can an energy saving bulb be run off DC ?

[Leo Bodnar]

LOL.  We are speaking in different terms.

What is the colour temperature of purple or green?  It just does not exist.
Look at the below diagram.  The curvy line in the middle is all colour temperatures from -273C to infinitely hot.
Any colour outside this line does not have a colour temperature.

[Simon]

Simon, I don't know if I agree. I recall the horrible high colour temperature blue CFL being dimmer than the warm white lamp round your house, despite being the same power rating.

Nope, have a good look at that graph above, 5500K is quite near blue. Warm white is the con-mans version of yellow - again look at the graph. the first CFL's that were produced were certainly not 2700 K or they would have never got out. Now the horrible yellowness of them is so apparant that people think the orange street lights are CFL's. Modern CFL's are not liked for a very good reason but people are stuck with them. Ask anyone over the age of 40 what they think of these new fangled bulbs....

[alm]

There is such a thing as correlated color temperature, which is something like the color temperature of a black body radiator that is the closest to the spectrum of the light source. As the spectrum gets farther from the ideal black body radiator spectrum, this figure tends to become less meaningful.

Incandescent lights for domestic use have a color temperature of something like 2700-2900K, so I don't understand Simon's issue with 2700K CFLs (unless they're actually lower than 2700K). Anything between say 2700K and 10000K can be perceived as white by the human eye, depending on the circumstances. Whether it looks yellow or blue mainly depends on the colors of the objects around you and whether other light sources are present. Early fluorescent lights used to have a fairly high color temperature, they actually lowered it (and reduced the efficiency somewhat) for home use.

[Zero999]

Nope, have a good look at that graph above, 5500K is quite near blue. Warm white is the con-mans version of yellow - again look at the graph. the first CFL's that were produced were certainly not 2700 K or they would have never got out. Now the horrible yellowness of them is so apparant that people think the orange street lights are CFL's. Modern CFL's are not liked for a very good reason but people are stuck with them. Ask anyone over the age of 40 what they think of these new fangled bulbs....

What do you mean?

Colour is subjective. To me the high colour temperature lamp round your house is a horrible dim blue/greyish lunar colour and the warm white is a more pleasant warm and creamy colour. I don't know how you can claim the 2700k lamp is less efficient because it seemed much brighter and used less power than the horrible blue light but whether you preferred it or not is your opinion. A sodium street light or green LED would probably be more efficient than either CFL but would look horrible.

In my opinion halogen lights give the best light quality, much better than normal incandescents, LEDs or CFLs.

[Simon]

The truth is that both those lights in my (or rather my dads now) house are not nice. It seems that CFL's have a narrow spectrum and it centres very closely around the main colour temperature. Because you are used to the 2700K one and also because you saw them in contrast you really noticed the defects of the 5500K. If the 2700 had not been there (illuminating a smaller space I might add) you would have not seen the stark contrast and you brain would have adapted more easily to 5500K.

The 2700K bulbs make me drowsy in the winter and i need way more "wattage" than claimed to be satisfied. I have bought 3500K compact strip lights (which I assume are also CFL's) and they were much much better, they looked white and the illuminated area was white.

The human eye will adapt within reason to different colour temperature lighting, it will autmoatically make the whitest object white and balance the rest accordingly. but 2700K is still so bad that the human eye can't quite do it. Most people don't notice or think about it as they are used to it or accept it, unfortunately i do notice and don't like it.

[Zero999]

We're all different. I can't stand 5500K because it burns my retina, is not a very flattering light to be seen in,and if it's too dim it appears grey. I have no problem with colour differentiation under 2700k as long as it's bright enough, if it's too dim colour perception is harder but at least it's not gloomy like higher colour temperature light.

[Simon]

yes each person will perceive light colour a little differently. I'm no fan of the 5500K light. it almost makes a natural light in my bedroom that is painted green (so does have some use when paired with the decor). I can manage with 2700K lighting but I need 2-3 times the equivalent power I would need from an incandescent. so that kind of almost makes CFL's not such an energy efficient option.

When i moved into my house i found 3 and 5 bulb chandeliers in the downstairs rooms. These are quite aptly fitted. It requires that many bulbs to correctly illuminate the rooms, that is rooms that according to the bulb manufacturer can be lit with one bulb. So in effect I am using up to 100W of CFL where I would otherwise use 100-150W of incandescent. Some energy saving

[Leo Bodnar]

It seems that CFL's have a narrow spectrum and it centres very closely around the main colour temperature.

I officially give up!    

By the way mid-day daylight is around 5500K.

