Lumens vs candlepower: How to know the brightness of a light
beingzoe 4 months ago // 6 responses // Potluck Tips, tricks, and advice

So: for us old guys who cling to the English measurement
system. How many candle power equal a lumen?

Well Dave, your comment has sent my head spinning with regret about posting this video. After reading your comment I began to realize that the statements in the video, further expanded hastily by myself in the accompanying text, were vague and ambiguous.

So, like I should have done in the first place, I set out to corroborate and cite the facts on luminosity measurements.

Big mistake.

Dave you ask: How many candle power equal a lumen?

They don’t, sort of. This is where the confusion begins and ends. Let’s just look at some of the definitions I was able to find out there on the interweb.

Candela (from wikipedia) : 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.

The definition above is the accepted modern standardized definition. Prior to the 20th century each country had it’s own measure of light. And a wiseguy from wisegeek.com says this about the candela:

Initially, the candela was defined as the luminous emission of a Planckian radiator, a type of blackbody, at the temperature of freezing platinum (2045 K). This correlated roughly to the light emitted by a typical candle, making it a suitable measure. In the late 1970s, it was determined that the experimental difficulties in creating a Planckian radiator at such high temperatures made the existing definition of candela less than desirable. The new breakthroughs in radiometry allowed scientists to have a more specific definition, and so the current hertz/watts definition of candela was adopted.

One reason the watt was not initially tied to the candela is the eye’s differences in seeing various wavelengths of light. Since the human eye is less observant of blue and red light, more wattage is needed to produce the same result in the brain as with yellow or green light. This is why the definition of candela is defined as a very specific hertz, unachievable in actual common lighting. There are those who critique the modern definition of the candela for this reason, arguing that it has removed any common-sense understanding of the candela as an actual emission of light.

So from this I get that while we can measure candela precisely, we are measuring a frequency that is “unachievable in actual common lighting,” or, “we know exactly how much candela but it doesn’t mean a thing when I’m buying a light.”

So what about candlepower (the actual measure being denounced in the video):

Candlepower (from the wikipedia) : An obsolete scientific unit of luminous intensity based on the light emitted from a candle made to a specified formula.

Okay now we’re getting somewhere, and maybe the video makes a little bit of sense after all. Candlepower is an obsolete measurement unit. But wait a minute, I just found at a Yahoo answers answer for the question, “Explain candle power?”:

Simply put: it is the luminous intensity expressed in candelas. ( the brightness of light compared to a candle)

Um, but according to the wiki candela is a precise measurement however questionably relevant to the purchase of a flashlight. To the individuals credit who responded they included the entire definition and history of the candela in their answer.

Just to be safe I dug a little deeper on this whole candela / candlepower topic. I found this information at brightguy.com in a FAQ under the topic light measurement:

All light measurements can be traced back to the Candela, which is the unit of luminous intensity. The Candela is also sometimes called candlepower…

But I thought we just learned that candlepower is an antiquated and obsolete unit. Back to the wiki entry on candlepower:

Calibration of lamps: The candlepower of a lamp was measured by judging by eye the relative brightness of adjacent surfaces, one illuminated only by a standard lamp (or candle) and the other only by the lamp under test. The distance of one of the lamps was adjusted until the two were judged to give equal brightness. The candlepower of the lamp under test could then be calculated from the two distances and the inverse square law.

Since 1948 the term candlepower was replaced by the international unit (SI) known as the candela. One old candlepower unit is about 0.981 candela. Less scientifically, modern candlepower now equates directly (1:1) to the number of candelas — an implicit increase from its old value.

Now we’re seeing more clearly. Something we measured by “eye” to calibrate was given a precise value that we can the convert as approximately 0.981 candela. Right. Not to be a cynic, but I was given the impression as a child that we had this science thing down to a science and the older I get the more disllusioned I become (ask me sometime about how my views on school teachers changed as I grew up).

I’ll grant that in modern scientific vulgate candlepower can be translated as a 1-to-1 relationship with candela. Just be sure to not compare specs on your flashlight from the early twentieth century and the one you bought today and you should be safe.

Brightguy.com goes on to say:

Total Luminous Output may be expressed in candela or in lumens. This is a measure of the entire light output of the flashlight regardless of beam focus. It is almost solely a function of the lamp, and for practical purposes is equal to the lamp output which lamp manufacturers rate in Mean Spherical Candela (Sometimes called Mean Spherical Candlepower which is not the same as peak beam candlepower), or in lumens.

Finally a common denominator we can convert on: Total Luminous Output. Remember the comment on “peak beam candlepower” though, we’ll get back to that in a bit.

Now that we’ve covered candela and candlepower and are getting close to a conversion let’s look at the Lumen.

Lumen (from wikipedia): The lumen is the SI unit of luminous flux, a measure of the perceived power of light...If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, its total luminous flux emitted into that angle is one lumen. The lumen can be thought of casually as a measure of the total “amount” of visible light in some defined beam or angle, or emitted from some source…That is, a light source that uniformly radiates one candela in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.

