gives a surface of one m2 see the section on candela. From this, it follows that at a measuring distance of 1 metre, the values for candela lumen per steradian and lux lumen per m2 are the same.

Convert Lux to Footcandles

In general, measurements in lux can be converted to and from candelas if the measurement distance is known. Note that when measuring LEDs, the virtual origin of the light source lies a few millimetres behind the physical point source because of the lens of the LED —this becomes relevant when measuring LEDs at a short distance.

Luminance Luminance is a measure for the amount of light emitted from a surface in a particular direction. The measure of luminance is most appropriate for flat diffuse surfaces that emit light evenly over the entire surface, such as a computer display. Luminance and illumination "Lux" are related, in the sense that luminance is typically used for light-emitting surfaces and illumination for surfaces that are being lit.

relationship between lux and footcandles

These older units are easily converted to candela per square metre by multiplying them with a scale factor. For foot-lambert, the scale factor is 3. Apex angle Since the lumen and the candela measures are related through the viewing angle or apex angleit is useful to know how this angle is defined.

The CIE has standardized the relation between luminous intensity and perceived brightness as a cubic root; other sources claim that a square root better approximates this relation.

See also the page on colour metric. These are often compared in their efficiency of turning electrical energy to luminous energy. Lighting efficiency is often expressed as a percentage, based on the theoretical maximum value of lighting efficiency of While this may seem low, LEDs are actually quite efficient in comparison with other lighting methods.