[tsmz]

Usually, CFLs (or generally, all lights) will have a CRI (Colour Rendition Index) printed somewhere. You will absolutely need to get the highest CRI available if you're interested in good colour reproduction. You probably won't find a CRI of 90 or more in CFLs, but at least T8 fluorescent tubes with good CRI are readily available and don't cost an arm and a leg (that is, they're about twice to thrice the price of standard T8 tubes, but they're still reasonably inexpensive). The situation is a bit worse with T5 tubes, but improving.
I changed my lighting to a couple of 965 (i.e. 6500 K, CRI > 90) tubes and really can't complain. Of course, you'll have to like blueish-white sunlight or else it'll drive you nuts, but the real trick with high colour temperature is decent light intensity. Dim daylight just doesn't work and feels really nasty.
I've got an odd 865 T5 tube somewhere because I couldn't find an available 965 type, and the difference in colours is stunning. Some colours really do look slightly off, but it should nonetheless work quite well in areas where you just need light and not really good colour rendition.

The problem I've found with commercial LED lamps is that they're crap. Almost all of them have horrible cooling design, which will make them overheat and dim. I've seen quite a few supposedly long-lasting LED light strips with partially dimmer LEDs, so I don't quite think we're there yet when it comes to LED lighting. Sure, you can get some really nice high-power LEDs, but cooling the stuff is important as well and that's where the problems begin.

[Simon]

well really I suppose colour temperature is a bad definition of a light source to start with. I mean like you say the perfect source emits a flat range of many colours. basically putting a colour temperature on a light source (and so giving it a colour) is removing that definition and is calling it a coloured light, or a varied coloured source that outputs mostly the declared colour (temperature) 5500K (blue) on it's own is useless as it 2700K (yellow), what we need is an average of 5500K that comes from a span ranging from 2700K to 9000K, we see colour as reflected light, white because it reflects all colours and each colour because it reflects that colour in the light. If the colour is missing in the light you won't be seeing it so well if at all

[Simon]

Of course, you'll have to like blueish-white sunlight or else it'll drive you nuts, but the real trick with high colour temperature is decent light intensity. Dim daylight just doesn't work and feels really nasty.

exactly. Same goes for so called warm white (yellow) as I said above, i need 5X the power to get a correct lighting, so goodby energy saving

[Zero999]

exactly. Same goes for so called warm white (yellow) as I said above, i need 5X the power to get a correct lighting, so goodby energy saving

Try halogen downlighers and go for 12V which is higher efficiency and lasts longer than 230V lamps because the filaments are thicker and can burn hotter.

I changed my lighting to a couple of 965 (i.e. 6500 K, CRI > 90) tubes and really can't complain. Of course, you'll have to like blueish-white sunlight or else it'll drive you nuts, but the real trick with high colour temperature is decent light intensity. Dim daylight just doesn't work and feels really nasty.

Sunlight is 5,800K but that's only at meridian in the tropics, it'll be a slightly lower temperature than that most of the time. Daylight averages at 6500K because of the sky being blue due to the refracted light.

The problem I've found with commercial LED lamps is that they're crap. Almost all of them have horrible cooling design, which will make them overheat and dim. I've seen quite a few supposedly long-lasting LED light strips with partially dimmer LEDs, so I don't quite think we're there yet when it comes to LED lighting. Sure, you can get some really nice high-power LEDs, but cooling the stuff is important as well and that's where the problems begin.

Yes I agree. I'd be interested to know what sorts of LEDs they used at the swimming pool I was talking about previously. The problem is domestic consumers will often choose products based on price. It took a few years for CFLs to have decent efficiency and life at life at a reasonable price and it'll take awhile for the same to happen with LEDs but when it does happen, they'll be better than CFLs.

[tsmz]

well really I suppose colour temperature is a bad definition of a light source to start with. I mean like you say the perfect source emits a flat range of many colours. basically putting a colour temperature on a light source (and so giving it a colour) is removing that definition and is calling it a coloured light, or a varied coloured source that outputs mostly the declared colour (temperature) 5500K (blue) on it's own is useless as it 2700K (yellow), what we need is an average of 5500K that comes from a span ranging from 2700K to 9000K, we see colour as reflected light, white because it reflects all colours and each colour because it reflects that colour in the light. If the colour is missing in the light you won't be seeing it so well if at all

You seem to be having a slight problem in understanding colour temperature.
There is no such thing as "white" light. How do you define white light? Same amount of radiated power in every frequency? The human eye has a high sensitivity in the greenish-yellow part of the spectrum, so this "white" light would probably look quite greenish or yellowish.
That's why we're using the definition of blackbody radiation. An ideal black body will emit light in a certain distribution when it is heated to a certain temperature. The sun is approximately a black body at about 5000 to 6000 K surface temperature. If we have established a function between temperature of a blackbody and the distribution of its emitted wavelengths, we can now define a colour temperature! The colour temperature is simply defined as the temperature a blackbody would have to be heated to to create the same (or the closest matching) distribution of wavelengths (in the visible spectrum) as the light source you're measuring.

