So now we’ve looked at the dimming performance of the Philips SlimStyle bulb, and even peeled back the plastic cover to look at the LEDs and driver circuit. (And I’m not being editorially coy in using “we” – I appreciate the information and corrections!) What about the actual light the bulb produces, and how does that stack up to the competition?
The SlimStyle’s packaging is labeled with some basic light quality and color temperature information: A CRI (color rendition index) of 80 and a color temperature of 2700K, which are industry standards now for LED incandescent-replacement bulbs. But what do these numbers mean, and are they really enough to judge how a light bulb will perform?
We can dig a lot more deeply into CRI with a spectrometer and related analysis software. I’m using the very impressive handheld spectrometer, the UPRtek MR350 with MoreSpectra software from Moreland Lighting.
(More on all the advantages of using a powerful, sophisticated instrument like the MK350 and MoreSpectra software in a later post.)
With CRI, higher numbers are better, with 100 being the ideal number of sunlight at 5000K. Also, you never round up, only down: A CRI of 81.7 is 81, not 82, because it shows what level the CRI (or R value) has reached, not what it’s closest to.
CRI and R values
CRI is a little more convoluted than just looking at the spectrum of light that’s present in a white light: It’s determined by comparing the color of sample color swatches under a test light source (in this case the SlimStyle) to a reference light source of the same correlated color temperature. The CRI value is then computed using eight colors, R1 through R8, that are unsaturated, pastel-like colors. However, pastel colors are only part of the visible spectral “information” that our eyes are sensitive to. Six supplementary colors are also tested to provide additional information. R9 through R12, for instance, are saturated versions of red, yellow, green and blue. LED light sources typically do a poor job of rendering reds, so to get a complete idea of the light quality of an LED bulb we need to know not just its CRI number, but also its R9 number.
Enough of the theory of CRI; Now let’s look at some examples.
Here’s the spectral power density (SPD) curve for a 60W incandescent bulb:
The spectral power distribution (SPD) is on the left, and the R values used to calculate the CRI are on the right. Looking at the amount of red showing in the SPD you can see why incandescent lights render red with such richness. The R9 value bears this out: It’s a whopping 98 out of 100.
Next, here’s the MK350/MoreSpectra rendering of SPD and CRI for the Philips SlimStyle:
The SPD graph shows the usual blue spike for white LEDs, falling off into the “valley of cyan” before ramping up into the peak yellow region. It falls off rapidly in the reds.
The SlimStyle’s CRI measures a bit better than its listed CRI: 81 rather than 80. But CRI doesn’t tell you what other colors are present in making white light, which is why R numbers, for example, are important in judging light quality.
The important number to see here is the R9 — saturated red — which is only 13. The take-away from these is that while you can expect a nice, all around white light from the Philips SlimLine, it won’t render reds very well – at least compared to an incandescent bulb.
Next up: How do Cree bulbs stack up in R9 values?
Cree has two LED incandescent-replacement bulb families: The original soft white (SW) series, released last March, included a 9.5W/800lm bulb and a 6W/450lm bulb, both in with an 80 CRI at 2700K. The more recent TrueWhite (TW) 11W/800lm bulb, also at 2700K, but with a CRI of 93, was released just last November.
The original Cree SW bulb is the head-on competition for the Philips SlimStyle in terms of performance: In addition to its CRI of 80 and color temperature of 2700K, it’s dimmable and omnidirectional. So what about it’s color quality?
As seen below, its SPD is quite similar to the SlimStyle’s with the blue spike, cyan valley, and lots of yellow.
Even though its CRI is the same as the SlimStyle’s. measuring 81, it’s R9 value for saturated red is an even weaker 6. So in terms of terms of light quality, the higher R9 value gives the SlimStyle the nod.
But what about Cree’s newer TrueWhite bulb, with its eye-popping 93 CRI? The Cree TW has a neodymium filter added to its glass cover that tweaks much of the yellow out of the bulb’s SPD. As seen below in the SPD, it’s no small tweak – it’s more of an optical notch filter.
By getting rid of some of the color spectrum that can make skin tones sallow, this filter also allows the R9 value to climb to an excellent value of 60, making the Cree TW the clear winner in color quality. However, it also has a list price at Home Depot of $20. Ouch.
So, which bulb should you buy? This is a gnarly question to answer. The only fault I could find with the SlimLine, which was actually a pretty serious one, was the noticeable humming it made when on a dimmer switch. However, that could have been a characteristic of the early bulb I had which was clearly a prototype. Let’s assume for the moment that production SlimStyles are quiet. Add that to the fact that the Cree SW costs more – it comes in at $13 vs the SlimStyle’s $10 – and you’d think, especially for lights that are not on a dimmer circuit, that the SlimStyle would be the clear winner.
Not so fast. I live in California where, as of January 1, no public utility can rebate a bulb which does not meet the standards of the California Energy Commission (CEC), the most significant of which is that LED bulbs (NOT, mind you, CFLs) must have a CRI of at least 90. And sure enough, last week Home Depot began selling the Cree TW in many (all?) California communities at a rebated $10. Given the choice of the SlimStyle at $10 or the TrueWhite at $10, for the gorgeous light quality alone you’d have to take the Cree TW. But you can only expect to see that kind of pricing through rebates, and probably only in California. Both the Cree bulbs and the SlimStyle at this point are sold exclusively through Home Depot.
Another considerable advantage for the Cree bulbs: Both the SW and TW series bulbs come with 10 year warranties vs. the SlimStyle’s 3-year.
So far the Philips SlimStyle is not available for purchase at Home Depot stores, although you can order them online. When they are generally available we can see about that dimmer-switch buzz. As things stand now, as a Californian able to take advantage of the rebated price, I will purchase Cree TW bulbs for my house at $10 each and enjoy the beautiful subsidized color quality. If I lived in a state without the TW rebate, I would buy a few TW bulbs for locations such as the entry and dining room, and then put SlimStyle bulbs in my (non-dimming) lamps.