# Lumens, lux, candela measurements for LEDs



## Steelwolf (Nov 15, 2002)

Hi all. The gist of this post is to find out total-light-output efficiency, not beam shape characteristics. ie, power in versus number of photons out.

IIRC, lumens is the measurement for total light output with no regard to displacement. Candela measurements are influenced by the concentration of the hotspot. What about lux?

With modern LEDs, how do they compare? Running them at the recommended power levels, or at commonly accepted levels, what is the total light output of each type of LED?

e.g. 5mm Nichia (insert model number) @ 4V 40mA?
Luxeon Star 1W (insert model number) @ 3V 350mA = 17 lumens?
@ ?V 400mA = ?
Any other commonly available LEDs?


----------



## rlhess (Nov 15, 2002)

A light puts out candelas. Footcandles and lux measure light falling on a surface.

One candela at one foot provides one footcandle.

One candela at one meter provides one lux.

If you measure lux at one meter, you're measuring candela. If you measure lux at two meters and multiply by four, you're measuring candela (inverse square law). If you measure footcandles at 10 feet and multiply by 100, you're measuring candela. 

Candela and candlepower are the same thing. (Although mean spherical candlepower is something different, I think).

All of these measure light in a particular location.

Lumens measures the total flux coming off the light source. A perfectly spherical radiator of one candela will output 4*pi lumens.

Sorry, I can't help with specific measurements.

Lumens are generally measured in an integrating sphere.

Cheers,

Richard


----------



## lambda (Nov 16, 2002)

But, it's not that easy for what your trying to do; measure total-light-output efficiency.

*"Luxeon Star 1W (insert model number) @ 3V 350mA = 17 lumens?"*

True, for some J and K ranked Luxeons, but for instance a P2H ranked Luxeon would produce 23 to 30 Lumens at 350ma and only 3 to 3.25V. 

And a Q2H ranked Luxeon produces a whopping 30 to 40 lumens at 350ma while still only requiring 3 to 3.25V.

With LEDs sometimes not even apples and apples can be compared.

And for instance, do R and S ranked 1W Luxeons exist?

See http://www.lumileds.com/pdfs/AB21.PDF for Luxeon BIN codes.


----------



## highlandsun (Nov 16, 2002)

So go buy an integrating sphere and tell us what you find.

Or just accept the nominal specs from the manufacturer. For LEDs spec'd in (m)cd with a given beam angle the lumens out is 0.00024 * cd * (beam angle)^2. This will give you numbers that can be directly compared. Most superbrite LEDs are only 1 lumen at best. Luxeons are already spec'd in lumens so that part is easy enough.


----------



## Jonathan (Nov 16, 2002)

Note that Howard's equation is _approximate_, and is only correct for small angles. The equation approximates the conversion of degrees in a cone to steradians. It is a good approximation for things like 30 degree LEDs, and probably close enough for 60 degree LEDs.

I've placed an equation and a table for the conversion of cone degrees to steradians at http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=3;t=003424#000007

-Jon


----------



## Steelwolf (Nov 17, 2002)

Sorry Lambda, I haven't kept up with the development of Luxeons. I didn't know that they had various rankings that went to 30 and 40 lumens. Are those white? As I understand it, only the cyans ever get that bright. Then the next step is the 5W LS, but that takes in proportionately more power.

I was just trying to see if it was worthwhile building a huge array of LEDs to replace the lamp in a halogen uplight, probably using the 6400mCd or some surplus 5600mCd white LEDs that I have lying around, or else a bunch of LS. 

I wanted a light that was as bright, photon for photon, but consumes less power and produces less heat. The uplight is a dimmable type and at full power, I can forget about switching on the heater even with an open door in the midst of a West Australian winter. (I think it is a 100W lamp.)

I was thinking of saving money and rather than using a Xitanium driver or something similar, just have 60 of the 5mm LEDs (if I can get them cheap) lined up in one line with a current limiting resistor per line (maybe 3 lines of them), hooked up to a power bridge rectifier and a smoothing capacitor. This way, everything is kept simple and the dimmer switch still works. (I'll probably use a line of amber to keep the colour).

Dumb idea or worthwhile? How many 5mm LEDs or how many Luxeons will I need? Will I save power? Will I be wasting money on using LEDs instead of sticking to the halogen tubes?

Thanks for all the answers so far.


----------



## Jonathan (Nov 17, 2002)

For that scale of light power, you are much better off using something like a Metal Halide HID lamp. The biggest halogen lamps have efficiency which is on par with the best LEDs, so you won't save any power switching from halogen to LED...with the following really big caveat:

Halogen lamps (and incandescent lamps in general) get less efficient when they are dimmed. LEDs get more efficient. A halogen lamp operated at 1/2 full power produces about 20% full light. A Luxeon operated at 50% power would produce more than 50% full light; one of the early reports on the LS5W suggested that operating at 10% full power would produce something like 15% full light. If you want a lamp which will regularly be operated on 'dim', then LEDs are a better bet than halogen.

