Technical - Photometric Data Guide

Close Projection Limit

The distance (m) from the fixture to a test board (veneered plywood painted black) is measured using 60C (ambient temperature of 30C) as the temperature for preventing discolouration and deterioration of fibres and other materials.

Terms used to describe the properties of light

TermUnit of measurementMeaning

Luminous fluxlm (lumens)Amount of light emitted from a lamp.

Luminous intensitycd (candela)Strength of light (amount of light emitted in a unit solid angle in a given direction).

Illuminancelx (lux)Brightness of surface being lit. Used as a basic guideline in lighting design.

Luminancecd/m2(candelas/square meter)Intensity of an object as seen from a given direction. (Where illuminance expresses how much light is reaching a given unit of area, luminance expresses the resulting visible brightness when seen from given direction.)

Terms used to describe lamps

TermUnit of measurementMeaning

Rated lamp powerW (watts)The power consumption of a lamp. Used on labeling and catalogues.

Lamp efficiencylm/W(lumens/Watt)Value derived by dividing the lamps total lumious flux by its power consumption (1amp power). This property indicates the luminous flux (measured in lumens) generated by 1 Watt of power.

Rated lifetimeh (hours)Lifetime published in catalogues, derived by average rating the lifetimes of multiple lamps tested under stipulated testing conditions. These stipulated test conditions vary by lamp type and are based on several standards, including the average operating time that elapses until the total luminous flux reaches a stipulated percentage and the operating time that elapses until a stipulated percentage of lamps stop working during a period of continuous operation.

Total luminous fluxlm (lumens)Amount of light discharged by the light source in all directions. Total luminous flux figures given as initial characteristics indicated the luminous flux following 0 hours of operation from standard and halogen bulbs, or following 100 hours of operation for bulb fluorescent lamps, fluorescent lamps, and high-intensity discharge lamps.

Colour temperatureK (Kelvin)Numerical representation of the colour of the light generated by the light source. Colour temperature values decrease as the lightbecomes redder and increase as the light becomes bluer. Light from different light sources may differ slightly (ie: appear to have a stronger red or blue cast), even if the colour temperatures are the same.

Average colourrendering indexRa (rendering average)Numerical representation of the reproductibility (way of seeing) of the colour generated by the light source. This property serves as an indicator of how faithful the colour appears when compared to the reference illuminant (Ra 1000) established by the JIS standard. The Ra unit is not synonymous with a colours desirability; pleasant colours may have low Ra values. 168 INDEX Photometric Data Guide

Calculating illuminance from product data

The illuminance of an illuminated surface is inversely proportional to the square of the distance from the light source.

Axial luminous intensity (cd) Square of distance (m) = Illuminance (lx)

Illuminance at point A 3.600 22 = 900 (lx)Illuminance at point A 900 2 = 450 (lx)(1/2 illuminance at point A)

Illuminance at point C 3.600 32 = 400 (lx)Illuminance at point D 400 2 = 200 (lx)(1/2 illuminance at point C)

Direct Horizontal Illuminance

One-half illuminance angleTop half of the graph

Indicates the spread of light when a light is shone downward onto a horizontal surface, the 1/2 illuminance angle refers to the angle at which illuminance directly under the light is reduced by 1/2.

Left side of the graphIndicates the relationship between the spread of the fixtures light and its illuminance (lx). The angle and light spread shown in the graph indicates 1/2 illuminance, indicating the 1/2 illuminance angle () and the centre illuminance for each height level.

The graph indicates that for a light source of 2m, illuminance directly under the fixture is about 950lx, with a 1/2 illuminance of 2550 x 2080.Indicates the lamp used for measurementsand the total luminous flux (lm) per fixture.

Right side of the graphThe direct horizontal luminance data indicates the range within which the horizontal illuminance expressed by the curve can be obtained, using distancefrom the fixture as the X-axis and horizontal distance from a point directly underneath the fixture is the Y-axis.

The graph indicates that for a light source height of 2m, an illuminance of at least 500lx can be obtained in an area with a radius of 1.2m.

This graph expresses the relationship between light spread for an upward facingfixture such as an outdoor floodlight and horizontal luminance (lx). (Hotizontalluminance differs from vertical luminance.)

Light distribution curve

This graph indicates the light distribution when the spread and strength of the light from the fixture are the same in all cross-sections.

