# Compact fluorescent bulbs: what's T2, T3, T4, T5? **URL:** \ **Category:** Factual Questions\ **Created:** [April 4, 2009, 4:31pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980 "2009-04-04T16:31:35Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![emmaliminal](https://avatars.discourse-cdn.com/v4/letter/e/7bcc69/32.png) [@emmaliminal](https://boards.straightdope.com/u/emmaliminal)\ **Post date:** [April 4, 2009, 4:31pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/1 "2009-04-04T16:31:35Z")
I’m guessing the T stands for “type,” but which T is what? Some Googling hints that T4 might designate “dimmable” but I’m not finding anything conclusive. Or does it refer to size, or type of ballast, or generation, or what? For examples, see [this eBay store](http://stores.shop.ebay.com/Ecoconceptz-One-Stop-Green-Shop_Compact-Fluorescent-Lighting_W0QQ_fsubZ564858013QQ_sidZ88448883QQ_trksidZp4634Q2ec0Q2em322). ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 4:43pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/2 "2009-04-04T16:43:02Z")
The “T” gives the diameter of the tubes in eighths of an inch, T-4 is 1/2". There are dimmable CFLs but I don’t know if there is a code or rating designating them as such. ---
**Author:** ![emmaliminal](https://avatars.discourse-cdn.com/v4/letter/e/7bcc69/32.png) [@emmaliminal](https://boards.straightdope.com/u/emmaliminal)\ **Post date:** [April 4, 2009, 4:46pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/3 "2009-04-04T16:46:44Z")
Huh. Thank you. Do you know – in the context of compact spiral bulbs, does the diameter of the tube have an effect on the lumen output or anything? ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 4:56pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/4 "2009-04-04T16:56:36Z")
Not sure. I usually check the lumens/watt rating on the package. ETA: I haven’t purchased from them but I’ve heard good things about [this site for purchasing lamps online.](http://www.topbulb.com/) ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [April 4, 2009, 5:07pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/5 "2009-04-04T17:07:56Z")
The screw in type of CFL are not dimmable. For a florance to be dimmable requires a dimmable ballast. And yes the T stands for the diameter. To check to see how bright check the package for the lumens rating. ---
**Author:** ![emmaliminal](https://avatars.discourse-cdn.com/v4/letter/e/7bcc69/32.png) [@emmaliminal](https://boards.straightdope.com/u/emmaliminal)\ **Post date:** [April 4, 2009, 5:15pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/6 "2009-04-04T17:15:48Z")
OK: in other words, does anyone know what the significance of tube diameter is to end use in a compact spiral? There seem to be different tube diameters available for the standard common medium screw-in base; it looks like larger diameters tend to be larger wattages but not uniformly so. So if I have a choice between two 30W, 2700 lumens, medium-base bulbs, one T3 and one T4, should I decide based on price alone, or is there any qualitative difference I should know about? And **Snnipe 70E** , um, there are now [screw-in dimmable CFLs](http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&num=50&q=cfl+dimmable+screw+OR+screw-in&btnG=Search). They’re not cheap, often 2.5 to 3x the price of regular nondimmable CFLs, but they are widely available. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 4, 2009, 5:30pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/7 "2009-04-04T17:30:58Z")
> [@emmaliminal](#): > > I’m guessing the T stands for “type,” but which T is what? T actually stands for “tube”. It’s one of the industry-standard codes for [bulb shape](http://www.topbulb.com/find/incandescent.asp), such as the following common ones: A (the classic lightbulb shape), T (tube), G (globe) and F (flared). ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 5:32pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/8 "2009-04-04T17:32:47Z")
> [@emmaliminal](#): > > They’re not cheap, often 2.5 to 3x the price of regular nondimmable CFLs, but they are widely available. I’ve purchased some of the dimmable bulbs and they worked well but not as well as the dimmers for incandescent, the range wasn’t as good. I’m not sure but I think mine were GE, different makes might be better. Also, they don’t tell you this on the package but all fluorescent tubes including CFLs should be used in areas where they will be on for around 15 mins or so each time they are turned on. If they’re on and off too often their life span isn’t any better than regular bulbs. ---
**Author:** ![emmaliminal](https://avatars.discourse-cdn.com/v4/letter/e/7bcc69/32.png) [@emmaliminal](https://boards.straightdope.com/u/emmaliminal)\ **Post date:** [April 4, 2009, 5:40pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/9 "2009-04-04T17:40:15Z")
> [@Q.E.D](#): > > T actually stands for “tube”. It’s one of the industry-standard codes for [bulb shape](http://www.topbulb.com/find/incandescent.asp), such as the following common ones: A (the classic lightbulb shape), T (tube), G (globe) and F (flared). 😕 But… then… why are various CF spirals _of all the same shape_ variously labeled different T#s? ETA: That site, Top Bulb, does _not_ describe its spiral bulbs with T#s… but several other sites do, including the eBay store I linked in the OP. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 5:44pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/10 "2009-04-04T17:44:39Z")
I think it’s for the same reason you see transformers sold with ‘wattage’ ratings rather than volt-amps, the marketing and sales people don’t understand their own product. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [April 4, 2009, 6:28pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/11 "2009-04-04T18:28:04Z")
I hope this isn’t too much of a hijack, but does anyone have experience with 3-way compact florescent bulbs? ([e.g. here](http://www.buylighting.com/Compact-Fluorescent-3-Way-s/209.htm)) The images (which may be generic, who knows) show only a single tube, so I’m wondering if the color varies for the three settings. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 6:45pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/12 "2009-04-04T18:45:06Z")