Additionally, colored LEDs are more efficient than white LEDs. If you had several strings of different colors, which you mixed to get the desired color, then you could have both dimming and adjustable color






To get the equivalent lumen output of a single 100W incandescent globe (1600 lumen), you would need about 1600 of the 5mm Nichia LEDs, but they would be more efficient. You could also use something on the order of 15 5W Luxeons.

-Jon


----------



## Steelwolf (Nov 17, 2002)

Jonathan: Thanks. That is precisely the simple practical explanation I was looking for. But you mentioned 1600 lumens for a *incandescent* globe. So a halogen, which is more efficient, will put out even more? I usually have the lamp around 50%-75% most of the time, so it probably isn't as efficient as it could be. However, once the bulb burns out I will replace it with a lower wattage bulb which would better suit my needs.

So we are looking at approx. 1 lumen per white 5mm LED. And we need 1600. That is a lot of LEDs. Won't be cost effective. Neither would using 5W whites at their current stage of developement since they don't have that many hours of life. Some of the other colours might be possible, but still looking at quite a huge outlay. Ditto for 1W luxeons.

I wonder... Hotfoot converted a halogen worklamp to use a bunch of 1W LS. I wonder if the resultant light output was comparable or dimmer. Got to search for that thread. This brings to mind that perhaps converting some of the larger incandescent flashlights may not be all that useful either.

Up to what point would it be, generally speaking, worthwhile to convert standard incandescents to 5mm LEDs? What about converting to 1W LS? And what about converting from halogens? Remembering that I want the conversion has to make sense in terms of the cost outlay versus the energy savings. 

BTW, I am only planning these conversions for dimmable house lights as I wish to retain that function, or handheld flashlights. For most other household lighting applications, I have already switched from the frightfully wasteful incandescents to compact flourescents.

Wait a second... 1600 LEDs running at nominally 4V 40mA each, if they were all in parallel, would draw 64A. P=IV Power consumption is 256W??? Something doesn't sound right.


----------



## Jonathan (Nov 17, 2002)

The light output and power consumption for the Nichias are just ballpark numbers. I was thinking along the lines of operating them at their normal voltage and current rather than pushing them, so figure 3.6V 20mA each.

For a 100W incandescent globe, you are looking at about 1600 lumen. For a 500W halogen linear tube light you are probably looking at 12000 lumen. It would take a rather large bunch of current LEDs to replace this.

But if you operate that 12000 lumen light at reduced output to get 6000 lumen, it is probably using 400W of power. If you are dimming the bulbs, then you can save lots of money by buying smaller bulbs. Don't bother waiting for the bulbs to burn out, buy the new ones today and put the oversized ones into storage as backups. Because the electricity used costs much more than the bulb using it, it essentially always pays to replace inefficient bulbs, even if they still work fine. Additionally, look for the bulbs that use Infrared Recovery technology; they usually cost about twice the regular bulbs, but use about 30% less energy to produce the same light.

LEDs are really _not_ cost effective versus halogen at anything over 5W, unless you can take advantage of the ability to dim them or otherwise electronically control them. And at 5W you are looking at a rather expensive fixture at the present time. IMHO LEDs are great for dim 'night lighting', very localized task lighting (a book light, for example), or portable lighting.

If you want to save money replacing a 500W halogen, then look at a 200W metal halide fixture. These are not crazy expensive devices at this scale, and are _much_ more efficient than halogen or LED lighting. HID seems esoteric and expensive because the portable HID applications get all the press. But mains powered HID systems have been around for quite a long time.

-Jon


----------



## Steelwolf (Nov 17, 2002)

Jonathon: Thanks for the simple advice. I guess general homelighting still has a ways to go. I really wanted an efficient dimmable lighting feature that could swing easily between full-on and mood lighting. I guess for efficiency sake, I'll need 2 different lights, or at least have a switch to select between a LED array for mood light and something else for full brightness.

I understand that flourescents can be dimmed using a variable inverter, but it is not good for the tube. Ditto for compact flourescents and CCFL. HID can't be dimmed at all. So dimming lamps will still need either incandescent or halogen (do they make xenon lamps for house lights?) above 5W and can use LEDs below that.

Is all that correct?


----------



## Doug S (Nov 18, 2002)

> Originally posted by Steelwolf:
> *I understand that flourescents can be dimmed using a variable inverter, but it is not good for the tube. Ditto for compact flourescents and CCFL. HID can't be dimmed at all. So dimming lamps will still need either incandescent or halogen (do they make xenon lamps for house lights?) above 5W and can use LEDs below that.
> 
> Is all that correct?*


<font size="2" face="Verdana, Arial">Regular Hot cathode lamps can be dimmed by special ballasts that heat the filament full time rather than just during starting. These would be relatively inefficient at low power levels [relative to lamp rated] as the filaments take a fair amount of power. CCFL can be dimmed at will without life reduction.


