# Spiral vs straight vs U-tube



## Steelwolf (Sep 3, 2005)

Had a look at some new flourescent lamps and realised that I never really asked about the different tube styles that are available.

In terms of brightness to volume/size, I think the spirals pack the highest wattage in to the smallest form factor. But what about other aspects of the tubes and why manufacturers choose to use one over the other?

Which tube style is easiest to replace, has the longest running life, is least susceptible to blackening from dying cells, is most efficient, is most robust, ...?

Don't know if there are any other questions I should be asking. 

Does anyone have an answer?

Thanks.


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## jtr1962 (Sep 3, 2005)

In general the longer the arc length and the narrower the tube the more efficient it is, all other things being equal of course. This means spirals are usually the efficiency champs as far as CFLs go. I've seen overall efficiencies (including ballast losses) from spirals as high as 75 lm/W. However, there is only so much length you can pack even in a spiral, and small tubes in general run hotter than larger ones because they run at a higher power density to get more light. This means that plain, old 4 foot and 8 foot linear tubes are the efficiency champs, especially the T-8 and T-5 sizes which can exceed 100 lm/W. Of course, these can't be used as direct incandescent lamp replacements. Phosphors also influence efficiency. Some of the better types of phosphors not only give superior color rendering, but also superior efficiency.

As for lifetime, here again the linear tubes are the champs with lifetimes up to 40,000 hours (spirals rarely exceed 10,000). End blackening depends upon electrode design and ballast. Tubes with electrode guards can run for their rated life without much end blackening, provided they are run with a decent ballast. Cheap ballasts are notorious for causing blackened tubes as well as short life. Frequent starts can also cause these things.

If you're looking for the best possible fluorescent lighting then I'd recommend a T-8 or T-5 fixture which uses 4-foot tubes. Make sure it has a quality electronic ballast from one of the better known manufacturers (GE, Sylvania, Advance, Philips). For best color rendering and true to life appearance try full spectrum 5000K tubes, often designated 950. The 9 refers to color rendering (9 is greater than are equal to 90) and the 50 refers to color temperature in 100s of K (50 = 5000K). Note that there is no degrees used when measuring temperature on the Kelvin or absolute scale. 5000K is pronounced 5000 Kelvin, not 5000 degrees Kelvin. Decent tubes will cost under $10 online, and often under $5 in quantity (i.e. 100s). Here is where I buy mine.


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## Steelwolf (Sep 4, 2005)

Thanks for the insight, jtr1962. Unfortunately, I'm not sure what T-8 or T-5 mean in physical terms. I have seen thick straight tubes, thin straight tubes (typically used in portable lamps), circular tubes, U tubes with a tight pinch in the bend, U tubes where the bend retains the full tube thickness and spiral. My knowledge of nomenclature is really pathetic. 

I actually asked this in relation to portable/camping lamps like stuff from Coleman and the Energizer folding lamp, but it is good to know a little more about fixed/mains powered lamps too.


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## Lynx_Arc (Sep 4, 2005)

The smaller the value after the T the smaller the diameter of the tube. Usually as you go down in wattage the T size follows. The smallest standard fluorescent tubes I have seen are T5 , and the CCFL (cold cathode) fluoros have really tiny tubes which are not replaceable. You wont find any big lanterns with CCFL, they use the standard replaceable tubes. 
Typically you can go by the wattage of the bulb/tube as a good indicator of comparative brightness regardless of the type of fluorescent. As for color most battery fluoros tend to be cool white while a lot of AC powered fluoros tend to be warm white. 
Essentially a 15 watt twister tends to put out close to twice as much light as an 8 watt straight tube with a 9watt utube about the same output as an 8watt staight one. The advantages in the differing tube configurations are in design and space considerations as people don't tend to carry what looks like a fluorescent light saber to use as a lantern so bending the tube makes sense for compactness.


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## Zelandeth (Sep 4, 2005)

Right, gonna give this anotehr shot...wrote a really long reply here last night, but it refused to post. Got told I was trying to access an invalid thread, and ate it.

Brief points for all the main topics.

--> Efficiency: Linear win there, because size is less of an issue. Efficiency (as I recall) increases in a mercury vapour lamp as current density decreases (to a point), hence the wider diameter tubes are the more efficient.
Next bit is guesswork. The fact that the tube walls are futher from the discharge itself, hence run cooler in a wide-bore linear lamp will be benificial for the efficiency of the phosphor, as they generally operate at peak efficiency in a fairly narrow temperature band. Probably more than efficiency this will help prevent the phosphor from degrading so rapidly over time, hence improve lumen maintenance. Spirals and U tube design CF lamps don't seem to differ all that much in efficiency betwen each otehr, but are behind linear lamps.
Linear wins.

--> Durability: A lot of that's down to luminaire design really. In a well designed lamp though, a linear lamp will be the least suseptible to breakage, as it's not going to have forces concentrated on any one point. For instance if you drop a spiral lamp equipped lantern on its side, the weight of the whole tube is going to be concentrated on one side - wheras with a linear design it's supported at both ends. I guess the proof of this is that I've thus far managed to break at least five CF lamps in my time, but have yet to end up with a linear one in pieces.
Linear Wins.

--> Longevity: Linear's a very clear winner there (especially in mains powered situations, ratings in excess of 40K hours aren't uncommon). In a portable setting running lifetime of the tube (and lumen maintanance, electrode blackening and such) is more likely to be dependent on how good the ballast is at looking after the tube, than the tube itself. Electrode blackening really shouldn't be an issue with modern (decent brand) tubes at all unless they're being switched very regularly or within the last couple of hundred hours of theri lifetime. I've a couple of CF lamps here near to 10K hours of continuous runtime now, with very, very little blackening visible. 
Mains: Linear. Portable: Ballast determined.

--> Size vs. Output: This is one where the newer lamps get a look in. Spiral lamps manage to pack the most light output into the smallest space - because of the way the tube is folded up. U-tubes come a close second, but tend to require a longer fixture. And liniear tubes come up last there, as they are biggest. The downside of spirals here is that they do (because there's exactly the same amount of tube for a given wattage) run hotter. (The actual amount of heat's the same - just it's coming from a smaller area for the spiral. This is only really an issue for fully enclosed fixtures where heat buildup may be an issue for the ballast.

--> Upkeep: There's a clear winner here. Linear lamps. It's a form factor that's been around since the dawn of really commercial discharge lighting, and isn't likely to be going anywhere soon. Plug in CF type lamps (as used by a lot of portable lanterns) can be hard to track down in some areas (I can't get ANY of them in the Aberdeen area, unless I go and pay £10 for a 5W CF lamp from a professional lighting store...I don't THINK so!), but most can be found online. They are however more prone to being outdated. The other advantage here is that linear lamps are generally far, far cheaper than CF's.

--> Application matching: This is a bit of a tricky one. If you're making something from scratch, linear lamps are probably the best bet, as they're available from about 3W up to 140W (I think), in just about every colour known to man from crimson red to ultraviolet, and most things in between. Including a few zillion shades of white. CF's are harder to find in unusual colours and obscure wattages, and of course, they need control gear to match. Wheras a lot of the modern electronic ballasts are set up to power a number of different common wattages of linear lamps. Generally linear/CF control gear is compatible - depends how obscure your lamp ratings are.

Okay...not actually on my own PC...so don't have any of my references to hand (so that's all out my head...hence the no doubetly numerous errors)...but hopefully there's some useful data there!


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