I picked up a pack of 3-way bulbs and was very disappointed. I couldn’t see a difference from one level to the next. That was actually how I found out about the shortened lifespan when they are turned on for short periods. When I called the company to complain that was one of the things I found out. That was last summer, maybe they’ve improved since then but I’m waiting to hear a positive review from someone else. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 4, 2009, 6:51pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/13 "2009-04-04T18:51:37Z")
> [@HongKongFooey](#): > > I think it’s for the same reason you see transformers sold with ‘wattage’ ratings rather than volt-amps, the marketing and sales people don’t understand their own product. Marketing and sales people don’t design transformer rating labels (or, in fact, the rating labeling of any product that I’m aware of). I know this because I actually worked in the transformer manufacturing industry. I’ve never seen a bare transformer labeled with a wattage rating; I suspect you’re confusing them with wall-wart power supplies (which are frequently and incorrectly called transformers by the general public) which are (correctly) rated in watts. The exception is high-power travel voltage converters which are labeled in watts because consumer products are generally only labeled in watts and for most consumer products watts = VA +/- close enough. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 7:00pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/14 "2009-04-04T19:00:39Z")
I was referring to websites and certain stores that sell electronics (who should know better) where they use watts in the product description, not the markings on the transformer itself. God help you if you want to know the form factor or percent impedance of them. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 4, 2009, 7:07pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/15 "2009-04-04T19:07:57Z")
[QUOTE=Q.E.D.] The exception is high-power travel voltage converters which are labeled in watts because consumer products are generally only labeled in watts and for most consumer products watts = VA +/- close enough. [/QUOTE] Yes those, I didn’t notice this part when I replied. I know it’s close enough but I still don’t like it (I’m kind of an asshole that way :p) ---
**Author:** ![not\_alice](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/not_alice/32/7283_2.png) [@not\_alice](https://boards.straightdope.com/u/not_alice)\ **Post date:** [April 5, 2009, 1:38am UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/16 "2009-04-05T01:38:22Z")
Although as someone noted, the bulb diameter may be a factor, it is not the defining factor of the technology I was surprised to learn. I’ve been considering shop lights recently. A couple of weeks ago there was a very good display explanation of all this - had to do with ballast type and efficiency IIRC - at the local Home Depot - or was it Lowe’s? They are across the street here, not sure which it was. Get thee to the florescent fixture section of each, and then you will know all you need to know. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [April 5, 2009, 3:26am UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/17 "2009-04-05T03:26:27Z")
> [@emmaliminal](#): > > OK: in other words, does anyone know what the significance of tube diameter is to end use in a compact spiral? There seem to be different tube diameters available for the standard common medium screw-in base; it looks like larger diameters tend to be larger wattages but not uniformly so. So if I have a choice between two 30W, 2700 lumens, medium-base bulbs, one T3 and one T4, should I decide based on price alone, or is there any qualitative difference I should know about? No significance to the end user that I know of. Mostly an engineering decision by the designer of the CFL. So yes, you probably should decide by price alone. Or look at quality differences, but those are usually not influenced by T-diameter, but by company – some brands are less reliable than others. ---
**Author:** ![Pseudocode](https://avatars.discourse-cdn.com/v4/letter/p/59ef9b/32.png) [@Pseudocode](https://boards.straightdope.com/u/Pseudocode)\ **Post date:** [April 5, 2009, 4:00pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/18 "2009-04-05T16:00:09Z")
> [@HongKongFooey](#): > > I picked up a pack of 3-way bulbs and was very disappointed. I couldn’t see a difference from one level to the next. That was actually how I found out about the shortened lifespan when they are turned on for short periods. When I called the company to complain that was one of the things I found out. That was last summer, maybe they’ve improved since then but I’m waiting to hear a positive review from someone else. I’ve been using a 3-way CFL for about 18 months now (3000+ hours) with no problems. I have noticed that the warm-up time is fairly long though. Did you let your bulb warm up on the brightest setting and then compare it to the other settings? ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [April 5, 2009, 4:13pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/19 "2009-04-05T16:13:47Z")
They had plenty of warm-up time but you’re right they do take a while to settle in. I think, in the case of the 3-ways, I just picked a cheap brand (a mistake I’ve made a few times with the regular CFLs). ---
**Author:** ![emmaliminal](https://avatars.discourse-cdn.com/v4/letter/e/7bcc69/32.png) [@emmaliminal](https://boards.straightdope.com/u/emmaliminal)\ **Post date:** [April 5, 2009, 6:02pm UTC](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980/20 "2009-04-05T18:02:27Z")
> [@t-bonham@scc.net](#): > > No significance to the end user that I know of. Mostly an engineering decision by the designer of the CFL. So yes, you probably should decide by price alone. Thanks. > [@t-bonham@scc.net](#): > > Or look at quality differences, but those are usually not influenced by T-diameter, but by company – some brands are less reliable than others. Any recommendations? [Next page](https://boards.straightdope.com/t/compact-fluorescent-bulbs-whats-t2-t3-t4-t5/491980.md?page=2)