----------



## Jonathan (Nov 18, 2002)

Steelwolf, I think that you understand the efficiency issues correctly, although Doug S's comments on dim-able florescent lights apply.

In terms of 'xenon' lamps, I presume that you mean xenon/halogen fill incandescent lamps, and I'm sure you could get an appropriate Carley or Welch-Allyn lamp to use in ordinary 12V quartz-halogen home lighting fixtures.

One feature to remember with 'mood' lighting is that you want to change both intensity and _color_. A dimmed incandescent lamp has a really wonderful ruddy color that reminds one of sitting by a fire. Inefficient as hell in terms of photon production, but really pleasant in terms of setting the mood.

If your goal is this color of light at a dim level, then I think that LEDs might be the efficiency champs at the present time. Red and Red/Orange LEDs are currently the most efficient, and if you are dimming your light, you are already reducing the light output that you need to match. Lets go back to that 500W halogen:

The 500W halogen at full blast is putting out 12000 lumen. But we want to dim it down, so we are getting 2000 lumen of reddish light out of it. Using some halogen dimming equations (they are on the Welch-Allyn web site) I estimate that to get the 1/6 light output, you need 45% power, and have to operate the light at about 230W. 

That same 2000 lumen could be produced with 50 of the 1W red-orange Luxeon star units, which would cost about $500 or less in this quantity. However just selecting the red/orange LEDs would not produce a good enough spectral spread. You would want a mix of LEDs, heavy on the red, red/orange, and amber, with a few green and cyan thrown in (I have no idea what proportions, however.) I would bet that you could get the equivalent light and color of a 500W halogen dimmed to 230W, using less than 100W of LEDs at a cost of less than $1000 US, possibly less than $500 US. And next year it will be even cheaper





-Jon


----------



## Doug S (Nov 18, 2002)

> Originally posted by Jonathan:
> *That same 2000 lumen could be produced with 50 of the 1W red-orange Luxeon star units, which would cost about $500 or less in this quantity. However just selecting the red/orange LEDs would not produce a good enough spectral spread. You would want a mix of LEDs, heavy on the red, red/orange, and amber, with a few green and cyan thrown in (I have no idea what proportions, however.) -Jon*


<font size="2" face="Verdana, Arial">Reading this option, I wonder if mood-altering drugs might be cheaper


----------



## Steelwolf (Nov 18, 2002)

After reading all that, I think it would be cheaper, both in terms of running costs and setup costs, to use a few candles and a couple of hidden flourescents with colour filters. *sigh*

I might investigate the CCFL option a bit further. It would seem, though, that nothing beats a compact flourescent for home lighting, especially those sold as direct replacements for incandescents. Relatively cheap to install and run, and puts out plenty of light.

LEDs have to be currently confined to flashlight mods.


----------



## Doug S (Nov 18, 2002)

> Originally posted by Steelwolf:
> * It would seem, though, that nothing beats a compact flourescent for home lighting, especially those sold as direct replacements for incandescents. Relatively cheap to install and run, and puts out plenty of light.
> 
> *


<font size="2" face="Verdana, Arial">I agree with your conclusion. BTW, CCFLs do come in colors.


----------



## Steelwolf (Nov 18, 2002)

Confucious he say: Light of million stars drowned by light of single candle, but which is prettier?





Confucious he also say: Ugly women are hidden by dim light. Same for ugly men.





I wonder where to find CCFL lights in Australia.


----------



## Albany Tom (Nov 18, 2002)

Dimmable fluorescent isn't such a bad thing. Our new building has them in nearly every conference room, and they work great. The efficiency at full power is the same as a non-dimmable fixture, and they do use less power when dimmed than not. 

Relative efficiency isn't as important as you'd think, as people don't respond linearly to light anyway. Or, to put it another way, fiddling with the controls for the sake of personal taste makes way more difference than the lamp design in terms of power use when dimming the lights.

An advantage of dimmable fluorescent over incandescent is that the fluorescent don't change in color temperature. They stay white. Fluorescent are LOTS more efficient then incandescent. 

BTW - halogen IS incandescent. Halogen is slightly more efficient than non-halogen incandescent. 

HID hasn't taken off for home use probably because it's more complicated, and way hotter. The line power HID units I've seen usually have to be pretty far away from anything combustible. 

HID has an advantage over flourescent of being easy to put in a reflector unit, great or required for outdoor use, but not really needed for home use. HID also works much better in cold weather, but again not important for home use. Finally, HID allows more light in a smaller package, which is great for a football field, but not really needed in your den. HID's are the light of choice for arenas, stadiums, warehouses, supermarkets, etc - places that are best lit with floodlights.


----------



## highlandsun (Nov 19, 2002)

...it is better to light a single candle than to curse the price of Luxeons...


----------

