Candela, Lumen, Lux: the equations Candela, Lumen, Lux: the equations You are here: Home > Articles > Electronics > Candela, Lumen, Lux Navigation: Candela, Lumen, Lux: the equations Light is measured with different techniques, and therefore there is a handful of related, but different, units of measurements. This article gives a brief overview of the most widely used measures and presents a few equations for conversion of one measure into another. For convenience, a table with conversion calculators is at the bottom of this page. Optical radiation covers a broad spectrum, including infrared and ultraviolet light. For the sake of brevity, this article focuses on visible light (the field of photometry). Candela The candela (unit cd) has its origin in the brightness of a "standard candle", but it has received a more precise definition in the International System of Units (SI) —and at that time the unit was also renamed from "candle" to "candela". The candela measures the amount of light emitted in the range of a (three-dimensional) angular span. Since the luminous intensity is described in terms of an angle, the distance at which you measure this intensity is irrelevant.

For ease of illustration, in the picture at the right the three dimensions have been flattened to two. In this picture, screen B would catch exactly the same amount of light rays (emitted from the light source) as screen A —provided that screen A were removed to not obscure screen B. This is because screen B covers the same angle as screen A. The angular span for candela is expressed in steradian, a measure without unit (like radian for angles in a two-dimensional space). One steradian on a sphere with a radius of one metre gives a surface of one m2. A full sphere measures \( 4\pi \) steradians. See the section on lux for the relation between candela and lux. Lumen If you look at LEDs, especially high-brightness LEDs, you may notice that the LEDs with a high luminous intensity (in candela or milli-candela, mcd) typically have a narrow apex angle . Similarly, LEDs with a wide apex angle typically have a relatively low luminous intensity. The same is true for halogen spots with reflector: those with a narrow-beam reflector have a higher rating in candela than the "floodlight" spots of the same power.

The cause for this relation is the total energy produced by the LED. LEDs of a specific class (for example, "high flux") all produce roughly the same amount of luminous energy. However, when a LED emits its total energy in a beam with a narrow angle, the intensity will be greater (in the direction of that angle) than when the same energy had been emitted over a wide angle. The lumen (unit lm) gives the total luminous flux of a light source by multiplying the intensity (in candela) by the angular span over which the light is emitted. With the symbol \( \Phi_v \) for lumen, \( I_v \) for candela and \( \Omega \) for the angular span in steradian, the relation is: \[ \Phi_v = I_v \cdot \Omega \] If a light source is isotropic (meaning: uniform in all directions), \( \Phi_v = 4\pi\ I_v \). This is because a sphere measures \( 4\pi \) steradians. See the topic on apex angles to get the three-dimensional angular span \( \Omega \) from an opening angle.

As a frame of reference, a standard 120V/60W light bulb is rated at 850 lm, and the equivalent 230V/60W light bulb is rated at 700 lm. A low voltage (12V) tungsten halogen lamp of 20W gives approximately 310 lm. Lux Lux (unit lx) is a measure of illumination of a surface. Light meters often measure lux values (or footcandles, but these are directly related: one footcandle is 10.764 lx). 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: the more distant that the surface is from the light source, the less it will be illuminated by it. In the picture at the right, screen A has the same size as screen B.

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. 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 is a derived measure, expressed in Candela per square metre (\( cd / m^2 \)). An alias for the unit \( cd / m^2 \) (unofficial, but still commonly used) is "Nit".

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. Assuming a perfect diffuse reflecting surface, you can multiply the measure in "Nits" by \( \pi \) to get the equivalent value in Lux. That is, with \( L_v \) for Luminance and \( E_v \) for Lux: \[ E_v = L_v \cdot \pi \] As with Lux, there are several older units for luminance, of which the foot-lambert is probably the most common (because of its 1-to-1 relation with the footcandle on a Lambertian-reflecting surface). 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 3.425. Apex angle Since the lumen and the candela measures are related through the viewing angle (or apex angle), it is useful to know how this angle is defined. One measures the angle between the axis where the light source gives its highest luminous intensity and the axis where that intensity is reduced to 50%.