ANSI lumen (from pcmag encyclopedia): A measurement of light that has been standardized by ANSI. It is commonly used to rate the brightness of a data projector. An ANSI lumen rating uses an average of several measurements taken across the face of the light source.

Woah, an ANSI lumen? Let’s hop back to the wiki entry for lumen:

The light output of projectors (including video projectors) is typically measured in lumens. A standardized procedure for testing projectors has been established by the American National Standards Institute, which involves averaging together several measurements taken at different positions. For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in “ANSI lumens”, to distinguish devices that have been so tested from those tested by other methods.

This all sounds good, but I am always suspicious when I see the phrase, “for marketing purposes.” But I trust ANSI and we aren’t necessarily talking about projectors here so we’ll just move on to Kelvin for a moment.

Kelvin (from querycat): Basic unit of thermodynamic temperature, color temperature, measuring the white value of a lights output. Higher Kelvin values are more white and lower values are more yellow. Over 5000k a light starts turning blue and begins to look like daylight.

The color temperature of a light is vital due to the fact that a white light of the same lumen value will be percieved as brighter to the eye than the same lumen value in yellow light. The article Low Light Levels / Human Eye Response at Venture Lighting says:

Lumens are the standard measure of light output, but light is actually defined as energy evaluated by the eye. Standard lumen measurements define the light output response of a person only during high light levels (called photopic light), typical of daylight and interior lighting. The light meter measures photopic light as seen by the central region of the eye. When light level is very low, like starlight at night, the viewing conditions are referred to as “scotopic.” Under these conditions, the eye’s visual response changes dramatically. Sensitivity to yellow and red light is greatly reduced, while response to blue light is vastly increased. If lamp lumens under scotopic viewing conditions have been determined using photopic measurements, the lumen value does not accurately measure the true amount of light production as perceived by the human eye.

BrightGuy.com corroborates this:

If two equally intense light sources differ in color temperature, the eye will perceive the whiter source (higher color temperature) to be brighter than the more yellow one. Color temperature is expressed in degrees on the Kelvin scale. A very white flashlight lamp will be rated around 3200 K.

I bring the issue of Kelvin and color temperature of a light only to point out that despite identical lumen values different colored light will be percieved differently thereby affecting your percieved happiness with your flashlight purchase. Though admittedly this has little to do with the question of how to convert candlepower to lumens. However, it seems relevant when scrutinizing what measurements and specs should be considered when purchasing a light from any manufacturer.

I would like to return to the concept of Peak Beam Candlepower. According to BrightGuy.com:

Peak Beam Candlepower is a measure of the brightest spot in the focused beam. It is a function of both the output of the lamp and the efficiency of the reflector. The focused spot of light has the same intensity that a bare source of unfocused light of the same candlepower would produce on the same area from the same distance. For example, a flashlight of 20,000 beam candlepower would project, within its “hot spot,” the same amount of light on a wall as would a bare lamp of 20,000 candela at the same distance. Typical values of beam candlepower will run into the tens of thousands for powerful rechargeable flashlights.

Since the Total Output system does not take into account the focusing efficiency of the reflector, there is no way to convert between peak beam candlepower and lumens.

So there is your real answer Dave. They can’t actually be converted except using candlepower as the common non-scientific term for candela.

BrightGuy.com goes on to say about Peak Beam Candlepower:

Peak beam candlepower is a subjective measurement because it uses the brightest spot of the beam from both the lamp and the reflector.

I believe this is what is being referred to in the video above. Different light manufacturers will measure peak beam candlepower differently resulting in wildly different results that are not necessarily accurate.

On the other hand, lumens can be used when comparing brightness because it is a measurement of the entire light output regardless of beam focus.

Before moving on to the brightest part of our research I need to include the definition for foot-candle.

foot-candle (from wikipedia) : A foot-candle is a non-SI unit of illuminance or light intensity widely used in photography, film, television, and the lighting industry. The unit is defined as the amount of illumination the inside surface an imaginary 1-foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a 1-square foot surface of which there is a uniformly distributed flux of one lumen. This can be thought of as the amount of light that actually falls on a given surface. The foot-candle is equal to one lumen per square foot.

So now we’ve added into the mix a non-standard unit commonly used by the lighting industry, the foot-candle. This will become relevant in the following excerpts from the illuminating article Lumens, Illuminance, Foot-candles and bright shiny beads…. by Robert H (Doc) Bryant at theledlight.com

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....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.

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!

The article goes on to discuss radiance (how much energy is released from a light source) and illuminance (the result of using a light, or what is lit up by the light). Which becomes important for comparing measurments. Later in the article

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 confuser! 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…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.

Bryant then goes on to actual give a conversion between lumens and candlepower:

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.

