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Thread: LED fixture design

  1. #1
    Flashaholic* idleprocess's Avatar
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    Default LED fixture design

    After pouring over deadrx7conv's excellent index thread (I can only imagine how much effort went into compiling that list!), I've realized that building functional fixtures isn't a problem for most of us ... but making them somewhat presentable is.

    I see two common approaches:
    Repurpose an existing fixture
    -OR-
    Make that 2.00" x 0.125" barstock as pretty as possible (or hide it somehow)

    I have a problem with designing a decent-looking fixture myself and have tended towards the second approach. I have a long-languishing project that uses frosted plex diffusers sized 2" larger than the barstock in all directions mounted on standoffs ... and the longer it sits the more I realize it's just not going to cut it aesthetically in public areas in addition to its decidedly un-fun installation sequence. Here's a mock-up (before I re-cut the plex to be square).

    I had considered going with some conventional flushmount fixtures for incans / CFL's, but those are both enormous, dependent on a J-box in the ceiling that I don't want to install, and will have poor-at-best thermal characteristics.

    Is there any easy way to solve this problem short of obtaining and learning how to operate a mill and/or lathe? The fixtures I built for my more recent (and actually completed) outdoor lighting project work better, but aren't much prettier (and stay out of sight anyway).

    I like those fixture a bit better in the sense that they're smaller and capture the stray light from the side (which is a bit painful to look at on my interior lighting from the wrong angle).

    Perhaps I'm just not being imaginative enough. I have some ~2.5" OD / 0.25" wall clear tubing that I could experiment with as an alternative means for diffusing the LED if I could devise a means to neatly cut it in half lengthwise, but relatively little of it and no access to more.

    This project seems to make the most of barstock - and with some brushing would look really nice. Loses some effective lumens from the ceiling bounce but also diffuses nicely and seems to achieve some aesthetics through economy of design.



    Where am I going with all of this? I'd like to design some secondary lighting that can light a room with enough light to navigate, perhaps read a book by with a handful of watts per room - operated from DC, which might some day be fed by solar. These fixtures should be fairly low-key but also be pleasing to the eye should I need to sell the house someday or eventually have other occupants. No fans, no bare LED's, and diffused light. They don't need to be miracles of lumen extraction nor efficiency. The LED equivalent of the (apparently now-discontinued) Lithonia Cirkel would be grand, but that might be a bit much to shoot for.
    I apologize that this letter is so long; I did not have time to write a short letter

  2. #2
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    Default Re: LED fixture design

    I think the easier way to achieve a good looking luminary that captures side light is, as you imagined, with an semi cylindrical diffuser. A glass cylinder would be difficult to cut, but not a Plexiglass one. You just need a nice pair of screw ends to get a good looking.

    Indirect lighting gets a very nice looking effect in general. If you want general lighting and not high iluminance on a task plane, it is the way to go. Painting the ceiling with a high reflective, diffuse (Lambertian) paint would help in reducing losses and avoid hot spots. There are some available, but you can make it too with any matte white latex paint and barium sulphate (or cheaper, baritine).

    Another option you have not mentioned is using a light guide. LEDs on a side (or two), using a U aluminium bar, which fixes too the textured guide (to distribute light down uniformly). Problem with this option is I do not find them on sale to individuals, just OEM. But maybe looking harder...this option makes easy to do a very good looking lamp with excellent efficacy. It is possible to combine this with the indirect lighting: the base aluminium of LEDs for uplight acts as reflector for the light guide, allowing to use available light guides of simple materials,as the Plexiglass EndLighten.

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    Flashaholic* LEDninja's Avatar
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    Default Re: LED fixture design

    I think having the LEDs on top of the barstock doing a ceiling bounce is a good idea. You avoid looking at the LEDs. Even with a diffuser you will see brighter points of light. And the barstock is in the middle of the room getting lots of cooling air.

    Cree does something similar in their CR14, CR22, CR24 fixtures. The LEDs (05) are on top of the bar (04).

  4. #4
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    Default Re: LED fixture design

    Ceiling bounce, reflector bounce, through lens, or some kind of frosted diffuser is a must. It all depends on who you live with for an interior decorator.
    Presentable??? definitely needed to convince the prettier half.