In the picture at the right, this angle is denoted with \( \theta \). The apex angle is twice that angle (meaning \( 2 \theta \)). Observe that the reduction of intensity to 50% is based on a linear scale, but that our perception of brightness is not linear. 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 . The three-dimensional angular span for an apex angle, using \( \Omega \) for the angular span (in steradian) and \( 2 \theta \) for the apex angle, is: \[ \Omega = 2\pi \left( {1 - \cos {2 \theta \over 2}} \right) \] Lighting efficiency There are ample ways to illuminate a surface or a room: incandescent lamps, fluorescent tubes, LEDs, tungsten-halogen bulbs, electroluminescent sheets, and others. These are often compared in their efficiency of turning electrical energy to luminous energy. The official name for lighting efficiency is "luminous efficacy of a source".

This should not be confused with the "luminous efficacy of radiation", which disregards losses due to heat generation and others (and therefore gives significantly higher values). The lighting efficiency is measured in lm/W (lumen per Watt). Lighting efficiency is often expressed as a percentage, based on the theoretical maximum value of lighting efficiency of 683.002 lm/W (at a wavelength of 555 nm). For example, at the time of this writing, a white 1 Watt "lumiled" can reach an efficiency of over 100 lm/W, giving an efficiency of 15%. While this may seem low, LEDs are actually quite efficient in comparison with other lighting methods. Equations The equations in this sections are given without further explanation or derivation. For details, please refer to the technical literature, or Wikipedia .

From Base unit definitions: Candela Unit of luminous intensity (candela) Abbreviations : CGPM, CIPM, BIPM Originally, each country had its own, and rather poorly reproducible, unit of luminous intensity; it was necessary to wait until 1909 to see a beginning of unification on the international level, when the national laboratories of the United States of America, France, and Great Britain decided to adopt the international candle represented by carbon filament lamps. Germany, at the same time, stayed with the Hefner candle, defined by a flame standard, and equal to about nine-tenths of an international candle. But a standard based on incandescent lamps, and consequently dependent upon their stability, would never have been fully satisfactory and could therefore be only provisional; on the other hand, the properties of a blackbody provided a theoretically perfect solution and, as early as 1933, the principle was adopted that new photometric units would be based on the luminous emission of a blackbody at the freezing temperature of platinum (2045 K).

The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the "new candle" based on the luminance of a Planckian radiator (a blackbody) at the temperature of freezing platinum. This modification had been prepared by the International Commission on Illumination (CIE) and by the CIPM before 1937, and was promulgated by the CIPM in 1946. It was then ratified in 1948 by the 9th CGPM which adopted a new international name for this unit, the candela (symbol cd); in 1967 the 13th CGPM gave an amended version of the 1946 definition.

In 1979, because of the experimental difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by radiometry, i.e., the measurement of optical radiation power, the 16th CGPM (1979) adopted a new definition of the candela: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. candela (cd) | SI unit of measurement | Britannica.com SI unit of measurement Alternative Titles: candle, candlepower, cd, standard candle Similar Topics British thermal unit (BTU) Candela ( cd ), unit of luminous intensity in the International System of Units (SI), defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and has a radiant intensity in that same direction of 1/683 watt per steradian (unit solid angle).

The candela has replaced the standard candle or lamp as a unit of luminous intensity in calculations involving artificial lighting and is sometimes called the “new candle.” Learn More in these related articles: luminous intensity the quantity of visible light that is emitted in unit time per unit solid angle. The unit for the quantity of light flowing from a source in any one second (the luminous power, or luminous flux) is called the lumen. The lumen is evaluated with reference to visual sensation. The sensitivity of the... hertz unit of frequency. The number of hertz (abbreviated Hz) equals the number of cycles per second. The frequency of any phenomenon with regular periodic variations can be expressed in hertz, but the term is used most frequently in connection with alternating electric currents, electromagnetic waves... 5 References found in Britannica Articles Assorted References Corrections? Updates? Help us improve this article! Contact our editors with your feedback. MEDIA FOR: You have successfully emailed this. Error when sending the email. Try again later. Edit Mode Submit Tips For Editing We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.

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Close Date Published: April 29, 2004 URL: Access Date: January 19, 2017 Share Candela | Units of Measurement Wiki | Fandom powered by Wikia Since the 16th General Conference on Weights and Measures in 1979, the candela has been defined as follows: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian . Origin The candela was based on an older unit, the candlepower, which was referenced to the luminous intensity of a "standard candle " of known composition. Explanation Edit The frequency chosen is in the visible spectrum near green , corresponding to a wavelength of about 555 nanometers. The human eye is most sensitive to this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. (See luminosity function ). A common candle emits about 1 cd.

