# Presentation: 194966

## Ultra-Efficient Solid-State Lighting:Performance Frontier, Progress, Challenges

**Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Security Administration under Contract DE-AC04-94AL85000.**

***Jeff Tsao***
***Physical, Chemical and Nano Sciences Center***
***Sandia National Laboratories***

- After “The World’s Appetite for Light”
- (Tsao and Waide, submitted to the Energy Journal)

**Candles**

**Gas**
**Kerosene**

**HID**
**Fluorescent**
**Incandescent**

**SSL**

**10****1.5**

**10****2.8**

**Notes:**
HELLO.  Good afternoon.  I’d like to give you today a high-level overview of SSL.  I’d like to cover three things: first, what we know now about the performance frontier of SSL (what is the best that SSL can do); second, give you a progress report on the status of SSL relative to that performance frontier; and third, describe and quantify what the most important technology challenges are, the surmounting of which would enable us to hit important intermediate targets enroute to that performance frontier.
TWO DIGRESSIONS.  Before I do start, though, a couple of things.
First, I tend to spend a lot of time on my viewgraphs, so if it seems like I’m going slow, don’t worry, I actually don’t have very many viewgraphs.
Second,  I’d like to spend a few minutes on an introductory digression, and put SSL into a larger historical context.  To do that, here I show a plot of luminous efficacy (in lm/W) and cost of energy (in $/MWh) for various nations or groups of nations at various states of technological development over the past three centuries.
Luminous efficacy, of course, is a measure of the efficiency with which power (Watts) is converted into light (lumens) visible to the human eye.
The operating cost of light is basically the cost of that power divided by luminous efficacy, and so decreases from the lower right to the upper left of this plot.  From 1700 until now, the operating cost of light has decreased by an amazing 4.3 orders of magnitude.  Of that, 1.5 orders of magnitude has been due to a decrease in the cost of energy.  But a much larger 2.8 orders of magnitude has been due to the increase in luminous efficacy, from less than a tenth of a lumen per Watt to nearly 100 lumens per Watt!
Luminous efficacy, of course, is a measure of progress, so this plot just quantifies what we all intuitively know: that lighting technology has made enormous historical progress: from candles, to gas and kerosene, to incandescent, fluorescent and high-intensity-discharge lighting.
And, now, SSL is poised to add its contribution to this progress.  It’s not the biggest contribution: over the past 300 years that honor probably belongs to incandescent lighting.  But it’s a significant one (potentially more than half an order of magnitude), made all the more significant by the increasing current importance of global energy consumption and energy efficiency.
SEGUE TO PERFORMANCE FRONTIER.  With that as historical context, let me start by discussing the first topic: what is the performance frontier for SSL?  That is, from what we know about the human visual system and the colors of the objects in the world around us, what is the upper limit to the contribution that SSL could make to this historical increase in luminous efficacy?

## SSL Performance Frontier

**SSL Performance Frontier**

**400                500                  600               700**

**Wavelength (nm)**

**800**

**400**

**0**

**Human Eye**
**Response**
**(lm/W)**

**90**

**70**

**50**

**1.0**

**0.5**

**0**

**0**

**0.4**

**0.8**

**Color Rendering Index**
**(R****a****)**

**Relative**
**Power**

**Relative**
**Reflectances Munsell Samples 1-8**

**Luminous Efficacy (lm/W)**

**375             400               425             450**

**8**

**1**

**7**

**2**

**6**

**3**

**5**

**4**

**7**

**8**

**CRI 90**

**408 lm/W**

**90**

**70**

**50**

**Color Rendering Index**
**(R****a****)**

**463**
**nm**

**530**
**nm**

**614**
**nm**

**573**
**nm**

- CCT 3,000K, after “Ultra-Efficient SSL” (J.M. Phillips, et al, Laser & Photonics Reviews, 2007)