    I went through almost every thread in this fixed lighting forum. I know I missed a few here and there. Slow day and there was plenty of beer in the fridge. I was crushed when I found that earlier thread. So, instead of creating a new one thread, I just bumped it up like a newbie.

    I think many of us will be stuck with "re-purposing an existing fixture". Those of you creative enough not too re-purpose, just keep posting those pictures so that the rest of us can dream(learn).

    Not most efficiency, or coolest looking, but COB LED's might be worth looking into. http://www.led-tech.de/en/Chip-On-Board-Technic-c_134_0.html
    A rectangular cob strip, on bendable copper water tubing(for wire runs) might be worth building, similar to http://cgi.ebay.com/120789507956



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    Default Re: LED fixture design

    These remote phosphor panels are definitely worth looking at if you're making your own fixtures. I bought a few to play with. They seem to make a nice, even diffuse light when excited by a blue or royal blue LED.

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    Default Re: LED fixture design

    Quote Originally Posted by jtr1962 View Post
    These remote phosphor panels are definitely worth looking at if you're making your own fixtures. I bought a few to play with. They seem to make a nice, even diffuse light when excited by a blue or royal blue LED.
    Great! I didn't know chromalit is available in retail. Definitively, a very easy and effective way of building our own fixtures. Using the Cree XT, it could get very good efficacy figures.

    So have you checked there is no glare at all? At any distance from LED or it needs to be placed at some minimum distance?

  7. #7

    Default Re: LED fixture design

    Couple suggestions here.

    For DIY Rails, you can 'punt' and point them at the ceiling and this will provide a nice even room glow. The problem is that you need a *lot* of lumens, like several thousand so that you can read or work effectively, plus the light tends to be very ambiguous. Frankly I'd use a different design for this with fewer, higher powered LED's and radial heatsinks for better convection. It would look pretty nice with a bit of care and wouldn't be that expensive if you used XMLs or Bridgelux.

    For downfiring LEDs you can use acrylic diffusers used in fluorescent ceiling fixtures, and they work ok, but it's really utilitarian and doesn't rate high in the spousal approval zone. Then again neither do fluorescent ceiling fixtures. What I do for downfiring rails is space LEDs about 6-12" apart on 1-1/2 C-channel, then mount the widest optic I can find over the LEDs. I then get a piece of 3" wood trim and mount it over the rail with holes drilled over the LED openings and then stain the wood according to taste. This look pretty good in most living spaces, provides smooth yet direct lighting, and if you use thin wire to hang them they look like they are floating. Hint - use the hanging wires to power it. It's low voltage DC.

    Floor lamps are a bit trickier, but I've been working on some cool designer ideas and I'll pass along some basics. Years ago I found out the best light for a room was a large 3-4' photographic umbrella on a decent looking light stand held horizontal about 7' high, and then shooting CFLs on book clamps into it. LED's work even better because if you position them close enough to the umbrella you can use them without optics and gain maximum efficiency. The effect is a soft light source that's local to one area of the room and easily brighter than a table lamp, but also provides ambient light for the entire area. It also looks pretty nice, if not a modern 'art house' kinda feel. The light though is nothing less than amazing.

    This then goes into the zone of using cloth panels wrapped over wood to deflect light, or pass through, or other things. Another simple idea would be to build a nice looking frame of wood or metal about 2' high, set a 12" ceramic tile on top with the whitest and glossiest finish you can find, and mount upfiring LED's at it on radial heat sinks. This is similiar in concept to the umbrella lamp, but smaller for a table and will shoot more light down.

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    Default Re: LED fixture design

    Quote Originally Posted by Kinnza View Post
    Great! I didn't know chromalit is available in retail. Definitively, a very easy and effective way of building our own fixtures. Using the Cree XT, it could get very good efficacy figures.So have you checked there is no glare at all? At any distance from LED or it needs to be placed at some minimum distance?
    I haven't experimented too much yet with optimum distances but it seems 5 to 10 mm away might be optimum. There are application notes which go into all this in more detail. Provided you have a good mixing chamber design, it's possible to have evenly lit phosphor panels.