A 100 W lightbulb emits about 120 cd. Although the definition of candela is now based on the watt, which is a derived SI unit of power, the candela remains a base unit of the SI system [1] . Historically, the candela was defined in terms of the black-body radiation emitted by 1/60 of 1 cm2 of platinum at its melting point. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition. Candela | Define Candela at Dictionary.com candela noun, Optics. 1. a basic unit of luminous intensity in the International System of Units (SI), formally defined as the luminous intensity of a source that emits monochromatic radiation of frequency 540 × 10 12 hertz and that has a radiant intensity of 1/683 watt/steradian: adopted in 1979 as the international standard of luminous intensity. Abbreviation: Cd. Examples from the Web for candela Expand Historical Examples We then, guided by candela, hurried forward on our journey, keeping a sharp look-out lest the fugitive Indians should return.

British Dictionary definitions for candela Expand noun 1. the basic SI unit of luminous intensity; the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian cd Also called candle, standard candle Word Origin noun 1. Felix. 1910–97, Mexican architect, noted for his naturalistic modern style and thin prestressed concrete roofs Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Word Origin and History for candela Expand unit of luminous intensity, 1950, from Latin candela (see candle ).

Online Etymology Dictionary, © 2010 Douglas Harper SI Units Explained - The candela Go to the SI Units Explained Page How much brighter is a flashlight (torch) than a candle? Clearly it depends on how powerful the flashlight is, but there are other factors too. For example, the candle spreads its light in all directions whereas the flashlight is highly directional. This makes direct comparisons a little tricky, but not impossible. This is where the SI quantity of luminous intensity is useful. It has the SI unit candela and the symbol cd. The cendela (cd) How many candles are as bright as a flashlight? The next SI Unit is the kilogram (kg) . One of the extraordinary things about the kilogram is that it’s a real object, hidden in a nuclear bomb-proof bunker outside Paris. Other SI units are available from the menus at the top of the page. For much of history the candle was used as a measure of the intensity of light. This made sense in a world without light bulbs or other forms of artificial light, even if each country tended to use a slightly different kind of candle as its "standard".

Things started to get complicated not just with the invention of light bulbs, but also with a greater understanding of the different colours of light. For example, the Sun's light is composed of all of the different colours mixed together, but, as seen from space, is faintly yellow in colour, meaning that it shines slightly more brightly in yellow than the other colour components. For these and other reasons a better definition of luminous intensity was needed. The modern SI quantity of luminous intensity takes into account the colour of the light and its direction. It is based on a frequency (light colour) of 540 x 1012 cycles per second ( hertz ), which is roughly yellowish-green and is a colour that the human eye is highly sensitive to. Now the surprisingly easy bit. One candela is almost exactly equivalent to one candlepower (I did say it was easy!). So why is comparing a candle and flashlight tricky? To answer this we need to consider how the light spreads out from its source as well as taking into account a derived quantity called lumens . Candlepower and candela are both measurements of light at source, but neither tells us how powerful the light is some distance away from the source.

Instead, we measure the amount of light illuminating a surface area, which is called, naturally enough, the illuminance. The result is measured in lumens, with 1 lumen = 1 candela x steradian. For our purposes here we can think of the the latter term as an area, as the following example illustrates: Imagine a transparent 1 metre radius sphere surrounding a candle. Its surface area will be given by 4 pi r2, so the surface area of our sphere is: Seen from space the Sun is yellow in colour 4 pi 12 = 12.57 m2 The amount of energy passing through 1 square metre of the transparent sphere is 1 lumen, and so it follows that 1 candlepower is 12.57 lumens. Many bulbs are now rated by the amount of illumination given at a distance. In SI units the standard is 1 lumen per square metre and called 1 lux. Now let's return to flashlights, most of which are designed to be highly directional. If we have a 1 candela (i.e.

candlepower) flashlight which just happens to have a beam that illuminates 1 square metre from 1 metre distance then that square metre will have an illumination of 12.57 lumens, as compared with 1 lumen for the candle. This is why it's not straightforward to compare the output of a flashlight with a candle. In short, when we want to measure how powerful a light source is we need to consider its nature or purpose - a half candlepower flashlight will be more useful than a candle for finding something in the distance at night. In this case the candle has more candlepower (i.e. a high candela rating) at its source, but the flashlight's light is focused and so more useful. Finally, to put things into perspective, it's worth noting that a full moon on a clear night results in the Earth being illuminated by about a quarter of 1 lux at the Earth's surface. In comparison a typical living room will have an illumination of about 50 lux and direct sunlight results in anything up to about 130,000 lux at the Earth's surface. Seconds in a Day etc. Luminous intensity How much brighter is a flashlight (torch) than a candle?