**1/8**

**2/8**

**3/8**

**2/8**

- All linewidths
- 1-nm FWHM

**100 nm**

**200 nm**

**Notes:**
NEW RESULTS ON AN OLD QUESTION.  Now, this question has been asked several times over the past decade.  Here I show some new results based on a simulator developed by Yoshi Ohno at NIST.  This simulator optimizes the wavelengths and relative powers of four RYGB colors (that together create white) so as to maximize luminous efficacy for a given color rendering index.  Importantly, the simulator accommodates narrow-linewidth sources, and this turns out to make a difference.
INVERSE RELATIONSHIP BETWEEN LM/W VS CRI.  As you can see here, and as many of you already know, there is an inverse relationship between luminous efficacy and color rendering index.  Luminous efficacy is highest at low color rendering index, and lowest at high color rendering index.  The reason is that these two metrics are in competition with each other.
On the one hand, the human eye response peaks at 555 nm, with a FWHM of about 100 nm, so maximizing luminous efficacy means concentrating as much optical power near 555 nm as possible.
On the other hand, objects in the world around us have a wide range of colors, so maximizing the rendering of those colors means diffusing the optical power away from 555 nm.  To illustrate this, here I’ve drawn reflectance spectra of the 8 Munsell samples used to calculate the standard color rendering index, Ra.  You can see that the wavelengths at which their reflectances peak span a broader, 200-nm-or-so range from 450 nm to 650 nm.
So the result is that as CRI increases, the center wavelengths of the RYGB sources become more widely spaced away from 555 nm, and, as a consequence, luminous efficacy decreases.
PERFORMANCE FRONTIER.
Now, CRI’s of 70 are common for many SSL white light sources, like the ones found in flashlights, but they aren’t really good enough for general illumination.  CRI’s of 80 to 85 are considered good enough for many general illumination applications, but a CRI of 90 is considered good enough for all but the most demanding of general illumination applications.  So, to be on the conservative side, here I’ve drawn a dotted line at a magic CRI of 90.  At this CRI it is possible to achieve a luminous efficacy of 408 lm/W.
This is what we consider to be the performance frontier of SSL.  This frontier consists of four RYGB LEDs at the wavelength 463, 530, 573 and 614 nm, with power fractions of 1/8, 2/8, 2/8 and 3/8, and with linewidths of 1 nm.  Note that one could call this RYGB, but it’s really not.  The red wavelength, 614 nm, is so short that it is really orange.
HIGHER LM/W THAN BEFORE.  This is a higher luminous efficacy than was previously thought possible for SSL, all of which have been below 400 lm/W, most of them significantly below.  There are two main reasons for this.
The first is our use, as you can see here, of very narrow (1 nm) linewidth light sources.  Fatter linewidths, particularly in the red, cause a spillover of light into the wings of the human eye response and decrease luminous efficacy.  This is an important point, so let me place some emphasis on it.  There is no penalty, within the current framework for measuring color rendering, to using the narrowest linewidth light sources possible.  All of the Munsell samples, as well as the color receptors in the human eye, have relatively broad spectra, so LED light sources with linewidths less than 20 nm or so are behaving anyway as delta-function samplers of these spectra, so there is no penalty to making them even narrower.  These sources could be lasers, with even narrower linewidths.
The second is the use of a relatively low color temperature -- 3000K.  This is roughly the color temperature of an incandescent light bulb, which in home environments is quite pleasing.  If we had picked a higher color temperature, as many previous simulations did, the power fraction of the red component would go down, the power fraction of the blue component would go up, and because the blue component is further out into the wing of the human visual response than the red component is, luminous efficacy as a whole would go down.
CRI CAVEAT.  The only caveat I would make about all this has to do with the CRI metric.  Color rendering is based on a complex interaction between the human visual system, the environment that system is embedded in, and the illumination source.  There is a lot of ongoing work aimed at improved metrics for color rendering, and that work could easily modify somewhat this 408 lm/W figure.

## SSL Progress Report: Luminous Efficacy

**190**

**120**

**30**

**2006**

**2007**

**2008**

**lm/W**

**22**

**17**

**SSL Progress Report: Luminous Efficacy**

**RG****B****B**
**60% eff**

**400         550       700**

**R****B****G****B****B**
**60% eff**

**R****B****G****B****B**
**15% eff**

- SSL data from Luxeon K2 warm white lamps (Philips Lumileds 2008)

**RYGB**
**60% eff **

**400         550       700**

**400         550       700**

**400         550       700 nm**

**4x**

**1.6x**

**1.3x**

**14x**

- SSL performances calculated using white LED simulator 5-3 (Ohno, NIST)

