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Old 5th Jul 2020, 9:15 am   #1
ValvoStef
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Cool White LEDs

Morning fellows,
Probably stupid, but, do have white LEDs a spectrum? I know that white light is the sum of all colours, using a RGB LED, no matter what I can’t get pure white light. If I had a prism I could experiment with a white LED. It is not really pure white, it is light shade of grey from a white LED. Odd question, I know. Being bored I think.

Stef.
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Old 5th Jul 2020, 9:39 am   #2
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Default Re: White LEDs

There are two main methods. One is to have red, green and blue LED's in one package. The other is to use a blue LED and a phosphor, usually yellow. So the white colour is a mix of the blue LED coming directly through the phosphor, and the broadband yellow from the phosphor.

By tweaking the wavelength of the blue LED and the precise characteristics of the phosphor means that you can get different colour temperatures - for instance for LED ceiling lights, where you can choose between daylight, or something warmer in colour.

The downside (we found) is that when one lamp fails, you can't just replace one (because after five years the exact type is obsolete) - you have to replace the lot.

Craig
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Old 5th Jul 2020, 11:49 am   #3
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Default Re: White LEDs

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Originally Posted by Craig Sawyers View Post
The downside (we found) is that when one lamp fails, you can't just replace one (because after five years the exact type is obsolete) - you have to replace the lot.
Agreed. It's happened to me three or four times now.
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Old 5th Jul 2020, 11:58 am   #4
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Default Re: White LEDs

All white light has a spectrum, by definition. This was quite spikey and banded when white LEDs were first developed, but the phosphors have improved a lot and the best now give a reasonable approximation of black body radiation.
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Old 5th Jul 2020, 12:29 pm   #5
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Default Re: White LEDs

According to a speaker at an IET talk on LED lighting a decade or so ago, the spectrum of the then-current white LEDs that used phosphors was highly temperature-dependent. This had limited their use in buildings with ceiling-mounted ventilation systems as the fittings next to the vents differed in hue from the more distant ones. Presumably technology has moved on since.

We have a couple of mirrors with bevelled edges. The edges function as prisms. With a small light souce such as a torch bulb or a LED, standing on the other side of a darkened room and moving the reflection of the light source across the boundary between the flat glass and the bevel produces a spectrum. Recalling my scool physics optics practicals, for spectrum analysis experiments with prisms, the light source needs to be narrow to produce a decent spectrum, otherwise the colours overlap and you only get red and blue fringes to white light.
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Old 5th Jul 2020, 1:36 pm   #6
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Default Re: White LEDs

I’m constantly impressed by the subjective similarity between the light from today’s warm white LED lamps and that from incandescent filament lamps. The LED lamps seem so much better than the compact fluorescent lamps that we tolerated only a few years ago. I’d have expected there to be much better control of the large area fluorescent coating in a CF lamp than in a tiny LED, but clearly the LED manufacturers have got the spectrum remarkably well sorted. Could it be something to do with the relatively high consistent operating temperature of the LED chip?

Martin
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Old 5th Jul 2020, 4:21 pm   #7
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Default Re: White LEDs

Thanks, guys for enlighten me. A bit wiser now.
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Old 5th Jul 2020, 5:07 pm   #8
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Default Re: White LEDs

There's some interesting information on LED characteristics on the Osram website. https://www.osram.com/ds/highlights/...s_overview.jsp, third row of information.

The "Technical application guide - PrevaLED® Core G7 Food and Fashion LED modules" publication gives guidance on which LED modules are best suited to diffent applications in retail environments. The spectra seem to be based on the techniques described by Emeritus with a peak in the RGB primary blue region around 450 nm and another somewhere between 580 (Sodium vapour [589 nm]) and 650 nm (between helium-neon laser and RGB primary red). The types with a more even spectrum have a peak in the 520 nm area suggesteing some input from a green LED.

The "Technical application guide - Basic Linear G2" describes LED modules for applications where very high luminous efficacy is paramount - formerly the domain of low-pressure sodium lamps (SOX), but without the sacrifice of any pretension to colour rendition which SOX entailed. These LED modules have spectral peaks around 450 nm and 600 nm suggesting the sources are blue and red LEDs. Their colour rendition for saturated red is poor but otherwise is vastly superior to SOX.

The argument that SOX was the most efficient form of lighting seemed flawed to me once fluorescect white light sources for outdoor lighting became available. The argument for SOX was based largely on lumens per watt while the perception of scene illumination to the human eye/brain combination takes information from both luminace and chrominance.

Osram's Basic Linear G2 seems to offer a much better outdoor illumiance source than SOX ever did with a similar luminous efficacy and useful life three times better. LEDs also have a zero warm-up time against nearly 15 minutes for SOX. This means that LEDs can be switched on later, saving energy.

The quality of LED lanterns for street lighting seems to have improved since the earliest examples, some of which had abysmally poor uniformity.

Does/will anyone mourn the passing of low pressure sodium and its monchromatic orange light, after a warm-up period of up to 15 minutes?

PMM
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Old 5th Jul 2020, 5:25 pm   #9
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Default Re: White LEDs

In my limited experience the best compromise of spectra from (good quality) white LED's is available from those which are rated 5,000 Kelvin colour temp. or thereabouts.

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Old 6th Jul 2020, 9:57 am   #10
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Default Re: White LEDs

Have not seen any spectra for white LEDs for a few years, but when I had a professional interest in these things white LEDs were actually blue LEDs with a phosphor to convert some of the blue light to yellow, so the spectrum was a narrow blue peak and a broad yellow "hump", and the relative magnitudes gives the various colour temperatures available. Usually notably deficient in the longer wavelengths of the red. Obviously this is not true of the RGB type.
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Old 6th Jul 2020, 12:25 pm   #11
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Default Re: White LEDs

Quote:
Does/will anyone mourn the passing of low pressure sodium and its monchromatic orange light
Urban astronomers, easy to filter out the sodium lines.
 