Clearly it depends on how powerful the flashlight is, but there are other factors too. For example, the candle spreads its light in all directions whereas the flashlight is highly directional. This makes direct comparisons a little tricky, but not impossible. This is where the SI quantity of luminous intensity is useful. It has the SI unit candela and the symbol cd. The cendela (cd) How many candles are as bright as a flashlight? The next SI Unit is the kilogram (kg) . One of the extraordinary things about the kilogram is that it’s a real object, hidden in a nuclear bomb-proof bunker outside Paris. Other SI units are available from the menus at the top of the page. For much of history the candle was used as a measure of the intensity of light. This made sense in a world without light bulbs or other forms of artificial light, even if each country tended to use a slightly different kind of candle as its "standard". Things started to get complicated not just with the invention of light bulbs, but also with a greater understanding of the different colours of light.

For example, the Sun's light is composed of all of the different colours mixed together, but, as seen from space, is faintly yellow in colour, meaning that it shines slightly more brightly in yellow than the other colour components. For these and other reasons a better definition of luminous intensity was needed. The modern SI quantity of luminous intensity takes into account the colour of the light and its direction. It is based on a frequency (light colour) of 540 x 1012 cycles per second ( hertz ), which is roughly yellowish-green and is a colour that the human eye is highly sensitive to. Now the surprisingly easy bit. One candela is almost exactly equivalent to one candlepower (I did say it was easy!). So why is comparing a candle and flashlight tricky? To answer this we need to consider how the light spreads out from its source as well as taking into account a derived quantity called lumens . Candlepower and candela are both measurements of light at source, but neither tells us how powerful the light is some distance away from the source. Instead, we measure the amount of light illuminating a surface area, which is called, naturally enough, the illuminance.

The result is measured in lumens, with 1 lumen = 1 candela x steradian. For our purposes here we can think of the the latter term as an area, as the following example illustrates: Imagine a transparent 1 metre radius sphere surrounding a candle. Its surface area will be given by 4 pi r2, so the surface area of our sphere is: Seen from space the Sun is yellow in colour 4 pi 12 = 12.57 m2 The amount of energy passing through 1 square metre of the transparent sphere is 1 lumen, and so it follows that 1 candlepower is 12.57 lumens. Many bulbs are now rated by the amount of illumination given at a distance. In SI units the standard is 1 lumen per square metre and called 1 lux. Now let's return to flashlights, most of which are designed to be highly directional. If we have a 1 candela (i.e.

candlepower) flashlight which just happens to have a beam that illuminates 1 square metre from 1 metre distance then that square metre will have an illumination of 12.57 lumens, as compared with 1 lumen for the candle. This is why it's not straightforward to compare the output of a flashlight with a candle. In short, when we want to measure how powerful a light source is we need to consider its nature or purpose - a half candlepower flashlight will be more useful than a candle for finding something in the distance at night. In this case the candle has more candlepower (i.e. a high candela rating) at its source, but the flashlight's light is focused and so more useful. Finally, to put things into perspective, it's worth noting that a full moon on a clear night results in the Earth being illuminated by about a quarter of 1 lux at the Earth's surface. In comparison a typical living room will have an illumination of about 50 lux and direct sunlight results in anything up to about 130,000 lux at the Earth's surface.

Advertisement Lumens, Footcandles, Candlepower, Measuring Light Output CustSvc What today's consumers need to know about lumens The term lumen is a measurement of light output which consumers have a need to become more and more aware of. Back in the day, we went to the store and bought light bulbs. We had become used to what a 60 watt or 100 watt light bulb looked like and how much light they provided. We weren't concerned with lumens and didn't need to be. Things began to change with lower wattage incandescent lamps which provided the same light output, but with a bit less power consumption. It was a pretty easy adjustment to make from a 75 watt flood light to a 65 watt energy miser. Both were still incandescent bulbs and we got the idea that it's better to consume less energy. This is when lumen output was introduced to the consumer. Light bulb producers wanted to let us know that we were getting just as much light as we did before. Fluorescent tubes have been around for a long time, but when they were introduced in a form that could be used in a table lamp, we saw even lower watt consumption levels for equivalent light output.