**Incand**

**Fluor**

**HID**

**14**

**71**

**96**

**245**

**408**

***B LED***

***R LED***

***GY LEDs***

**Power**
**Spectra**

**Perf Frontier**

***614 nm***

**Notes:**
SSL PROGRESS REPORT: LM/W.  With that caveat, we consider 408 lm/W to be a reasonable estimate for the performance frontier of SSL.  So where is SSL now with respect to that performance frontier?  Here I draw a luminous efficacy progress line.
Over here at the far right is the 408 lm/W performance frontier.
Over here at the far left is the state-of-the-art in SSL for the past three years.  I’ll talk about this a bit more in the next viewgraph, but I’ve deliberately chosen data points for SSL lamps that are commercial, that have a high CRI, are most importantly driven fairly hard so they have a low cost of light.  These are the so-called warm white K2 LEDs from Philips Lumileds.
The state-of-the-art in mid-2008 for such lamps is a luminous efficacy of about 30 lm/W.  This is a factor 14x less than the performance frontier of 408 lm/W, so clearly SSL has a long ways to go.
TWO KINDS OF SUCCESS.  Of course, SSL doesn’t need to make it all the way to 408 lm/W to be successful.  In fact, there are two ways to define success, depending on what one’s perspective is.
If one’s perspective is that of the SSL lamp industry, then success means just beating traditional lighting.  And I emphasize just beating.  Once the SSL lamp industry has just beaten traditional lighting, it takes market share away from them, and presumably makes a lot of money selling lamps.  But assuming consumers are pretty happy with the amount of light they consume now, if the SSL lamp industry continue to increase luminous efficacy further, it could very well end up selling fewer lamps! Traditional lighting is a mix of incandescent (14 lm/W), fluorescent (71 lm/W) and HID (95 lm/W) lighting, so from this perspective it’s sufficient to achieve something like 120 lm/W.  That is something like 30% of the performance frontier.
But if one’s perspective is global energy consumption (decreasing it, or at least enabling it not to increase as much), then success doesn’t mean just beating traditional lighting, it means getting as close to the 408 lm/W performance frontier as possible.  That means trying to achieve much more: say, 245 lm/W, which is something like 60% of that performance frontier.
TECHNOLOGY CHALLENGES.  Either way, it is interesting to ask: what are the technology challenges that, if surmounted, would enable achieving these targets?  Well, here are what I think of as “the big three,” from most important to least important.
BLUE LED.  The first technology challenge is the blue LED – in particular, the challenge of maintaining high efficiency even in packages designed and driven at high currents.
To see the magnitude of this challenge, at the lower left I’ve drawn the power spectrum of the white LED corresponding to the 2008 SSL data point.  It has a 20 nm wide peak associated with the blue LED, and two much broader (roughly 100 nm) peaks associated with the phosphors that absorb the blue and emit in the green and red.  In fact, the commercial blue LED in this lamp is only about 15-20% efficient, again at the high drive currents necessary for low ownership cost of light.
One of the main reasons (but not the only reason) for this is the so-called droop problem, in which efficiency drops as drive currents increase.  This is an important challenge right now in SSL, and one that many groups are working on.  In fact, if one could get to 60% efficiencies, with exactly the same phosphors, then you’d have a power spectrum like this, 4x larger, and you could get to luminous efficacies of 120 lm/W.  This isn’t so difficult to imagine, though it certainly won’t be easy. Laboratory blue LEDs at low drive currents now have efficiencies in the 60-70% range.  And high-power semiconductor lasers now have efficiencies in the 80% range.
RED LED.  The second challenge is develop a red LED (or, perhaps, more accurately, an orange LED, since the red wavelength that optimizes the balance between luminous efficacy and color rendering is actually a very short wavelength red, around 614 nm).
The reasons this is important are twofold.  You eliminate the Stokes shift loss associated with transforming blue light into red with a phosphor, and you eliminate spillover of light into the deep red where the human eye isn’t very sensitive.
The power spectrum of a white light source based on such a red emitter is shown here, where all we’ve done is replace the 100-nm linewidth red phosphor with a 20-nm linewidth red LED.  But the efficiencies of the blue and red LEDs we’ve kept at 60%.
When you do this, you increase luminous efficacy from 120 lm/W to 190 lm/W.  This is only a factor 1.6x, so it’s much smaller than what you get from solving the blue LED problem.  But it’s still significant, and is the next most important challenge after the droop challenge.
I think of this as the forgotten challenge.  Many think that semiconductor red has been solved, since there are of course very good LEDs at 650 nm or so.  But the optimal wavelength for the red is 614 nm – that’s too short for the AlInGaP materials system, but too long for the AlInGaN system.
GREEN AND YELLOW LEDS.  The third challenge is to develop green and yellow LEDs.
Again, the reasons are twofold. You eliminate the Stokes shift loss associated with transforming blue light into green and yellow with a phosphor, and you concentrate your light into those wavelengths where the human eye is most sensitive.  These are exactly the same benefits that we received from replacing the red phosphor with a red LED, just a little less so.  The reason is that the Stokes shift loss from the blue to the green and yellow is less than that from the blue to the red, and the sharpening of the lines in the green and yellow, though it helps a little, is not as important as that in the red, since the green and yellow are reasonably well matched to the human eye response anyway.
The power spectrum based on such green and yellow emitters is shown here, where we’ve replaced the 100-nm linewidth green phosphor with 1-nm linewidth green and yellow LEDs, and replaced the 20-nm linewidth blue and red LEDs with 1-nm linewidth blue and red LEDs.  But the efficiencies of the LEDs we’ve kept at 60%.
When you do this, you increase luminous efficacy from 190 lm/W to 245 lm/W.  This is only a factor 1.3x, so it’s even smaller than what you get from solving the red LED problem.  But again it’s still significant, and is the next most important challenge after the droop and orange LED challenges.

