I recently got my hands on a couple of the new Cree linear LED T8 fluorescent-replacement lights. These lamps go into an existing fluorescent light fixture and run off of the fluorescent ballast. Cree says they are compatible with 90% of the existing fluorescent T8 fixtures, even dimmable ones.
Why should we care about fluorescent lights? Because they are the largest consumer of electricity in the US. In 2010 there were about 1B fluorescent luminaires, of which about 60% were T8 lamps. So that’s 600M T8 fluorescent tubes. At 2 lamps per fixture, that’s 1.2B T8 tubes.
No more T12 fluorescent lamps
But here’s another, more pressing reason: As of July 14, 2014, fluorescent lamps that can’t meet efficacy levels of 88 lm/W (or 89LPW for lamps with CT greater than 4500K) will no longer be allowed to be manufactured or imported into the US. This eliminates all T12 fluorescent lamps and many T8. If you’re like me, you’ll find several fluorescent light fixtures in your home that may suddenly have become obsolete because their lamps are no longer available.
With the July 14 deadline looming, it’s not so surprising that Cree recently introduced a linear LED retrofit lamp for T8 fixtures. (Philips introduced its InstantFit product last February and I hope to test it soon.)
According to CALiPer’s recent report on linear T8 LED lamps, benchmark numbers for good (not necessarily average) T8 and T12 fluorescent lamps are about 3,000 lm per lamp, with efficacy around 100 lm/W for the T8s and 80 lm/W for the T12. Here’s a quote: “Although there continues to be some development of linear fluorescent lamps, these performance characteristics remain fairly constant.” I interpret this to mean that fluorescent lamps have pretty much plateaued in efficacy, with many T8 lamps meeting the July 2014 requirement, while no T12s meet them.
The report also says that “…the basic F32T8 lamp is the most commonly used today…”
So I went to Home Depot and purchased two F32T8 32W Philips Alto II lamps, which are 48-in. T8 fluorescents with a listed light output of 2800 lm, a color temperature of 3500K, and a CRI of 78. At 87.5 LPW, these miss (barely) the July 2014 minimum efficacy of 89 LPW. But they are extremely popular and representative of existing T8 fluorescents.
Since all of the fluorescent fixtures in my home and office are T12s, I also picked up a T8 fixture with a non-dimming ballast, a Lithonia Diamond Plate fixture.
The ballast that this fixture came with was mediocre at best; Not because it wasn’t compatible with the Cree LED lamps, but because it made the fluorescents look bad. So back I went to Home Depot for a replacement ballast for a Philips AmbiStar T8 ballast. (I will cover the foibles of fluorescent ballasts in a later post; I want to keep the focus in this article on the Cree LED T8 lamps’ performance.)
Fluorescent T8 lamps performance
Let’s start with the test results of the fluorescent lamps. I think that the best way to evaluate light quality is with a spectrometer: I use the handheld MK350, with the MoreSpectra software package. As we’d expect, we see the peak-y spectral power density (SPD) graph that’s the signature of fluorescent light:
Note the pathetic R9 of −1, which is about the poorest R9 I’ve measured. The overall CRI = 81 is surprisingly high — higher than its specified number of 78. But don’t expect reds to render at all well. This reading was after a warm-up/stabilization time of 30 minutes. The reading immediately after turn-on was much lower.
Cree T8 LED performance
Now we’re ready to look at the light from the Cree LED T8 lamps:
Unlike fluorescent lights, the Cree T8s came on immediately at full brightness. The light quality is excellent, with CRI=91, and an R9 of 56. Cree calls its LED color-quality technology “TrueWhite,” which is based on the addition of red LEDs with white LEDs. I believe it dynamically ups the intensity of the red LEDs as the light dims, allowing the light to mimic an incandescent’s red-upon-dimming light characteristics, and matching our human predilection for warmer tones in dimmer light. [Nevermind, I'm not seeing this in the actual teardown.] I wasn’t able to test this, however, nor the lamps’ overall dimmability because my Home Depot doesn’t sell dimming fluorescent ballasts. (Like I say, the selection of a fluorescent ballast is a nightmare real challenge, and I will cover it in a separate post.)
This is a wonderful light quality, especially the R9 value.