Consumers became more and more conscious of lumens vs. watts. The 'ice cream cone' fluorescent lamps consumed even less than energy miser incandescent bulbs and produce the same amount of usable light. FACT LED light bulbs are brighter than ever, with excellent coverage and color rendering. Best of all, they've come way down in price. At last, the LED light bulb arrived on the scene. Now we are talking even lower power consumption for a comparable light output and those watt consumption numbers continue to go down. "Wattage" is no longer a valid reference point. "Lumens" is however, a valid reference point. That is a stable measurement of light output that will not vary as LED light bulbs continue to get brighter and more efficient. Lumens per watt is even more important. How much light output are you getting from a product and how many energy dollars (watts paid for on your electric bill) do you need to spend to get that light output? So here are some numbers for you to keep in mind when shopping for LED light bulbs. It won't be long before referencing incandescent bulbs is totally a thing of the past, so learn your lumen numbers now. The higher the number, the brighter the bulb.

For those of you who want to delve into the definition of lumens in a more detailed, technical manor, here is an article written for us some time ago by a professor, Robert (Doc) Bryant. It's entertaining while still very informative. Lumens, Illuminance, Foot-candles and bright shiny beads …. In defining how bright something is, we have two things to consider. 1. How bright it is at the source- How Bright is that light? 2. How much light is falling on something a certain distance away from the light. Lets' do some definitions now…… Foot-Candles - We're in America, so we are going to talk about units of measurement that concern distance in feet and inches. So, we will use some terms that folks in Europe don't use. We're going to talk about "foot-candles". This one's simple. Get a birthday cake candle. Get a ruler. Stick the candle on one end of the ruler. Light the candle. Turn out the lights. Sing Happy Birthday to Doc. It was his 47th on the 23rd. OK, quiet down. Enough of that nonsense.

One foot-candle of light is the amount of light that birthday cake candle generates one foot away. That's a neat unit of measurement. Why? Say you have a lamp. You are told it produces 100 foot candles of light. That means at one foot from the lamp, you will receive 100 foot candles of light. But here's where it gets tricky. The further away you move the light from what you want to illuminate, the less bright the light seems! If you measure it at the light, it's just as bright. But when you measure at the object you want illuminated, there is less light! A Physics teacher is going to tell you that light measured on an object is INVERSELY PROPORTIONAL to the distance the object is from the light source. That's a very scientific and math rich way of saying, the closer you are to the light bulb, the brighter that bulb is. Or, think of it this way. You can't change how much light comes out of your light bulb. So, to make more light on an object, you have to either move the light closer, or add more lights. Now, lets get to LUMENS.

A LUMEN is a unit of measurement of light. It measures light much the same way. Remember, a foot-candle is how bright the light is one foot away from the source. A lumen is a way of measuring how much light gets to what you want to light! A LUMEN is equal to one foot-candle falling on one square foot of area. So, if we take your candle and ruler, lets place a book at the opposite end from the candle. 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…….1 foot candle AND one LUMEN! Ahh, we've confused you. Let's split off from this and talk about the difference between RADIANCE and ILLUMINANCE. RADIANCE is another way of saying how much energy is released from that light source. 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. ILLUMINANCE is what results from the use of light. You turn your flashlight on in a dark room, and you light something up. That's ILLUMINANCE. Turning on a light in a dark room to make the burglar visible gives you ILLUMINANCE. 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!"). 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. Or you could use it to show how much light energy was reflected off the object itself. All this brings back two points. Well, three. The first point is if we measure the output of a light at the source that gives us one thing. The second point is that we use an entirely different unit of measure if we are measuring the results of that light's output. The third point 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.

