Christmas lights - another angle

[PetrosA]

I was reading the other Christmas lights thread today after I came back from getting some LED lights for the tree. The nerd in me and the saver in my wife love the fact that a 100 light string of LEDs only burns 8 watts. And, as the other thread shows, you can do a lot of customizing with LEDs.

But... In spite of the fact that I got name brand lights (GE) and not some cheapie one-hung-lows, we hit the wall again. My wife plugged them in and the color was SO anti-holidays and SO glow-in-the-dark nightclub that we're going to take them back. There's absolutely no way either of us could see using something like that inside the house. They would work outside, but we don't go too crazy with decorating the shrubs.

So in spite of a great idea, it's just not time yet, or maybe we're too old fashioned (we also have real candles on the tree). CFLs, LEDs, etc. have to come a long way yet before they really become a part of our life and not just a cheap light to leave on in the bathroom

[tinsmith]

You bought the wrong color, Q.E.D. all non-incandescent lighting has no place in the home other than as a night light?

I'm not sure I'm following you...not in the least because most "Christmas lights" are strands of small incandescent bulbs.

[Zero999]

Some stores have displays showing off their LEDs.

What colour were they?

I don't like the deep blue ones, but I think cold bluish white looks good and red is also a nice colour for Christmas tree lights. Multicoloured lights are also popular but I've noticed that blue is sometimes the dominant colour in a set with different colours, which isn't nice.

How about making your own? It may cost than buying them but at least you have some control over the colours and flash pattens, if you're into that kind of thing. If you do make your own then run them from a small mains adaptor, don't use mains voltage, unless you're sure you've insulated the connections well enough and used a suitable fuse.

[PetrosA]

You bought the wrong color, Q.E.D. all non-incandescent lighting has no place in the home other than as a night light?

I'm not sure I'm following you...not in the least because most "Christmas lights" are strands of small incandescent bulbs.

I got mini lights in both white and various colors (the string kind that you wrap around the tree). The white is actually pretty white by LED standards - only slightly on the blue side but it's enough that they look a bit surreal while the colored ones give off strange, almost fluorescent colors that look like something you'd see in a night club with black lights on. Maybe all red ones would work, but I think the screwy spectrum that the low cost LEDs have is what's making it so unpleasant.

I have tested high end LEDs for general lighting and I can say that some of the bulbs are great with color, but they cost over $100 each. I've also tested commercial outdoor LED fixtures that cost $190 for the cool white version and $200+ for the warm white versions each and unless I was going for a moonlight effect, only the $200 version even comes close to giving pleasant light.

As far as CFL goes, until you get into the really expensive ones, I haven't seen any that give good color. I dunno, maybe most people don't have artwork on their walls or anything with natural or non-printed colors in their homes so they really don't see the difference that bad metamerism makes.

[tyblu]

...the difference that bad metamerism makes.

I had to lookup metamerism; it's not even in my dictionary.

In colorimetry, metamerism is the matching of apparent color of objects with different spectral power distributions. Colors that match this way are called metamers.

For other mortals.

[PetrosA]

...the difference that bad metamerism makes.

I had to lookup metamerism; it's not even in my dictionary.

In colorimetry, metamerism is the matching of apparent color of objects with different spectral power distributions. Colors that match this way are called metamers.

For other mortals.

Sorry 'bout that I've worked with graphics and photography for a long time and I forget that some terms aren't very common. Anyway, here's a better explanation of what I mean.

The blue light of a typical LED doesn't bother us or look unnatural just because it's blue but because it's a blue spike in the color spectrum and other colors don't look right in that light. It's only when you get to the really high end super expensive LED luminaires that the balance is good enough that we don't notice a problem.

[tinsmith]

Yeah, thermal sources emit more of a blackbody spectrum (for reasons that should be obvious). They're also inherently less efficient for the same reasons. On the other hand, it's a bit easier to tune the spectra of other methods and a bit easier to get more luminous flux given reasonable installation parameters - so you see a lot of fluorescents in offices, kitchens, and other work areas (non-compact fluorescent have been here for ages and aren't moving). Still, the whole "ban incandescent bulbs" movement is...well, at least not something that's clear-cut. White LEDs can be made pretty good in terms of intensity and the degree to which their initial spectra accommodates the expectations of the human visual system, but they fade quickly and don't keep their original spectra.