The light distribution curve is a graph that expresses strength (luminous intensity) of light from the fixture in all directions. Values read from this graph are for a lamp with a luminous flux of 1,000lm; the actual luminous intensity can be calculated using the following formula:

(Luminous intensity)Indicated luminous intensity X Lamp luminous flux(cd)

1,000

Wall luminance distribution(incl. Wall washer models)

This graph indicates illuminance distribution for a fixture installed 0.9m from a wall, as shown in the diagram to the right.

Utilisation factor

The utilisation factor table indicates how much of the luminous flux produced by the lamp in the fixture enters the work plane under a variety of conditions.

Room index = W x L(W L) x H

W : Width (m)L : Depth (m)H : Height of light source from work plane

The room index obtained from the above formula is used in combination with the reflection rates of the ceiling, walls and floor to obtain the utilisation factor from the utilisation factor table.

The following formulas are used to calculate average illuminance andthe required number of lights for a given set of conditions using the luxmethod.

Average illuminance E = F x N x U x M

A

Required number of lights N = E x A

F x U x M

E : Average Illuminance (lx)F : Lamp luminous flux (lm)U : Utilisation factorA : Floor area (m2)N : Number of lampsM : Maintenance factor

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Introduction to Lighting Design

How To Read A Photometric Report

Published:May 2010ByCraig DiLouie

In a perfect world, a lighting manufacturer would respond to interest in one of their products by assuming the cost of installing samples in an exact mockup of the actual space being designed. Then, the manufacturer would hire people to work there for a while and conduct a postoccupancy survey on their satisfaction with the lighting.

In the real world, we have photometric reports. Commonly available for specification-grade lighting products, these reports are found on the catalog sheet. What a report says about a lighting fixture can be used to predict how it is likely to perform in a given application and help us choose the right fixture. Specifically, we can determine how the light is distributed, how efficiently it is distributed and how likely it is to produce glare or unwanted patterns.

What are the basics that we need to know, so we can properly read and interpret photometric reports? The most important items on the report are the candela chart and the candela distribution curve, which give us a picture of the fixtures distributed lighting pattern. All the other items on the report, such as zonal lumen distribution, fixture efficiency and fixture spacing criteria are derived from the numbers in the candela charts table.

(Note that this article focuses on Type A photometry, which covers indoor general light fixtures, not Type B photometry, which is used for floodlighting and other outdoor fixtures.)

FundamentalsImagine that we are looking directly at the cross-section (end) of a pendant-mounted linear direct/indirect lighting fixture in an open office. The fixture has a vertical axisan imaginary line running through its center from nadir at 0 (a point on the ground directly below the fixture); up to 180 (a point on the ceiling directly above the fixture). It also has a horizontal axis that runs through its center from 90 to 270. From our position at the end of the fixture, we can take measurements of light intensity, measured in candelas, at any angle of elevation from 0 to 180, which are called vertical viewing angles. In practice, these measurements are taken in manufacturer or independent testing facilities using a device called a goniophotometer.

We have now determined the light intensity values for a single vertical plane intersecting the cross section of the fixture at its center. If the fixture emits light in a perfectly symmetrical pattern in all directions, this would be enough to evaluate the fixtures lighting distribution. But most fixturesfrom 2-by-4 troffers to linear direct/indirect pendants to wall washersdo not.

This means we need to repeat the process of measuring light intensity at 0 to 180 from different positions around the fixture to create more vertical planes and get the complete story. Instead of looking at the fixture from the side, now we must look at it from the top and draw an imaginary line through its center. Typically, measurements are repeated at an angle parallel to the lamp axis (0), 22.5, 45, 67.5 and perpendicular to the axis (90), with 0, 45 and 90 being the primary angles. These are the horizontal viewing angles. Ninety degrees gets us a quarter of the way around the fixture and is enough to give us a complete picture if the fixture has a standard symmetrical geometric shape.

The result is a mapping of light intensities at different combinations of vertical and horizontal viewing angles. Visually, this three-dimensional pattern would look like an oddly shaped bubble. Change the fixtures reflector design, shielding and even just its lamp/ballast combination, and this bubble will morph into a new shape.