More Confusion! Candlepower! 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? No. That's illuminance. Is it a measure of how well we see an object that is illuminated by that light source? No. That's something all together different, and we are not going there! Nowadays we use the term CANDELA instead of candlepower. Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up. Further confusing the matter is beam focus. That's how much candlepower can be focused using a reflector/lens assembly. 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. And here comes the confusion! A candlepower as a unit of measure is not the same as a foot-candle. A candlepower is a measurement of the light at the source, not at the object you light up.

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: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. The above from the National Institute of Standards Reference on Constants, Units, and Uncertainty. 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 12.57 square meters of surface area in such a sphere. Remember your Solid Geometry classes? That one candle (1 Candlepower/Candela) is illuminating equally the entire surface of that sphere. 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 12.57 times of that which fell on the square meter area. 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 12.57 lumens. And for you figuring out LED equivalents, first you must know how many lumens your LED's each produce. Then divide that value by 12.57 and you have candlepower of the LED. You don't have foot-candles, remember foot-candles are illuminance. And we are measuring radiance. Summing it all up: What’s the Difference Between Candela, Lux, and Lumens?

— 1000Bulbs.com Lighting Blog Lighting Terms It happens far too often. You walk into a store, pick up light bulb or a handheld spotlight and look at the specifications- only to find that one says lumens, another says lux, and the last reads something about candela. Inside, you question why no one uses the same system for measuring these things, and take your best guess based on the other items on the shelf. The short answer is that they’re all separate standards for measuring different kinds of lighting effects. Let’s break it down into the long answer to dispel a few more shadows. Candela Image used courtesy of Andrew Adam We’ll start with candela (cd) – also commonly called candlepower. Candela is the base measurement for describing luminous intensity. It tells you how bright the light source is which shows how far away from an object you can be and while still being able to see it. Any light source eventually becomes too dim to see the further away you are. This is different from lumens (a total light output) because it’s the value of light intensity from any point in a single direction from the light source.

Laser pointers or spot lights have the highest candela rating since the majority of their light is focused in a single direction. A simple comparison is that 1 candela is roughly equivalent to the light from a single candle. If you have a light bulb generating 1 cd and block part of the light, every direction not obscured still produces 1 cd. This is because the same intensity of light can be seen from any non-obscured direction at the same distance. Again, candela is the luminous intensity which describes how bright a light source is. Lux The Las Vegas hotel right? Wrong, that’s the Luxor. Lux (lx) measures illuminance, which is the amount of light on a surface per unit area. A single lux is equal to one lumen per square meter. If the lamp displays its brightness as a measurement of lux, it usually lists a distance from the bulb since any change in distance or bulb type changes the lux level. As an example, if you place a 100 lumen bulb in a flood light that shines on only one square meter of surface, that surface will be lit at 100 lx.

However, if you back the flood light away to shine on four square meters, the surface is now lit with 25 lx. Fun Fact: A full moon on a clear night can shine up to a single lux. In full daylight, indirect sunlight can illuminate a surface by between 10,000 to 25,000 lux. This is why seasonal affective disorder (SAD) lamps should be rated at 10,000 lux minimum. Lumens Image used courtesy of Roland Tanglao Lumens are now the most common measurement for a light bulb. The lumen (lm) is a measurement of luminous flux, or the total amount of visible light. To put it simply, the lumen rating is how much total visible light is produced by a light source. To show the difference between lumens and candela, let’s go back to the example previously used for candela with the partially obscured light bulb. For a bulb emitting 1 cd, that bulb would also have a luminous intensity of 12.57 lm. Obscuring half the bulb (making it a hemisphere instead of a full sphere), a 1 cd bulb will emit only 6.28 lm.

This is because lumens measure the total amount of visible light from a light source. So why all of the different ratings? Since candelas, lux, and lumens are all measuring something different, you can gain insight into how a lamp is useful. A laser pointer will have an extremely low lumen value but a very high candela rating, since a laser pointer doesn’t give off very much light but is visible from great distances. Light bulbs are usually listed in lumens to show how much illumination the uncovered bulb produces. And lamps will often display a lux value for a set distance to give you an idea of how bright your surface will be for task lighting. A good way to remember the differences between terms is: Lumens are how much light is given off Lux is how bright your surface will be Candela measures the visible intensity from the light source. Care to add your own insight, or just have a few questions on the math behind converting between measurements? Then comment below, or contact us using Facebook , Twitter , Google Plus , LinkedIn , or Pinterest !

Question: A candela is a measure of what?
Answer: Luminous intensity