But for something like decorative strands of colored lights, the emission spectra is sort of irrelevant since we're talking about narrow-band radiation. Any narrow-band source with a similar center frequency, fall-off profile, and luminous intensity will look fairly similar. There's something to be said for emission vs. absorption spectra (did it come that way or was it filtered), but that's not really something you can distinguish without a spectrometer and some lateral thinking.

[slburris]

My biggest problem with LED Christmas lights is the fact that they are
usually run directly off the mains and I can *see* the 60Hz flicker.
I just couldn't take that inside the house, but I'm slowly going all
LED outside.

Now as for color quality, I'm not any kind of expert, but the LED
Christmas lights look more saturated than I would expect.  I'm not
saying that necessarily bad, but it's different than what I grew up
with.  I've seen some new LED lights that pretend to be C7 style
bulbs, i.e. they are white LEDs with C7 color cases.  I may try those
this year and see how I like them.

The white LED lights range from hideous to acceptable, usually because
they still tend to have a bluish cast.  I've been spoiled by the Home Depot
LED light bulbs which have a 3000K color temp, and I've seen Feit Electric
bulbs with 2700K.  Why can't I get white LED strings at that temp?

Scott

[Zero999]

The blue light of a typical LED doesn't bother us or look unnatural just because it's blue but because it's a blue spike in the color spectrum and other colors don't look right in that light. It's only when you get to the really high end super expensive LED luminaires that the balance is good enough that we don't notice a problem.

Only LEDs made from blue LEDs will have that horrible blue spike, mainly white, pink and some turquoise LEDs. Other colours such as red, orange, yellow and green won't have any blue in their spectrum.

The thing I think you like about incandescents is that they don't produce monochromatic light the way LEDs so. The filter isn't perfect so other wavelengths get through so a red incandescent light will appear more orange/pinkish than an LED and a blue will appear more greenish than deep blue. The only way to get this effect using LEDs is to use an RGB LED for each light and experiment with the amount of red green and blue until you're happy.

[Zad]

I'm glad it isn't just me. Many new cars now PWM drive LED clusters. This is fine for brake/stop lights, but I am sensitive to strobed lights (not epilepsy, just perception) and Fiats in particular seem to drive their rear running lights quite slowly, so in motorway traffic I often get this flickerflickerflickerflickerflickerflicker effect which is the visual equivalent of a small child screaming in my ear.

The EU/UK government has banned the import and manufacture of 100W incandescent lamps, and 60W ones are due to follow some time soon. This has meant that many people have had to move over to high efficiency CFLs, which always seem a very harsh light to me, maybe because I can subconsciously see the 50/100Hz flicker. Aside from that, we have the issue, discussed above, of colour reproduction. This spectrum explains the point quite well:



And this shows the problem (aside from 50/100Hz strobing) which LEDs suffer.



A big reduction in emissions in the cyan/green area, caused by the gap between the primary blue source and the secondary yellow phosphor emitter.

[tinsmith]

The situation in the power spectrum plot isn't a terrible one to have since it can be solved with a filter (well, addressed).

What's the circuit like in these LED bulbs that you're seeing flicker? LEDs are basically DC devices regardless of what happens with PWM or whatever (they make light when there's current), so it's not likely that they're being driven off raw AC. Is there just really crappy ripple rejection, or what?

[Simon]

I once found some 3500K strip lights that were a  nice white, I've got a couple of 5500K cfl's and they are not nice, personally I'd go for 3500/4200K cfl's when I come to changing bulbs. we also need to realize how wide a range of light intensity the human eye can cope with, the bulb is nothing compared to natural light so you see every little defect, I hate 2700K cfl's but put 5 in a lampshade and it's nice and white instead of yellow, but then that is nearly 500W worth of light

[Zero999]

The situation in the power spectrum plot isn't a terrible one to have since it can be solved with a filter (well, addressed).

That will only help with the white, pink and turquoise LEDs. A filter won't do any good for red, orange, yellow and green because they simply don't emit the wavelengths required to make the lightly faded warm colours he likes.

[alm]

Filters will also decrease efficiency, since they work by converting part of the light to heat. Unless you use fluorescence, but that's not really a filter.

