As LED bulbs come down in price and up in performance, the viability of LEDs as a replacement for incandescent, halogen, and fluorescent lighting approaches a tipping point. I've been into LEDs for a while and have made many of my own bulb replacements and specialized LED fixtures. However, it is still nice to be able to drive my Escalade 7 blocks down to the local big box home improvement store and fork over some cash for a commercially made LED bulb while sipping my half caf mocha latte and chatting loudly and profanely with my buddies about the epic powder experienced during my recent ski weekend on a bluetooth headset, brah.
Toward that end, I recently rounded up a number of the LED bulbs I've bought over the past years and measured their power usage and light output using a kill-a-watt, integrating sphere and light meter. The following details those measurements and presents a look into the economics and efficiency of today's LED bulbs. As this is a rapidly changing field I hope to keep these measurements somewhat current as new products are introduced. Due to the wide range of bulb types out there, I am concentrating on the most common style, standard A-series type screw in bulbs.
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Step 1: Method
Taking brightness measurements is not as complicated as you might think. The most accurate method is through the use of a device called an integrating sphere. An integrating sphere is a sphere (duh) coated with a diffuse reflective coating inside to allow scattering of the light input to the sphere such that the illumination at any point on the interior is uniform. This minimizes the effect of directionality due to optics or reflectors present on the bulbs being measured. The sphere has two holes in it, one for the bulb being measured, and one for the measurement. A baffle is placed inside the sphere as well between the light source and the measurement port so that light cannot directly impinge on the measurement port. The measurement is made by a light meter inserted into the port, and the system calibrated by placing a calibrated light source (with a known light output) in the sphere and taking a reading with the light meter. The light meter outputs a lux measurement that can be converted to lumens via the known calibrated light source. This generates a lux to lumens calibration factor for the system. I don't have a calibrated light source but the good news is that comparison among the bulbs doesn't require conversion to lumens, we can stick with comparing the lux readings.
So the method for this study is to simply screw the bulb into a light bulb socket on the end of a power cord, plug the cord into a kill-a-watt or similar power meter and stick the bulb into the integrating sphere. Making note of the lux measured on the meter and the power on the kill-a-watt. The higher the lux reading, the more lumens output by the bulb. The factor we are interested in though is the light output per watt, a measure of the efficiency of the bulb. Since LEDs and CLFs change their light output with temperature, I also took and initial measurement and then waited 5 minutes and took another to compare how the bulbs performed once they got hot. In the case of LEDs the effectiveness of the heat sink will be shown by how much or little the light output drops over time. A bulb that looses a lot of intensity with runtime signals a bulb with a poor thermal design and which will not last long and is not suited for use in an enclosed fixture.
One caveat is that this method does not correct for color, meaning lumens are typically scaled by the sensitivity of the human eye. So a red bulb would have to output more red photons to reach the same lumen output as a green bulb since the human eye is more sensitive to green light. Since these are all nominally "white" bulbs of various color temperatures (mixes of wavelengths) we'll assume that the lux meter and the human eye is nominally equally sensitive to the light output by the bulbs.
Step 2: Diversion: Making an Integrating Sphere
The following is a diversion from the main topic at hand, taking measurements, into the weeds of how to make an integrating sphere to take measurements.
As you can see in the pictures, the integrating sphere I have is not too sophisticated. Basically it is a large "papier mache" sphere painted inside with flat white paint and with two holes cut into it and a baffle between the holes to prevent light at the input from reaching the output port without bouncing off a few walls. That is it.
The process started with sourcing a large beach ball without seams, then I tore up a bunch of newspapers into inch wide strips, diluted a bunch of wood glue, soaked the newspaper strips in the wood glue, and applied them to the beach ball. I did this many times to develop a multi-layer shell of glue and paper over the ball. I let the whole thing dry after each coating, and built up about 5-6 layers. Letting this dry completely for a few days resulted in a very hard and strong sphere. I cut a hole in the sphere to serve as the input port, popped the ball and then cut the whole sphere in half to allow me to paint the interior and install the baffle. Before painting you'll want to cut the output port in one of the halves and any other port you might want. Placement of the input and output ports is kind of arbitrary, but separated by 90 degrees on the sphere is a good place to start.
For the interior, there are lots of different materials and methods to achieve a truly diffuse and highly reflective coating. Some of which are expensive purposed made coatings, and others dustings of special powders or other materials. The goal is to have a surface that reflects all wavelengths equally well (a flat frequency response if you will) and is stable over time (to maintain calibration). After reading a bunch about it online, I came to the determination that flat white ceiling paint was good enough for me. Oh, and I had a gallon or two sitting around. It is stable, pretty flat frequency wise, and dirt cheap. Once painted, install the baffle, which also needs to be painted.
The baffle should be big enough to prevent light from your biggest light source to be tested from reaching the light meter port directly, as in without bouncing around the sphere a bunch, but not too big as to impact the uniformity of the light in the sphere. For regular household sized bulbs, I put a baffle roughly 5 inches in diameter in the sphere. This will help to normalize your readings since you won't be able to easily position the bulb in the same place every time. Placing a 1000 lumen spherical bulb in the integrating sphere should give you the same lux reading as a 1000 lumen spot light if your baffle is well placed and your sphere coating uniform. In practice I found that to be largely the case although 5% fluctuation due to placement of the bulb is observed, indicating some directionality. Oh well, the price was right.
Once the baffle is placed, you need to glue the sphere halves back together. I used tape and hot glue. Be careful to keep the seam nice and tight so that the interior is uniform. In the photo you can see a view from the input port into the sphere where the baffle is visible.
That is pretty much it.
Now back to the measurements.
Step 3: Measurements
For comparison, I've also put in a standard daylight (3700K) CFL and one of my home made DIY single Cree XML bulbs. The XML bulb does pretty well, with a good thermal design keeping light output steady over time but total output limited by the single emitter and low drive current. The CFL takes some time to warm up, but has OK efficiency once warm.
The take away from all this, is that the bulbs are getting better. The latest bulbs at home depot from Cree are especially nice, with a decent price, nice warm omni-directional light, and great efficiency. The downside to the Cree is a glass dome with silicone coating that attracts dust like black pants. Yuck whose idea was that? Crud sticking to the silicone coating will impact efficiency over time, so I hope these bulbs are dishwasher safe. Cleaning stuff off of them is not easy.
The Philips 420240 is also a good choice with a very compact form factor and decently warm light. Even at the elevated prices of today ($10-15 per bulb) these LED bulbs compete well with CFLs in short duration, high cycle applications where a CFL would burn out quickly.
|Cree BA19-08027OMF-12DE26-1U100||Start up||8||17300||0.07||0.97||2163|
|LG 593260||Start up||5||11040||0.04||1||2208|
|Philips 422154||Start up||11||15740||0.12||0.78||1431|
|Philips 420240||Start up||10||19180||0.1||0.8||1918|
|Tess T-67004S||Start up||6||9600||0.09||0.59||1600|
|DIY XML||Start up||2||3800||0.03||0.74||1900|
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