## SSL Progress Report: Ownership Cost of Light

**SSL Progress Report: Ownership Cost of Light**

**0.01             0.1                   1                   10               100**

**Operating Cost ($/Mlmh)**

**Capital Cost ($/Mlmh)**

**100**

**10**

**1**

**0.1**

**0.01**

**Incandescent**

**Fluorescent**

**HID**

**SSL**

**2006**

**2007**

**2008**

**1:6 Ratio**

- Traditional lighting data from survey of 2001 U.S. market (Navigant)
- SSL data from Luxeon K2 warm white lamps (Philips Lumileds 2008)
- Cost of light calculated following “Light Emitting Diodes (LEDs) for General Illumination” (J.Y. Tsao, Ed., OIDA, 2002)

**60% eff RG****B****B**

**60% eff R****B****G****B****B**

**60% eff RYGB**

_**Costs**_**:**
**Ownership  =  Operating  +  Capital**

***Electricity***

***Luminous***
***Efficacy***

***Power***
***Delivery***
***Cost***

***Lifetime***

**Notes:**
SSL PROGRESS REPORT: COST.  So that’s a progress report on where SSL stands with respect to luminous efficacy.  But luminous efficacy isn’t the only important metric for success.  There is cost as well, and one often hears that the cost of SSL is what is keeping it from being competitive against traditional lighting.  In fact that’s true, but only partially so.
TWO COSTS.  To see this, here I’ve plotted the two main costs associated with lighting.
The operating cost – how much it costs you to run your light bulb, day after day – is the bottom axis of this plot.  It’s basically the cost of electricity ($/MWh) divided by luminous efficacy (lm/W).
The capital cost – how much it costs you to buy your light bulb or lamp, amortized over its lifetime – is the left axis of this plot.  It’s basically something I like to call the power delivery cost ($/W), which is how much it costs you to make a lamp that sinks a certain amount of power at the chip, divided by luminous efficacy (lm/W), the lumens you create given the power you’re sinking at the chip, and lifetime (Mh).
Both of these costs have the same units, $/Mlmh, as does their sum, the ownership cost of light.  The ownership cost of light, to zeroth order, is what counts in terms of market competitiveness.
TRADITIONAL LIGHTING.  To orient you to these costs, here I’ve plotted traditional lighting as the circles in red, blue and green.  The data are from a comprehensive survey by Navigant of lighting in the U.S. in 2001.  Each circle represents a lamp type, and the areas of the circles are the market sizes of the lumens produced by that lamp.
Take incandescent lighting, in red.  Its luminous efficacy is pretty low (14 lm/W), so its operating cost is relatively high.  Delivering power to light bulbs is pretty cheap (about a penny per Watt), but because its luminous efficacy is so low (14 lm/W) and its lifetime so short (1,000 hrs), the capital cost amortized over the life of the lamp ends up being pretty high. Now, to enable us to compare incandescence to other technologies, this red curve represents a constant ownership cost of light curve, where the ownership cost of light is the sum of the operating and capital costs.  Points up here have very high capital cost but very low operating cost.  Points down here have very high operating cost but very low capital cost.  But they all have the same ownership cost of light as incandescent lighting.
Now take fluorescent and high-intensity-discharge lighting.  Their luminous efficacies are higher (71 and 95 lm/W), so they are further to the left in operating cost than incandescence.  Delivering power to fluorescent and HID lamps is more expensive (about a nickel a Watt), because these lamps are much more sophisticated.  But because their luminous efficacies are so much higher and their lifetimes so much longer (10,000 hours), they are down in capital cost, again amortized over the life of the lamp, compared to incandescence.  And, just as for incandescence, these blue and green curves represents constant ownership cost of light curves, but for fluorescence and high-intensity-discharge lighting.
CAPITAL TO OPERATING COST RATIO.  One interesting thing to note is that for traditional lighting, the capital cost of light is about 6x lower than the operating cost of light – most of these data points lie on a line offset downwards on this log-log plot by 6x.  Basically what this means is that lighting is a true energy service, for which the dominant cost is due to consumption of energy.  The lamp itself is only a small part (1/6) of the ownership cost of light.
LM/W AND $/W.  Another thing to note is that the operating and capital costs respond differently to improvements in technology.  Luminous efficacy improvements cause both operating and capital costs to decrease, and so cause movement on this plot diagonally down and to the left.  Power delivery cost improvements cause only capital cost to decrease, though, and so cause movement vertically down.
SSL.  So where is SSL on a plot like this?
Here I show the last three years of SSL, again drawn from state-of-the-art commercial devices driven hard so that they have low ownership costs.  You can see that most of the progress has been vertically down.  That is, most of the progress has been in reducing power delivery cost.  In fact, that progress enabled, in 2007, the capital cost to be, for the first time, lower than the operating cost, and the ownership cost to just nudge below that of incandescence.  And if the economics of SSL ends up being similar to that of traditional lighting, power delivery cost has a factor 2.2x to go before the capital cost exactly reaches 1/6 of the operating cost, and at that point the ratio is so low there isn’t much point in decreasing it further.
The remaining progress, for SSL to catch up with and exceed fluorescent lighting, has to be diagonally down and to the left – that is, in luminous efficacy.  So the major remaining challenge in SSL is increases in luminous efficacy, not decreases in power delivery cost.  These are the factors of 4x, 1.6x and 1.3x that we illustrated on the previous viewgraph and that would enable getting to the three targets illustrated here.