But there are hardly any ideal blackbody radiators out there. The atmosphere will filter out some of the longer wavelengths of sunlight, making it appear more blueish, so "apparent" sunlight is usually defined somewhere around 6000 to 6700 K. Normal incandescent bulbs are mostly blackbody radiators run at quite a low temperature, wasting most of the power as infrared radiation, and that's why they're so terribly inefficient.
Of course, color temperature can be defined even if you've just got a few peak wavelengths in your spectrum, but this will get increasingly silly the more you deviate from a continuous spectrum. That's why there's the CRI, which exists in countless variants and has the strangest definitions, but the idea behind it is to give a measure of the blackbody-like quality of your light source.

[Leo Bodnar]

How do you define white light? Same amount of radiated power in every frequency? The human eye has a high sensitivity in the greenish-yellow part of the spectrum, so this "white" light would probably look quite greenish or yellowish.

Eye is only sensitive to green/yellows because there are more of them around then other wavelengths. I reckon your flat spectrum "white" light will look pink (excessive amount of red + blue.)

[Zero999]

I think for comfortable viewing, the colour temperature should correlate with the intensity. At sunrise and at sunset when the light intensity is low, the colour temperature of sunlight is also low. Light with a higher colour temperature at this intensity will look horrible. The only time you want dim high colour temperature light is when it's very dark (moonlight) when the the eyes switch to scotopic vision which is why LED torches are so good as the rods work better at shorter wavelengths.

[Leo Bodnar]

The only time you want dim high colour temperature light is when it's very dark (moonlight)

Moonlight should have spectrum identical to the direct sunlight (around 5500K.)

when the the eyes switch to scotopic vision which is why LED torches are so good as the rods work better at shorter wavelengths.

Where do you get all this from? Rods sensitivity has a peak bang in the middle of green parts of visible spectrum.  I should say that this is purely my opinion.

[Simon]

yes and it will be no where near as bright as your localized artificial light

[Zero999]

The only time you want dim high colour temperature light is when it's very dark (moonlight)

Moonlight should have spectrum identical to the direct sunlight (around 5500K.)

Yes, i agree.

when the the eyes switch to scotopic vision which is why LED torches are so good as the rods work better at shorter wavelengths.

Where do you get all this from? Rods sensitivity has a peak bang in the middle of green parts of visible spectrum.  I should say that this is purely my opinion.

From Wikipedia, other sources are available.

Human vision has two modes:
Photopoic; this occurs in bright conditions, when the cones are dominant and has a peak sensitivity of 555nm.

Scotpoic: this occurs under low light conditions, when the rods dominate and has a peak sensitivity of 498nm. The eye's sensitivity to shorter wavelengths increases slightly but the sensitivity to the red end of the spectrum diminishes.


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


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

[Leo Bodnar]

Scotpoic: this occurs under low light conditions, when the rods dominate and has a peak sensitivity of 498nm. The eye's sensitivity to shorter wavelengths increases slightly but the sensitivity to the red end of the spectrum diminishes.

Ah, OK, not that much difference then.  It looks like night vision is not affected by deep red light part of the spectrum beyond 600nm.
This is probably why red light is used for temporary night light sources e.g during astronomical observations.  
You don't want somebody flash a white light onto their map and blew away your night vision that took few hours to adopt to.

[johnwa]

So you mean they just omit the filter capacitor? That would make the power factor better at the expense of flicker so it looks like a lamp powered from a traditional iron ballast.

Sort of. The half bridge driver runs from filtered DC, but the other end of the lamp is coupled back to the unfiltered supply. Have a look at the circuit 'SINECAN 5 2x 26-30W' on the page I mentioned. I guess it works by running the positive half-cycle of the HF waveform from the filter caps, and the negative half-cycle direct from the rectified mains. I have attached another circuit of a CFL that I traced out a while ago - it uses the same design.

[Zero999]

It looks like a strange thing to do.

How will it improve the power factor?

I don't see how it can. The fluorescent tube is connected to the unfiltered supply via a 47nF capacitor which makes no difference at DC and has an impedance of 68k at 50Hz so only allows the higher frequency tube current through.

The tube is powered in cold cathode mode which makes starting harder and is not good for tube life.

[johnwa]

It looks like a strange thing to do.

How will it improve the power factor?

I don't see how it can. The fluorescent tube is connected to the unfiltered supply via a 47nF capacitor which makes no difference at DC and has an impedance of 68k at 50Hz so only allows the higher frequency tube current through.

I sort of thought, running it from completely unfiltered DC will improve the power factor, so running it from unfiltered DC part of the time would give some improvement too. Perhaps I have misunderstood the circuit, although I can't see why else they would connect it like that.

The tube is powered in cold cathode mode which makes starting harder and is not good for tube life.

Yes, that is how I drew it, I think there were only two wires going from the PCB to the tube, although there might have been some other components off the board. I really can't remember, it was quite a while ago I traced it out.