The candela chartAll of the above data is available in the photometric reports candela (cd) chart. The horizontal viewing angles (0, 22.5, 45, 67.5, 90) are the column headings and the vertical angles (0 to 180 in increments) are the row headings. Figure 2 shows an example for a direct/indirect fixture. If we are facing the fixture perpendicularly (90 horizontal viewing angle) with our eyes at a 55 vertical viewing angle from the fixtures 0 line (nadir), then the relative light intensity at that point is 234 cd. If we then circle the fixture until we are facing its cross section (the end of the fixture, a 0 horizontal viewing angle) at a 55 angle with the center of the fixture (vertical viewing angle), then the relative light intensity at that point is 109 cd. At a 55 viewing angle, the fixture emits more than twice the amount of light intensity on a 90 vertical plane than a 0 vertical plane.

The candela chart is important because it can be used for detailed analysis of a fixtures distribution of light and its impact on lighting levels and potential glare conditions using lighting design software. For this purpose, many manufacturers make the data available as downloadable electronic files on their Web sites. These files are typically based on a standard format created by the Illuminating Engineering Society (IES), which is why they are often called IES files.

Note that the candela chart is generated based on a specific fixture and lamp combination, so a three-lamp T8 fluorescent fixture report will not apply to the two T8 version of that same product, nor will it apply to a three-lamp T5 fluorescent model. Further, the ballast used in the test is a reference ballastwhich means the lighting output is reported as though the ballast were delivering 100 percent of the rated test lamp lumensso the actual ballast factor will have to be applied as a light loss factor. Similarly, if the lighting output of the specified lamps is different than those used in the photometric test on which the report is based, further adjustment will be necessary.

The candela distribution curve is a graphical representation of relative light intensity for a single vertical plane based on candela readings across the vertical viewing angles (0180) for a single horizontal viewing angle (see Figure). Since the distribution of light intensity varies based on the horizontal viewing angle, several patterns may be overlaid on top of each other; in this example, the pattern at a 90 horizontal viewing angle is shown as a solid dark line, a 45 angle as a lightly shaded line, and a 0 angle as a dashed line. The lines radiating from the center of the fixture are the vertical viewing angles from 0 to 180. The concentric circles represent candle-power, with each progressive outward circle being a larger candela value.

While not as precise as the candela chart, the candela distribution curve can provide much of the same useful information and in an at-a-glance visual format. For example, looking again at Figure 2, suppose we would like to avoid a light intensity exceeding 300 cd at 55 to 90 vertical viewing angles because of glare concerns. Doing some simple eyeball estimating, candle-power is around 200 cd at 55 on a 90 vertical plane, 150200 cd at 55 on a 45 vertical plane, and less than 100 cd at 55 on a 0 vertical plane.

Useful interpretationsLooking at the photometric report, probably the easiest thing to note is whether the fixture is direct (the light is emitted below the horizontal axis), indirect (the light is emitted above the horizontal axis), or direct/indirect (a mix of the two, and to what degree). The fixture in Figure 2 emits 64 percent of its light output up and 36 percent of it down.

We can also tell whether distribution is symmetrical (light output is emitted in a roughly equal pattern on both sides of the fixture) or, as is common with cove lights and similar fixtures, asymmetrical (light output is restricted to one side or the other). If the fixture has symmetrical distribution on both sides, only half of the drawing may be shown, as in Figure 2.

Additionally, we can tell whether the fixture has a spot distribution (narrow pattern), narrow and medium flood (fuller pattern and a flatter bottom), or wide flood (wide pattern and possibly a batwing shape where peak distribution is on each side of the center instead of directly above or below the fixture). We can tell whether the fixture is likely to produce a smooth light pattern (smooth, rounded candela distribution curve) or streaking on walls or the floor (striations in the pattern). And we can tell whether the fixture is likely to produce glare (a high concentration of direct light intensity is being emitted above a 60 vertical viewing angle). An experienced eye can learn even more than that at a glance.

Other interesting data in the photometric report are derived from the light intensity measurements, such as zonal lumen summary and fixture efficiency.

The zonal lumen summary table lists the fixtures light output, in lumens, in specific zones and then summarizes for all light emitted down (090), up (90180) and total (0180). These values are used to calculate the fixture efficiency, the percentage of lamp light output in lumens that exit the fixture relative to the total lamp lumens that go into the fixture. Fixture efficiency is the sum of all zonal lumens nominal lamp light output 100. The fixture portrayed in Figure 2, for example, has an efficiency of 90.4 percent. But while higher efficiency is generally better, we must consider where that light is going to determine if the emitted light is actually useful. Unshielded fluorescent striplights can be as efficient as 95 percent, but would be considered a glare bomb by office workers. There is often a tradeoff between fixture efficiency and optical control: The more the fixture works to deliver light where it is wanted and block it where it is not wanted, the lower its efficiency will be.