## Key Points

**Lighting technology evolution in perspective**
**Over 3 centuries, 2.8 orders of magnitude improvement in luminous efficacy**
**SSL has potential to add final 0.6 orders of magnitude**
**Performance frontier of SSL**
**408 lm/W at CRI 90 and CCT 3,000K**
**1/8 Blue 463 nm, 2/8 Green 530 nm, 2/8 Yellow 573 nm, 3/8 Red 614 nm**
**Narrower lines better, particularly in the red**
**SSL Minor Challenge: Power Delivery Cost**
**4x    from 2006-2008 has reduced capital/operating cost ratio to 1/3**
**2.2x from higher drive currents and lower manufacturing costs will easily bring capital/operating cost ratio to 1/6, same as traditional lighting**
**SSL Major Challenge: Luminous Efficacy**
**1.8x from 2006-2008 has been only modest improvement**
**4x    from maintaining high efficiency at high flux in blue LED droop will beat traditional lighting**
**1.6x more from 60% efficient red LED at 614 nm**
**1.3x more from 60% efficient green and yellow LEDs at 530 and 573 nm**

**Notes:**
Let me close by summarizing the key points.
First, it’s good to have some humility with respect to SSL.  Over 3 centuries, lighting technology has improved its luminous efficacy by 2.8 orders of magnitude.  SSL has the potential to add a final 0.5 to 0.6 orders of magnitude to that.  That’s not unimportant, of course, but in the larger sweep of history is relatively minor.
Second, the performance frontier of SSL is 408 lm/W, higher than previous estimates have been.  That means there is more headroom than we thought before.  The characteristics of this perfect SSL light source are: 1/8 of its power is in the blue at 463 nm, 1⁄4 of its power is in the green at 530 nm, another 1⁄4 of its power is in the yellow at 573 nm, and a final 3/8 of its power is in the orange at 614 nm.  The narrower the lines the better, particularly in the red.
Third, most (about 4x) of SSL’s progress these past three years has been in power delivery cost.  There’s still more progress to be made here, but soon SSL will reach a capital to operating cost ratio of 1/6, and there won’t be much point in decreasing that ratio further.
Fourth, less (about 1.8x) of SSL’s progress these past three years has been in luminous efficacy.  So there’s a lot more progress to be made here.  Eliminating droop would get you 4x, developing an efficient orange LED would get you another 1.6x, and developing efficient green and yellow LEDs would get you another 1.3x.
Thank-you.