Old 6th Jul 2020, 2:24 pm   #12
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Default Re: White LEDs

Yes, the broader spectrum of mercury lamps and LED lighting is far more difficult to remove from astronomical images - at least by means of physical filters.
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Old 6th Jul 2020, 4:18 pm   #13
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Default Re: White LEDs

Quote:
Originally Posted by pmmunro View Post

The argument that SOX was the most efficient form of lighting seemed flawed to me once fluorescect white light sources for outdoor lighting became available. The argument for SOX was based largely on lumens per watt while the perception of scene illumination to the human eye/brain combination takes information from both luminace and chrominance.

Osram's Basic Linear G2 seems to offer a much better outdoor illumiance source than SOX ever did with a similar luminous efficacy and useful life three times better. LEDs also have a zero warm-up time against nearly 15 minutes for SOX. This means that LEDs can be switched on later, saving energy.

The quality of LED lanterns for street lighting seems to have improved since the earliest examples, some of which had abysmally poor uniformity.

Does/will anyone mourn the passing of low pressure sodium and its monchromatic orange light, after a warm-up period of up to 15 minutes?

PMM
Street lighting lanterns had reached a high degree of sophistication with low pressure sodium lighting, giving a remarkable uniformity of illumination of the road with their careful prismatic design. Personally, I find the orange monochrome light less glaring than white light, particularly on a wet night. Maybe it's something to do with reduced dispersion where a greasy windscreen film is involved.

When LED lighting came along, it seemed that all the design archives of optimum optical design of lanterns had been sent to the skip and an entirely new team of naive optical engineers had to learn how to do it all over again. For example, we used to have a sodium light at the end of our drive: its good lateral light distribution easily illuminated the way to the house. Now that it's been replaced by a new white light, the pedestrian has to grope in darkness once they've left the pool of light immediately under the lantern.

Martin
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Old 6th Jul 2020, 5:24 pm   #14
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Default Re: White LEDs

Almost all white LEDs used for general illumination consist of a blue LED and a yellow phosphor, the resultant spectrum has a narrow blue peak and a wider yellow peak. Very little red.

By tweaking the exact wavelength of blue, and by careful selection of the phosphor, different shades of white may be produced.
The colour rendering will remain at least somewhat poor due to the lack of significant red.

Improved colour rendering may be obtained by adding a red phosphor to the mix (uncommon as cost is increased and efficiency reduced) or by adding some red LEDs to the light fitting (uncommon due to added complexity)

The best LED lamps on general sale have good colour rendering, comparable to fluorescent lamps.
For almost perfect colour matching, either "colour matching" fluorescent lamps, or specialist LEDs are needed.

Good colour rendering may be obtained by mixed red, blue, and green LEDs, but this approach is rare for general lighting. The different coloured LEDs react differently to high temperatures, they also depreciate at different rates with prolonged use. Therefore the light fitting has to monitor the colour of the light produced, and to continually adjust the drive current of the different coloured LEDs in order to produce a good white.
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Old 6th Jul 2020, 5:31 pm   #15
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Default Re: White LEDs

Re #13, we had the same problem at my late mother's. She had a 30' front garden, and the sodium street lights used to throw enough light into her recessed porch to illuminate the keyhole and the combination lock of the key safe that her carers had to open to get in. The highly directional LED replacements left the porch in total darkness, resulting in a, fortunately failed, burglary attempt at forcing both the front door and the front window. I had to fit a porch light for the benefit of the carers and to discourage burglars, as well as drastically pruning the front garden shrubbery next to the window to eliminate the deep shadow that hadn't previously existed.

Last edited by emeritus; 6th Jul 2020 at 5:40 pm. Reason: Typos
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Old 9th Jul 2020, 1:54 pm   #16
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Default Re: White LEDs

Here's a fun video demonstrating the spectrum of some different white lamp technologies:
https://youtu.be/nycAujdp708?t=320
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Old 9th Jul 2020, 3:37 pm   #17
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Default Re: White LEDs

The interesting thing in this video is the CFL spectrum, I did not expect the spikes.
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Old 9th Jul 2020, 3:46 pm   #18
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Default Re: White LEDs

CFLs have an awful spectrum, worse than the worst LEDs. It's why their colour rendition is so bad, and one of the reasons that they usually seem dimmer than their paper performance would suggest.
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Old 9th Jul 2020, 3:54 pm   #19
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Default Re: White LEDs

Yea, CFL is odd.
Given it's light is mostly from a phosphor, as are LED's, it must be down to the different phosphor or the that the energy gap is much bigger for the CFL (UV -> visible). Existing in a vacuum and being whacked by UV no doubt limits the choices

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Old 9th Jul 2020, 4:27 pm   #20
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Default Re: White LEDs

Quote:
Originally Posted by paulsherwin View Post
CFLs have an awful spectrum, worse than the worst LEDs. It's why their colour rendition is so bad, and one of the reasons that they usually seem dimmer than their paper performance would suggest.
Yes: I wonder why they seemingly never managed to achieve a proper "daylight white" 6500K rendition with CFLs? Is there perhaps some factor associated with commervcially-viable phosphors that make those at the spectrally-whiter end less energy-efficient (given that the reason for promoting CFLs was 'greenness'?)
I always found the things horribly yellow, and as soon as decent daylight-spectrum LEDs appeared I consigned all my CFLs to the local WEE-skip.

I've got some Ultra-Violet LEDs here: at one time I was looking at repainting the needles on some meters with UV-fluorescent paint and fitting the UV LEDs as extra illumination so the needles appeared brighter than the scales - something Ive seen done in aviation applications (and Lexus cars).
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