Initial cost, aesthetics, ability to provide target light levels, and lamp/ballast efficiency are all important considerations when choosing a lighting fixture. But they say nothing about how the fixture will actually perform in the space, and what impact it will have on the people who use the space for work or leisure. What we really need to know is how the light is distributed, how efficiently it is distributed, and how likely it is to produce glare or unwanted patterns.

Its all in the photometric report.

DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached atwww.zinginc.com.Luminous Measurement Graphic RepresentationThe collection ofluminous intensityemitted by a source of light in all directions is known asluminous distribution. The sources of light used in practice have a more or less large luminous surface, whose radiation intensity is affected by the construction of the s ource itself, presenting various values in these scattered directions.

Special devices (like the Goniophotometer) are constructed to determine the luminous intensity of asource of lightin all spatial directions in relation to a vertical axis. If luminous intensity (I) of a source of light is represented by vectors in the infinite spatial directions, a volume representing the value for the total flux emitted by the source is created.

Such a value may be defined by the formula below:

Photometric solid is the solid obtained.Fig. 1shows an incasdescent lamp photometric solid.

Figure 1 - Incandescent lamp photometric solid

If a plane passes through the symmetric axis of a source of light, for example, a meridional plane, a section limited by a curve, knownas photometric curve, or luminous distribution curve is obtained (SeeFigure 2).

Figure 2 - Photometric curve for an incandescent lamp.

By reviewing the photometric curve of a source of light, luminous intensity in any direction may be determined very accurately. This dataare necessary for some lighting calculations.Therefore, spatial directions through which luminous radiation is irradiated may be established by two coordinates.

One of the mostfrequently used coordinate systems to obtain photometric curves is the C y represented inFig. 3.

Figure 3 - C - y coordinate system

Photometric curves refer to an emitted luminous flux of 1 000 lm. Generally speaking, the source of light emits a larger flux. Thus, thecorresponding luminous intensity values are calculated by a simple ratio.

When alampis housed in a reflector, its flux is distorted, producing a volume with a marked shape defined by the characterist ics of thereflector. Therefore, distribution curves vary according to different planes. The two following figures show two examples where distributioncurves for two reflectors are represented.

Fig.4reflector is symmetric and has identical curves for any of the meridional planes. This is the reason why a sole curve is enough for its photometric identification.

Fig. 5reflector is asymmetric and each plane has a differentcurve. All planes must be known.

Figure 4 (left) - Symmetric photometric distribution curve; Figure 5 (right) - Asymmetric photometric distribution curve.

Another method to represent luminous flux distribution is the isocandela curve diagram (Fig. 12). According to this diagram, luminairesare supposed to be in the center of a sphere where exterior surface points with the same intensity are linked (isocandela curves).

Generally, luminaires have, at least, one symmetric plane. This is the reason why they are only represented in a hemisphere.

Figure 6 - Isocandela curves

This representation is very comprehensive. However, more experience is needed to interpret it.

The flux emitted by a source of light provides surface lighting (illuminance) whose values are measured in lux. If those values areprojected on the same plane and a line links the ones with the same value, isolux curves are formed (Fig. 7).

Figure 7 - Isolux curves

Finally, luminance depends on the luminous flux reflected by a surface in the observers direction. Values are measured in candelas persquare metre (cd/m2) and are represented by isoluminance curves (Fig. 8)

Figure 8 - Isoluminance curves

Luminous measurement summary chart

Chart 1. Luminous measurement summarySOURCE:Lighting Engineering 2002 Indalux

Lux to Total Lumens or Foot-Candles to Total Lumens Converter:

Need to Convert Foot-Candles to LUX or LUX to Foot-Candles Instead? Click on THIS link for That Converter.

Click on this Link to Use Converter and/or Download Converter

NOTE:Converting between geometry-based measurement units is difficult and should only be attempted when it is impossible to measure in the actual desired units. You must be aware of what each measurement geometry implicitly assumes before conversion. Any results with this converter must be considered approximate.

Image from Chapter 7 - Measurement Geometries -Light Measurement Tutorial

STEP 1:Enter theORIGINALmeasurement taken in lux or foot-candles with yourlight meterandsensor. This number can be entered in decimal format (i.e. "0.000341") or in scientific notation (i.e. "3.41e-04"). Select what units (foot-candles or lux) the measurement was taken in.

