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Multi-colored exterior wall washer lights. Courtesy led24.de
LEDs are quickly expanding into the world of architectural lighting into many specific applications once occupied by metal halide or tungsten halogen lamps (think MR16 sized) such as spot lighting, marker lighting (stairs, railings, doorways etc..), and wall washing. Its this last one that I want to talk about here – particularly the pitfall of color temperature as it applies to white LEDs.
Wall Washing? I don’t even clean my windows!
Wall washing is when a light or series of lights, usually but not always colored, are positioned and often aimed (physically or through the use of lenses) to spread a wide swath of light onto a wall. Colors can be used for moody effects and colors can be blended from lamp to lamp even within a single fixture. You’ve seen this a lot on the exteriors of large buildings – particularly modern designs or highly stylized buildings such as theaters, casinos, hotels, arenas, etc… Sometimes wall washing – really a sub- type of effects lighting – is used to great effect indoors as well – not only on larger expanses of vertical walls (you typically see larger metal halide or halogen lamps used for very large walls) but equally well on smaller walls, particularly when using LED fixtures.
Why use LEDs for wall washing?
Well for several reasons really. Generally the same reasons for using LEDs in most any application they can fit into – cost savings on both maintenance and energy use, heat reduction, ease of dimming/cycling, and size reduction. I’m not going to go into a full-on discussion of LEDs versus lamps (I’ve already done that) here but I will talk briefly about their advantages in wall washing applications. Thanks to the LED manufacturers (particularly Cree and Osram) and their latest generation of high powered LEDs with their much increased power output – LEDs can be put to the task of replacing the typically hot, energy hogging, high maintenance metal halide and halogen spot lamps. Here are some comparisons in no particular order:
LEDs are much more efficient in their energy use than halogen or metal halide lamps leading to a cost savings in energy over time.They are much smaller even when considering the use of additional focusing lenses for wall washing/spot applications, they do produce heat but far less heat and what is produced is thrown out the back and can be easily directed away from the fixture (the fixtures themselves – often extruded aluminum can be heatsinks in their own right) – so the light itself is not hot whereas a metal halide spot light tightly focused on a wall could melt the paint right off if care isn’t taken. This also means they can easily get wet outdoors since an LED wall washer throws no heat in the light, the lens or window sealling off the LEDs won’t heat up and crack when hit with cool water like it can with lamps throwing lots of heat.Due to the really small size of LEDs (even high powered ones), one can create substantially smaller fixtures – even when considering the relatively small MR16 type halogen lamps the LED fixture is generally smaller.LEDs last, on the whole, waaaaaayyyy longer than lamps. Halogens typically last under 2,000 hours of continuous use and metal halides typically under 10,000 hours but almost all LEDs have lifetimes of 100,o00 hours of continuous use. While I’m on that subject – the lifetime of a halogen or metal halide lamp is defined as “when it’s dead and won’t turn on anymore” where as lifetime for an LED is defined as 70% or less of it’s initial output” meaning LEDs won’t typically just blink out but will dim and/or color shift when they are considered at the end of their life. So, due to the extreme lifespan of LEDs – you get a huge boost in the maintenance savings since your not out replacing bulbs all the time. Kewl
Since LEDs are solid-state devices, they can be turned on and off at will with no ill effects whereas turning lamps off and on can over and over again shotens their lifespan. They can also easilly be dimmed but so can lamps – in fact lamps typically only need a small rheostat to dim whereas LEDs need a more complicated pulse width modulation module (PLM module) for dimming. Since the power supplies (called drivers) are solid-state themselves, they are typically small and can allow for all sorts of new, innovative features such as computer control and real-time feedback on LED parameters plus LEDs are all low voltage DC where as lamps, especially high-powered wall washers, tend to be AC although a lot of low-voltage halogen lamps are used but their output is not particularly ideal for large wall washing applications.
What are some of the pitfalls of using LEDs in wall washing applications?
Well, the initial cost of LEDs is generally much higher than traditional halogen or even metal halide fixtures and due to that initial cost, it can take a long time in cost savings from efficiency to pay for the expense. Power output – lets face it, it takes way more LEDs together in an array (at much greater cost) to make the same power output of a modest halogen spot lamp which costs like a tenth of what the LED array would. The more LEDs you use, the more drivers you will need and if your utilizing dimmers or computer control, more LEDs increases the complexity of the wiring. Lastly, color temperature – which isn’t really a problem with lamps, can be a BIG problem with LEDs. Colored LEDs aren’t as much of a problem but color temperature variation really comes into play with white LEDs. Non-colored lamps tend to be very consistent in color temperature between lamps but due to the complexity of manufacturing and how white LEDs are made, you can get considerable variance which must be delt with as I recently learned.
