The difference is 1/8" in diameter. That's all. I would choose the one for which replacements are more readily (and inexpensively!) available.

I guess that it would be easy to assume that a smaller diameter bulb would yield less light, but that does not appear to be the case...at least between the T4 and T5 of equal lengths.

David knows more about fluorescents than I do, but I'd look at availablity of different bulb qualities. It may be that the larger diameter bulbs, having been around longer, are available in a wider range of color temperatures and CRI ratings.

In my search, I've found that most manufacturers offer a choice of bulbs (temperature/K) but seem to have cool white as the default choice. When the manufacturer offers one type of bulb only (or at least shows only one choice), it is also usually the cool white.

That's probably because cool white bulbs are cheap. If you care about the quality of the light, you should look for better bulbs.

Given the choices, what bulbs give the best work light without muddling the colors of things:

3000K - warm white
4100K - cool white
6400K - daylight

I haven't seen price differences between the choices.

Thanks,
Rob - Long Beach, NY

Almost certainly the daylight, but you're really looking at the wrong attribute. You want to look for a number called the CRI or Color Resolution Index. The closer that number is to 100, the better.

Cool white is cool-colored with lousy color resolution.

Warm white is warm-colored with lousy color resolution.

Daylight is cool-colored but probably has much better color resolution.

What Jon says was true years ago, but not any more. "Cool white" was often used (and still often is) to refer to the old halophosphate phosphor T12 lamps, which had low CRI (in the 60 range or less). These do yield rather unpleasant light and poor color rendering.

"Warm white" also used to refer to modified halophosphate lamps. Often these had even lower CRI than the cool white lamps, in the 40s or 50s. These did an absolutely horrible job on colors.

However, modern T8 lamps have a blend of 3 types of rare earth phosphors. This gives a much better light and a wide range of color temperature (CT) and color rendering index (CRI).

You can get T8s in 4100K color temperature with high CRI (82 or so). These have a somewhat chilly light but it's much improved over the dreary light from the old T12s. They still feel like living under a perpetually cloudy sky, but at least the color rendering is better. They're incredibly bright, BTW.

The 3000K T8 lamps with 82+ CRI produce a very pleasant warm light which blends well with incandescents. The pinkish cast of old warm whites is gone, and color look good and warm under them. They're so far ahead of the old warm white that there's no comparison.

When buying T8 lamps, look at the product code on the lamp. It depends on the manufacturer, but most adhere to this convention. You'll usually see something like "F32T8/741" or "F32T8/830." There may be some other letters in there, but what you're concerned with is the last 3 digits (after the slash).

The first digit designates the CRI, the last two stand for the color temperature. Thus an 830 lamp is 82CRI, 3000K and a 741 lamp is 74CRI and 4100K. You usually want the first digit to be as high as possible, and the last two are more a matter of taste.

I use 830 lamps in living spaces, and 835 or 735 lamps for the cellar and garage. If you're fitting out an artist's studio or a laundry room, you might want 850 or 865 (bluish-tinged daylight).

There are also a few very high (90+) CRI lamps available. Osram-Sylvania and/or Philips (sorry, don't recall which) make 930, 950, and 965 types. These are fairly expensive and usually have to be special ordered, but they should produce excellent color rendering.

Hope this helps.

Some additional information. Here's a table showing the CT, CRI, and roughly equivalent T12 and T8 lamps.CT
CRI
T12
T8
3000K
52-53
Warm White
none
3000K
74-79
Deluxe Warm White
FxxT8/730
3000K
82-85
Proprietary
FxxT8/830
3500K
57-60
Medium White
none

3500K
74-78
Proprietary
FxxT8/735
3500K
82-85
Proprietary
FxxT8/835
4100K
62-67
Cool White
none
4100K
74-78
Proprietary
FxxT8/741

4100K
82-89
Deluxe Cool White
FxxT8/841
6500K
75-79
Daylight
none
6500K
75-79
Daylight
FxxT8/741

6500K
82-85
Proprietary
FxxT8/841

"Proprietary" indicates that most manufacturers supplied a lamp meeting this standard, but used a proprietary name for it (Designer, Chroma, Ultralume, etc.). The only names that were used industry-wide were warm white, medium white (or just white), cool white, and daylight. Later "deluxe" versions of some of these became part of the standard names.

Notice that there are essentially no T8 equivalents of the old low-CRI T12 lamps (though some of the cheap Chinese import T8 lamps may fall into that category). AFAIK, all the T8 lamps use triple rare earth phosphors and have CRI at least in the 70+ range.

The table is not exhaustive; many other types of lamps have been manufactured.

By the 1970s and 1980s, it was quite possible to obtain high-CRI T12 lamps. What has changed is that in those days high-CRI lamps were hard to find. Today, the standard low-priced T8s available from stock in most hardware stores are about equivalent to the special order midrange T12s of 30 years ago. Thus, with T8s, you're much more likely to get good light than you ever were with T12s.

David, thanks for bringing us (me) up to speed. Do you happen to have similar information about the smaller bulbs currently used in UC lights?

For the most part, for every quality T4 and T5 slim strip light I saw, the CRI index for the bulbs were stated to be 82+.

The bulb most recommended (or at least the default choice on manufacturer sites) seems to be the 4100K cool white with the CRI of 82+.

Unless I am convinced or reeducated otherwise, I am assuming that these bulbs will play best in a kitchen with halogen incandescent recessed lighting.

Am I on the right track?

Thanks, Rob

The color temp of the halogens (if not dimmed) will be around 2900, so the cool white fluorescents will look cool by comparison. The difference will increase as the halogens are dimmed.

I don't know about the science, I just know that when I was picking under-cabinet fluorescents, I was able to see various GE bulbs "in action" and ended up choosing the 3500s over the 3000s. We like them, but don't have much halogen in the kitchen if that makes a difference.

A 75-100 watt conventional incandescent (A19) lamp is around 2800K. A halogen lamp runs hotter and thus is typically 3000K - 3400K, depending on wattage. Jon is spot on in saying that these CTs will drop drastically (the light becomes warmer) when the lights are dimmed. (FWIW, dimming fluorescents usually doesn't have the same effect on their CT. Sometimes they even get a bit cooler rather than warmer.)

I don't know of any incandescent that even approaches 4100K, so "cool white" probably won't blend well with anything incandescent. I suspect that the reason this is the default CT is that it's what most people think of as fluorescent light - a darn shame, if you ask me, since it seems to make them think of offices, not homes.

You can get 2700K in FxxT8, though it's usually a special order item. It's available in T5 and T4 too.

Fantastic information.

We are in the process of choosing undercabinet lighting. I like the look of xenon illumination but now am trying to learn about fluorescent options to cut down on the heat.

Now I understand that xenon is pleasing to me because its CRI is close to 100.

DavidR mentioned some T5's do have CRIs above 90. I tracked one down on the Philips web site. Their T5 with 90+ CRI has a 5000K color temperature though.

Here is a link that might be useful: Philps T5 lamp link

Well David,Just a couple of minutes back we were discussing about the diff. betn.T4-T5 lamp.We require T5 RGB lamp for a fixture.Since T5 RGB is not available we planned using T4 RGB and we tested too. The scenario is a fixture having DALI controler requiring 3 RGB lamps that will dim continuously.Now the question is what would the output be on the long run? Will the ballast for T4 and T5 creat any difference, the limitation of the lamps....., the drawback - Can u please share some ideas.