STEP 2:Enter theMEASUREMENT DISTANCEat which the reading in Step 1 was taken, in this box. This number can be entered in decimal format (i.e. "0.000341") or in scientific notation (i.e. "3.41e-04"). Select whether the distance is in meters or in feet. If you are measuring in units less than meters or feet (i.e. millimeters or inches) you will have to convert these numbers up to meters or feet first.

STEP 3:Enter theSHADOW DEGREES (solid angle)in the numerical stepper here. This is the solid-angle of light that is occluded or blocked by the lamp base.This angle can be any number between 0 (perfectly isotropic source) to 120 degrees. This can be a difficult property of a light source to measure, which is a contributing factor for why any conversions done with this calculator are to be considered approximate and should only be used as a last resort when measuring lumens in an integrating sphere is not possible. In many cases, the default of 30 degrees is sufficient for the expected accuracy of this conversion process.

OUTPUT:This is theAPPROXIMATE TOTAL LUMEN OUTPUTof the lamp. Again, this calculator makes several assumptions which directly affect the accuracy of the conversion. Since this is a measurement geometry conversion - this cannot be helped. This calculator assumes that the light source is a point-source and is isotropic (output is the same in all directions) in nature with the exception of the losses due to the lamp base which are again, assumed, to be about 30 degrees solid-angle. The lamp is also assumed to have a clear envelope. Variances from these assumptions will lead to additional error in the conversion process and could invalidate any results (i.e. in the case of trying to convert an illuminance reading from an area source).

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 have to be. Things began to change with lower wattage incandescent lamps which provided the same light output, but with a bit less power consumption. 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.

At last, theLED light bulbarrived 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 toLUMENS.

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.1foot candleAND oneLUMEN!

Ahh, we've confused you. Let's split off from this and talk about the difference betweenRADIANCEandILLUMINANCE.

RADIANCEis 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.

ILLUMINANCEis what results from the use of light. You turn your flashlight on in a dark room, and you light something up. That'sILLUMINANCE. Turning on a light

in a dark room to make the burglar visible gives youILLUMINANCE. It also gives you another problem when you note the burglar is pointing your duck gun at your bellybutton.

Illuminanceis 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 infoot-candlesagain! And when people talk aboutLUX, it's illuminance measured in metric units rather than English units of measure. To reinforce that,LUXis the measurement of actual light available at a given distance. Aluxequals onelumenincident 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 afoot-candle. Being lazy, we use it all over the place.

More Confusion!Candlepower!

Candlepoweris 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'silluminance. 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 termCANDELAinstead ofcandlepower.Candlepower, orCANDELAis 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 isbeam focus. That's how muchcandlepowercan 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 theilluminancewill be higher.

And here comes the confusion! Acandlepoweras a unit of measure is not the same as afoot-candle. Acandlepoweris a measurement of the light at the source, not at the object you light up.

And acandelais 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: Thecandelais 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.

Candlepoweris a measure of light taken at the source-not at the target.Foot-candlestell us how much of that light is directed at an object we want to illuminate.

Now, lets convert thelumens, a metric unit of light measurement, tocandlepower.

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 (1Candlepower/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 onelumen. 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.

LUXis an abbreviation forLumens per square meter.Foot-candlesequal the amount ofLumens per square feetof area.

So, thatone candlepowerequivalent equals12.57 lumens.

And for you figuring out LED equivalents, first you must know how manylumensyour LED's each produce. Then divide that value by 12.57 and you havecandlepowerof the LED. You don't havefoot-candles, rememberfoot-candlesareilluminance. And we are measuringradiance.

Summing it all up:

Candlepoweris a rating of light output at the source, using English measurements.

Foot-candlesare a measurement of light at an illuminated object.

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

Divide the number oflumensyou have produced, or are capable of producing, by 12.57 and you get thecandlepower equivalentof that light source.

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 ofilluminationto a measure ofradiation!

This has been an ideal proof of the superiority of the metric system. Then again, the metric system is a product of those wonderful folks that brought us:

Renault, Peugeot, Citroen, and Air busses. Not to mention simply awful Bordeaux.

And, if you're happy with this, send those little gems to:

Robert H (Doc) Bryant 3408 Thomas Ave Midland, Texas 79703-6240

I hope you have enjoyed this as much as I have. You ought to see me up in front of a classroom. My classes are absolute laugh riots. But people learn!

Doc Bryant is not an employee of TheLEDLight.com