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    Flashaholic* JohnR66's Avatar
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    Default Re: LED fixture design

    Quote Originally Posted by jtr1962 View Post
    I haven't experimented too much yet with optimum distances but it seems 5 to 10 mm away might be optimum. There are application notes which go into all this in more detail. Provided you have a good mixing chamber design, it's possible to have evenly lit phosphor panels.
    Very cool product! I need to get an order in. The strip seems cost effective as you can cut it down to several squares, circles or whatever is needed. If as good as claimed, these can be a real positive force for LED lighting acceptance.

    I'm unclear about the 50K hours storage life. It seems to be use it or lose it as it has a shelf life. At least it is not very expensive and can easily be replaced.

    Jtr, please post some pix if you can.

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    Default Re: LED fixture design

    Quote Originally Posted by JohnR66 View Post
    Very cool product! I need to get an order in. The strip seems cost effective as you can cut it down to several squares, circles or whatever is needed. If as good as claimed, these can be a real positive force for LED lighting acceptance.
    Yes, the strips seem to be the most useful, although the circles would work well for downlights.
    I'm unclear about the 50K hours storage life. It seems to be use it or lose it as it has a shelf life. At least it is not very expensive and can easily be replaced.
    I think that 50K hours might apply under worst-case storage conditions (95°C and 90% humidity). This is just plastic with phosphor coating on one side. I can't see that it would degrade in 50K hours under normal storage conditions.
    Jtr, please post some pix if you can.
    Will do as soon as I get the chance (kind of busy lately). So far I haven't really played around much. I bought 10 royal-blue Rebels (LXML-PR02) along with the panels just for some fun. I might try my hand a maybe making a strip light, or perhaps a down light, when I have some more time. I think the Cree XT would work even better with these, but unfortunately Cutter was out of them last time I ordered.One thing I like about the remote phosphor concept is that it should bring down the cost of LED lighting. I've noticed royal blue emitters selling for less than half what whites go for. Even adding in the cost of the phosphor panel, overall total fixture cost is still lower, plus you get inherent diffusion.

  11. #11
    Flashaholic* JohnR66's Avatar
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    Default Re: LED fixture design

    How thick is the plastic? I'm either missing it or it isn't given in the datasheet. Thin enough polycarbonate can be cut with ordinary scissors, otherwise roto-tooled.

    Getting the phosphor off the LED die can mean good things (more than just fixture cost) as I'm sure you are aware.
    Last edited by JohnR66; 10-07-2011 at 08:52 PM.

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    Default Re: LED fixture design

    0.08"-easy enough to cut with either a rototool, hacksaw, or maybe even scored/snapped. Just thought of another advantage to these-if I make the panel easily removeable, I can alter the CCT of the lamp at will.

    On my wish list-high CRI, 4000K or 5000K panels.

  13. #13

    Default Re: LED fixture design

    The spec sheets claims an efficiency of 89 lumen per watt (depending on blue source efficiency), but I'm not sure if that's a spec invented by their marketing dept, or engineers. Light bulbs based on remote phosphor technology range from mediocre to OK, so the panacea is clearly relative. I would be curious to see how well the remote phosphor panels worked with Cree XTE's though compared to an XPG.

    Yes, the strips seem to be the most useful
    The circles in my opinion have the most potential because they could be applied over the top of existing and inexpensive reflectors. Not to be buzz-kill on the strips, but why not just get a single T5 fixture? Cheaper, and just as efficient.

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    Flashaholic* JohnR66's Avatar
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    Default Re: LED fixture design

    Quote Originally Posted by blasterman View Post
    The spec sheets claims an efficiency of 89 lumen per watt (depending on blue source efficiency), but I'm not sure if that's a spec invented by their marketing dept, or engineers. Light bulbs based on remote phosphor technology range from mediocre to OK, so the panacea is clearly relative. I would be curious to see how well the remote phosphor panels worked with Cree XTE's though compared to an XPG.



    The circles in my opinion have the most potential because they could be applied over the top of existing and inexpensive reflectors. Not to be buzz-kill on the strips, but why not just get a single T5 fixture? Cheaper, and just as efficient.
    I think we should give them the benefit of the doubt until someone with the measuring equipment can verify some of the claims.