Color Temperature Variations
Well, this is the crux of this application note and the reason I am writing today. Color temperature. Color temperature (given in degrees Kelvin) refers specifically to the visual rendering of a white light as compared to that of an ideal black-body source where the temperature of the black body source causes it’s light output to match that of the compartive light source. The “warmer” the color temperature, the more yellow it appears (but the lower the actual color temperature 2700 – 3300 K – kind of weird huh?) and the “colder” the temperature, the more bluish it looks but the higher the color temperature is (+5000 K). What may look white to you at first glance may not be – it could be bluish, yellowish, or variations in between. Halogen lamps are black body sources and tend to be yellowish and metal halide lamps tend to be quite bluish but they tend to be quite consistent from lamp to lamp due to the physical properties of how they generate light (really, REALLY, hot elements and gasses) . LEDs being solid-state devices and there can be considerable variation in the manufacturing process. “White” LEDs are typically made by taking a blue LED chip and coating it with a yellow phosphor that glows or phosphoresces when hit by the blue light but phosphors of various colors (green, red, blue etc…) can be stacked in various thicknesses on top of various base LED chips (sometimes near UV chips are used) to create “white” LEDs. It’s precisely this amount of variability that can lead to color temperature problems in white LED applications. Generally, you would want to stick to one LED type from one LED manufacturer to eliminate possibly using different types of phosphor coated LEDs but even then the LED manufacturing process can produce considerable variability even among one LED type from the same manufacturer. Generally, LEDs are produced in large batches of 10's of thousands of LEDs and all the LEDs from a single batch tend to but not always have similar color characteristics. Through the use of automated spectrophotometric equipment, LED manufactures can futher sort or bin their LEDs by their specific color temperatures – putting the more white LEDs together, the yellowish LEDs together, and the bluish LEDs together in smaller batches or bins. Depending on the manufacture and how sophisticated their equipment is and how demanding their customers tend to be – even amongst individual color bins you may still find some variation. The quality of binning tends to vary from manufacturer to manufacturer and it is often directly proportional to how many LEDs you order.
Prominence of color temp variation in LED color rendering varies with distance between fixtures & between LEDs themselves
Generally, you can get more picky about color binning when ordering huge numbers of LEDs and you have no say at all really when order less than a 1,000 pieces – something to consider. Generally, however, the variability from LED to LED within the same lot or reel tends to be ok with a few deviants. Its when you start mixing and matching from various reels/lots that you can really run into problems. In practical use, you may not ever notice that a
“white” LED isn’t quite white depending on it’s use – with spot illumination applications like down lighting and task lighting you may not notice particurlarly where you tend to have one LED or a very tight cluster of LEDS all emitting a narrow cone of light or the spot fixtures themselves aren’t that close to each other but the color shift is particularly pronounced in wall washing applications
where the light fixtures are very linear and you may have several inches between LEDs (preventing their output mixture until several inches to feet up the wall) and the fixtures tend to be put one after another in long strings. Walls tend to be like a blank canvas and where the light tends to be right up against that canvas, you really start to notice the color.
Practical Lesson
I know all of this since I recently worked on an LED wall washing application where the color of binned LEDs shifted all around even though the LEDs all came out of the same bin. The binning wasn’t terrible (it could have been better) but our contract manufacturer didn’t consitently use all of the LEDs from a reel before moving on to the next when populating the module boards. We had hundreds of linear LED modules with 6 high power LEDs on each and each with a prismatic, directional lens on them. Each was supposed to be a nice warm color around 3000 K. However, what we got were many, many LEDs that produced amber even dark amber LEDs that when placed up against a white wall, looked yellow – I mean you’d call these “yellow” LEDs
The effects of color temperature variation of white LEDs in wall washing applications
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Plus we got some that were pinkish in color, greenish, and even orangish in color yet they all came from the same bin. These colors, even many inches above the actual LEDs would mix and create unattractive hues on the wall but were particularly noticable right above the LED. See my ugly illustration to the upper right to get an idea of what happens. Anyway, since the boards were all made up with the LEDs on them through no fault of our own, we quickly had to use our expertise and start pulling these surface mount LEDs off them and doing our own “binning” to get boards with uniform color outputs from each of the six LEDs. Was this a nightmare? Yes, but a lesson learned – check LED color temperatures wherever possible even if the LEDs all come from the same bin when the application demands it, like color sensitive wall washing applications. Make sure that lot/bin numbers are carefully checked when assembling devices from LEDs and stick to one lot or bin when placement of LEDs will allow for color mixing such as a linear array.
Conclusion
Well I hope my thoughts or rants made some sort of sense to someone. Let me know what you think. Have you had your own experiences with lousy color temperature variation in white LEDs – let me know?
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