All of which makes a lot of sense and is summed up by Bryant:

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.

Divide the number of lumens you have produced, or are capable of producing, by 12.57 and you get the candlepower equivalent of that light source.

This all well and good, but at the LED center we find the most informative information of all in the instructions and disclaimer for their candela (millicandela) to lumen conversion wizard

To briefly explain the math the wizard does, it simply applies the conversion 1 candela = 1 lumen/steradian. To do this, it converts the beam angle you supply into a solid angle in units of steradians.

The likely thing for a manufacturer to do is to list their luminous intensity spec in the most favorable way possible. So it can be the case that this wizard provides inaccurate values—this happens because the assumption in the wizard is that the luminous intensity value you supply represents an average value across the beam angle you supply. It’s too tempting for the suppliers to list a maximum value instead, since there is not a standard for measuring this. Beware especially of wide-angle LEDs, which will probably be distorted the most by this calculation.

And I think that sums it up the best. Any measurement, like statistics, are reliant on the conditions and definitions of measurement. There are measurable equations and units to calculate light at the source and light at the thing you are illuminating. But due to shifting uses of these terms and units to describe these values, variable test conditions, and the uncontrollable urge for manufacturers to make their product look as good as possible, everybody is going to use whatever number is best not necessarily the standard or even the same exact testing environment.

In the video above the topic is candlepower versus lumens in terms of a finding a useful value to compare between individual lights and different manufacturers. The claim was made and is corroborated by my research that the term candlepower can and is used ambiguously. What exactly does one company mean when they say their light produces X candlepower? Do they mean candela? Or are they talking about Peak Beam Candlepower? This is what I think the video is trying to say however cursory.

But I’m no longer convinced that the lumen value as compared between manufacturers is necessarily 100% accurate either. I think it is going to be closer than candlepower or even candela comparisons, but I need more information on how different manufacturers are taking their measurements. I am running on the assumption that while not entirely subjective, the stated lumen value of any light is unlikely to be 100% accurate or directly comparable between manufacturers. But close enough in most circumstances.

And I say close enough is good enough if only because of the Kelvin color temperature issue. If only because of the Low Light Levels / Human Eye Response article that discusses how different colored eyes react differently to light. If only because a light will appear differently in varying lighting conditions. If only because human beings are so irrational sometimes that someone might say a light is brighter somehow than another in side by side comparison due to an unreasonable bias towards one manufacturer over another.

The bottom line becomes anytime you are comparing specs on a product to bear in mind how much you trust the source, how much you trust the manufacturer, how much you trust the context the information is being presented, how much you trust the industry those specs are being produced for, and how much you understand the science behind the measurements being compared.

By posting this video above it raised many questions in my mind, especially after Dave responded with his candlepower to lumen conversion question. Trader Dan and the CoTradeCo team might not know it all. And we might even get it wrong from time to time (hard to believe I know). But know that we trust our vendors and we are doing our absolute best to provide useful valuable information through the trading post. And this community blog post is a great example of how we can work together to learn something new. I will be contacting our vendors to get more information about how they measure the specs they present, especially for the various lights we sell.

I am not a scientist, math is not my strongest subject, and sometimes I interpret things wrong. If you have anything to add or correct from the information provided and cited here please post your comments and join the discussion. I hope eventually this discussion will turn into a definitive article (or at least the source for one) on measuring light especially as it relates to comparing flashlights and other types of lighting. Until then, I hope you understood all of this more than me.

Hey UK Case Lady, maybe you could shed some light on how Underwater Kinetics measures lumen values and other specs on their lights?

I want to thank the websites cited in this post. The fact is I went did far more than cite them, borrowing a lot of content to get a clearer view of this topic. Below are the domains that I used information from. There is a lot more good information to be found at all these sites (even at brightguy.com – one of our competitors) and if you want to learn more about any of these topics you should visit their sites.

• http://en.wikipedia.org
• https://www.brightguy.com/
• http://www.pcmag.com/
• http://www.querycat.com/
• http://www.venturelighting.com.au/
• http://www.theledlight.com/

And again I invite and encourage you to share your knowledge, insight, and expertise with the community here at CoTradeCo regarding this topoic. I feel like there is a lot more to know and can probably expressed far better than has been done here so far.

So, Pelican lights are bright sons of guns!

You make everything so concise and clear Dave! :)

Wow! That is a lot of information.

Zoe: Were there any follow-ups to this?

Just wanted everyone to know that I have a tentative appointment to interview the CTO (Chief Technical Officer) from UK Underwater Kinetics about the topics addressed in this post. I will also be contacting other sources including Pelican and other light manufacturers (and who knows maybe a scientist or two ;) in an attempt to shed some more light on Lumens and measuring light. I’ve been busy though and have not had a chance to follow-up yet.

If anyone has any other thoughts or information to share in the meantime, post your responses and comments below.