    I ordered the 4000K 80CRI strip. I like the strips mainly for tinkering. I can get a lot of small circles out of it as well.

    I don't use a lot of LEDs for lighting. They are expensive and the home improvement store stuff has lackluster efficiency. Some are just now catching up with CFL. I just bought a twin F32T8 fixture with 2950 lumen 4100K 86 CRI bulbs. I figure it will be a few years until LEDs can match it at a reasonable price.

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    Default Re: LED fixture design

    Quote Originally Posted by blasterman View Post
    The spec sheets claims an efficiency of 89 lumen per watt (depending on blue source efficiency), but I'm not sure if that's a spec invented by their marketing dept, or engineers. Light bulbs based on remote phosphor technology range from mediocre to OK, so the panacea is clearly relative. I would be curious to see how well the remote phosphor panels worked with Cree XTE's though compared to an XPG.
    That efficiency is for 3000K. There is a pair of studies linked which shows conversion efficacy is much lower for lower CCT (up to near half).

    On the datasheet you have the conversion efficacies from 2700 to 5000K for the different CRI types. It ranges from 160 lm/Wrad (typical figures) of 927 tone (CRI>90, 2700K) to 230 for the 750 tone. It do not specify if such conversion efficacies involves the use of highly reflective mixing chamber (I suspect it does). But from calculating the LER of the spectrum achieved under 450nm peaked blue LED (333lm/Wrad according to my calculations on the datasheet's spectrum) and derating it to the conversion efficacy stated, phosphor conversion efficacy is about 69% (230/333) for the 5000K (the only one I have calculated right now, I'll do the others on next days), in line with literature data about this topic. That figure is good, but for warmer tones it seems to be lower and not a good advantage compared with traditional methods. So for warm tones it seems to not offer a great advantage, I agree with that.

    But this topic arose as consequence of looking for a method of DIY nice looking lamps. Using this remote phosphor system allows you to integrate the diffuser in the phosphor layer. This avoid getting an additional diffuser, and having a yellow diffuser is a great help for a DIYer because it hides the inner parts of the lamp from the view. And allows to use cheaper blue LEDs instead of white ones and save on the additional diffuser.

    I imagine Future will carry soon CerFlex's mixing chambers, allowing to build nice looking and effective DIY home lamps at decent prices. With an XT-E of 500mW (350mA) (50% radiant efficiency), you would get a 115 lm/W (CL-750) down to 80 lm/W (CL-927). Taking into account it is a final figure of light exiting the whole luminary, they are very good figures for residential lighting.


    Quote Originally Posted by blasterman View Post
    The circles in my opinion have the most potential because they could be applied over the top of existing and inexpensive reflectors. Not to be buzz-kill on the strips, but why not just get a single T5 fixture? Cheaper, and just as efficient.
    T5 fixtures are not just as efficient, at least when comparing standard ones (not expensive design ones) and not too warm tones. T5 fixtures usually have efficiencies below 70%. So the actual figure when taking into account the whole fixture and not just the tubes is often below 70lm/W. If you replace a 70 lm/W lamp by a 115 lm/W one, you get an improvement of 164%. So cheaper currently, but not just as efficient.

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    Default Re: LED fixture design

    Quote Originally Posted by jtr1962 View Post
    On my wish list-high CRI, 4000K or 5000K panels.
    Mine is for 3500K high quality color (not high CRI, its not the same for me).

    But a nice way of accomplish it is having a normal CRI (70) coolwhite, about 6500K, with highly efficient conversion, and mount a cyan and red LED together with the blue one. As phosphor is almost transparent to both, you obtain a warmer white tone of excellent CRI, and if you use a dimmable system for each color, with tunable CCT. And all well covered by the yellow diffuser. Likely, some manufacturer could offer a Blue-Cyan-Red (RCB) package directed to remote phosphor tunable CCT systems.