And just for reference, here’s what the sun was emitting at noon yesterday on a gorgeous bright spring day day in California:
You can see at a glance that the fuller light spectrum covered by the Cree is much closer to a natural lighting scenario.
There was a question posted on the announcement of the Cree T8 here last week about the lower lumen output of the Cree compared to a fluorescent lamp: 2100 lm for the Cree vs 2800 lm for the fluorescent T8. Comparing lumens for fluorescent lamps is a deceptive measure: Fluorescent tube lamps are an omnidirectional light source, emitting light uniformly around the axis of the lamp. LEDs are a directional light and their light pattern is at most about 180 degrees.
However, directional light is for most fixtures actually an advantage over omnidirectional, because much of the omnidirectional light must be reflected from the top of the fixture and back down towards the work area, introducing an inefficiency. Since virtually all of an LED lamps light goes down in a horizontally-mounted fixture, there is much less inefficiency in its light-pattern. We can see this in the Lux numbers for each lamp, since Lux is an areal rather than a spacial light measurement: The Cree measured 5997 lux, while the fluorescent lamp only measured 4943 lux. So, the Cree wins out in light output as well as light quality.
Slightly different shape
The Cree lamps are different in shape from traditional fluorescent tube lamps: They are slightly oval and their 1 1/-in. cross-section is slightly larger than the 1-in.diameter of a fluorescent T8.
This can be a problem for some fixtures, as I found to my dismay. The first fixture I bought at Home Depot would take two fluorescent T8 bulbs, but not the Crees. Here is the fixture with two fluorescent lamps:
…and here are the too-big Cree LED T8s:
(I returned the fixture and bought the Diamond Plate.)
Cree says the lamps are compatible with 90% of the installed fluorescent fixtures. Initially I assumed the 10% that were incompatible were due to ballast weirdness; now I think at least some incompatibilities are probably due to too-narrow tube mountings. It shouldn’t be a problem for commercial fixtures.
It’s important to keep in mind that, unlike LED incandescent replacement bulbs, LED T8 linear lights are not intended primarily for the consumer home, but for commercial installations. In these installations lights are turned on and left on for 6-12 hours and longer, so their lifetime is long. Even the low-end fluorescent lamp that I used in this evaluation has a rated lifetime of 30,000 hours. So it’s difficult to make a case that LEDs last many times longer than fluorescent lights do in a commercial setting.
The Cree T8 claims its lifetime is 50,000 hours and has a warranty of 5 years. (This 5-yr warranty is half the 10-year warranty that many of Cree’s other lighting products carry. I don’t know why: I’ll ask.)
Oh, right, the price…
In general, fluorescent lamp technology is currently fairly competitive with LEDs as you can see by comparing the 100 lm/W efficacy of the Cree T8 lamp with the 89 lm/W mandated fluorescent lamp efficacy of the July 2014 deadline. Over time we can expect LED lighting to continue to improve its efficacy numbers, while fluorescents plateau. But for now, the sticking point for fluorescent-replacement LED linear lights is the price: The Cree is $30/ea, while the fluorescent lamp can be had for well under $5/ea. However, when you remember how fast the price of LED A19 incandescent-replacement bulbs fell between 2011 and the present — from $50 to well under $10 — a sizable drop in LED T8 pricing is reasonable to predict also. In the meantime, utility rebates will help to push prices down in the commercial sector.
In the meantime, for any commercial application where color quality is a factor such as office space or retail lighting, the excellent color quality of the Cree LED T8 lamps makes them the winner. Plus, I’ll go out on a limb here and predict that linear LED lamps such as the Cree T8 will be the gateway product to introduce linear lamps more widely into residential lighting, where their color quality makes them a cost-effective way to get virtually full-spectrum light into the home.
Next up: Tearing down the Cree T8 linear LED lamp.
|Lithonia Fixture, Philips AmbiStar ballast|
|Cree T8 LED lamp||T8 fluorescent|
|Watts, each, spec’d||21||32|
|Watts, measured, 2-tube fixture||52||61|
|Color Temp, spec’d||3500K||3500K|
|Color Temp, measured||3304K||3217K|
|Warranty||5 years||3 years|