[slburris]

The situation in the power spectrum plot isn't a terrible one to have since it can be solved with a filter (well, addressed).

What's the circuit like in these LED bulbs that you're seeing flicker? LEDs are basically DC devices regardless of what happens with PWM or whatever (they make light when there's current), so it's not likely that they're being driven off raw AC. Is there just really crappy ripple rejection, or what?

The are strung together in series directly on the AC line and conduct for only half
a cycle, at 50/60 Hz.  Or the more expensive ones use a diode bridge and conduct
on every cycle both positive and negative, so a 100/120Hz flicker.  I suspect every
set I have is the half cycle variety.

From wikipedia at:

http://en.wikipedia.org/wiki/Christmas_lighting_technology

Additionally, low-end sets do not contain power supplies (or have only a transformer instead of a SELV), and so the bulbs flicker in sync with the alternating current, being completely off when the voltage is negative. This produces a noticeable stroboscopic effect when an individual happens to move the lights across his or her field of view quickly, as when moving the eyes or turning the head rapidly. Higher-quality strings include a bridge rectifier to supply full-wave direct current to the lamps, making the lights brighter and greatly reducing the flickering (though there is still a small amount because diodes need a minimum voltage to begin conducting). Cheaper sets with two circuits connect each in the opposite polarity, which minimizes flicker in the combined light reflected from walls, and also keeps power consumption symmetrical so as not to affect the electrical system.

Scott

[Zero999]

If you build your own Christmas tree lights which work from a DC power supply, they will be ripple free.

[tyblu]

you wouldn't believe the R&D going into making LEDs emit blackbody at different temperatures. It's similar to the art of making plastics nowadays, mixing this and that, putting different concentrations of holes at different energies, using feedback loops from the traps to drive lower energy bandgaps... Amazingly boring stuff, but managed to pass the class.

[tinsmith]

If you build your own Christmas tree lights which work from a DC power supply, they will be ripple free.

There's a good chance that you could build a supply which buffers the strand and supplies it as-is with a higher-frequency drive signal. Increasing the PWM frequency will quickly make any variation imperceptible to humans.

Sometimes they still show up on video etc. though, which can get a bit weird.

[TheDirty]

The flicker drives me crazy as well.  Most noticeable on the white lights.

[Zad]

It is obviously an immense market, so I can imagine the sort of money going into it. I recently read an article on how they are producing exotic phosphors to try and produce a more even emission spectrum. It sounds potentially cheap and efficient.

Different Christmas lights use different methods of driving. I have seen them use a DC power block and use cheap transistors to do PWM, but I also have a set which takes the non-rectified input and fires 4 thyristors. The PIC12 controller is fed power via a resistor, rectifier 1N4148 with a zener and electrolytic. This is very flickery indeed when viewed out of the side of your eye.

[slburris]

OK, so for those of you who like the LED Christmas lights you have, what did
you buy and where did you get them?  How about ones with a DC power pack
for those of us who can see the flicker?

I can probably be convinced that the mini white LEDs put out comparable
brightness to the mini incandescent lamps.

But when I've looked at C7 and C9 LED lamps, they just look dim to me.
Not surprising since all I've seen appear to be a single LED with a large
plastic shell of the appropriate size.

I've seen some kinds of LED lights so pathetic that I'm convinced the
LED's were swept up rejects from the factory floor.  Wouldn't be too
surprising since the LED lights aren't commanding that much of a price
premium over the incandescents right now.

Aren't there any decent LED lights?  Say made with a major supplier[s
LEDs instead of no-names?  Nichia anyone?

Scott

[DJPhil]

If anyone would like to play with spectroscopy on the cheap there are a bunch of projects online for making a simple spectroscope with a CD or DVD (like this one).

Just in case anyone finds it useful.

[Zero999]

There's a good chance that you could build a supply which buffers the strand and supplies it as-is with a higher-frequency drive signal. Increasing the PWM frequency will quickly make any variation imperceptible to humans.

That would work but you'd need to check to make sure they don't use an inductive or capacitive ballast.

It would be a pain to make though. I suppose a crude way to do this would be to use an electronic 12V halogen transformer with a home made ferrite cored step up transformer to boost it back up to mains voltage but at a higher frequency.