  17. #17

    Default Re: LED fixture design

    T5 fixtures are not just as efficient, at least when comparing standard ones
    T5 with a proper reflector, not some matte white Walmart junk is just as efficient as remote phosphor, if not more so, and the 115 lumen per watt comment is ludicrous from anything other than bare Cree's or top flux Rebels. If anything it's the LED fixtures that are lucky to score above 50 lumens per watt in independant tests like the DOE caliper. The few LED bulbs that score above 80 don't seem to be using remote phosphor. Speaking of which, why doesn't Cree use remote phosphor panels in their fixtures if they are indeed have such high efficiency levels?

    I'm also not going to give anybody the benefit of the doubt until I see the product actually tested. Micro-etched acrylic diffusers have something like 90% transmissive efficiency and accomplish the same light distribution. This seems like a typical solution in search of a problem with the typical Candlepower attention span of a couple months.

    ....Maybe use a really big Tesla coil and stimulate the LED die with radio frequency across the room like I used to do with lasers. Hey, if moving the phosphors off the die is a good thing from an IPO perspective lets move everything off the die.

    Glowing panels in walls might work for the cafeteria or restroom, but I simply don't see it solving any mainstream lighting applications for the same reason back lit milk plexi isn't good for anything other than shadow boxes. More solving the 'lets mess around with it rather than do yard work' factor.
    Last edited by blasterman; 10-10-2011 at 01:00 PM.

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    Default Re: LED fixture design

    The 115 lm/W figure comes out directly from the datasheet statement of conversion efficacy of 230lm/Wrad supposing applied to a Cree XT-E of 500mW output (minimum for a power typical of 997.5mW, so over 50% efficiency at 350mA). If you convert 0.5 Wrad/Winput at 230 lm/Wrad, you get 115 lm/W. Of course, that is giving credit to the conversion efficacy stated in the datasheet. Well, say the phosphor sheet gets at 80ºC, then the conversion efficacy lowers down to 225 lm/W, and then you get 112.5lm/W.

    I give credit to that figure as Intematix is a serious manufacturer, leader on its field and supplying other manufacturers. It serves mainly to OEM, so I'm sure that info is accurate. Other thing is there is no info on the conditions at it was measured. I suspect, as I said earlier, it involves using a mixing chamber with very high reflectivity.

    I don't think remote phosphor is a panacea for all situations, but for sure it offers some advantages and we are going to see several products using it soon. Phosphors suffer a lot for the increased temperature. So when applied over the chip, it goes very hot. Additionally, the losses on the phosphor adds to the heat load of the chip. And often it results on halo effects. Separating the phosphor from the chip reduces halo effects, reduce heat load of the chip and allows to stable working temperature of the phosphor as it is uncoupled from the hot chip and have a huge dissipating surface. And avoiding the use of an additional diffuser, which have losses independent of how good it is, results on reduced optical losses. When you consider it from the perspective of the whole fixture, those advantages seems more than enough to get remote phosphors in wide use.

    One thing is an OLED with reduced total light output and other thing a lamp with remote phosphor, which can emit a very large amount of light. The goal is getting the balance of light emitted per surface area to avoid glare (thus avoiding aditional components), but there is no any fundamental limit to the light emitted using remote phosphor. Do not associate them with low light level ambient panels.

    I would like to remark that the topic raised as a solution suggested for doing DIY nice looking lamps. I feel in this case these phosphor panels offer many advantages. There is other ways of doing nice looking DIY lamps for our homes, of course, but this one is easy, straightforward and probably with excellent efficacy. Going further on a debate of remote phosphor technology would hijack idleprocess's thread, so I suggest to open a dedicated thread for the topic.

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    Flashaholic* idleprocess's Avatar
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    Default Re: LED fixture design

    Quote Originally Posted by blasterman View Post
    Not to be buzz-kill on the strips, but why not just get a single T5 fixture? Cheaper, and just as efficient.
    Sure. Cheaper and far more cost-efficient. Just ... not necessarily fitting my (admittedly somewhat arbitrary) requirements for secondary lighting operating off of DC, definitely not suitable for quick on/off, and also decidedly lacking in the "roll your own" factor which is most of the appeal of these projects.

    As for the remote phosphor, while intriguing, it seems that one would need to buy quite a bit of it - negating the cost advantages of cheaper blue LED's when fixtures are being built in small batches at best.
    I apologize that this letter is so long; I did not have time to write a short letter

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