A controller system using consumer LED lamps to provide color temperature control and management alongside daylight dimming.
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The first results that were collected from the system were the normalized responses of the lamp light output relative to the power input. Rather than list the measured power level I've simply used the abstracted power levels used by the lighting controller board (a range from off at 66 to full on at 0). Although it was possible to calculate the measured lumens of each lamp once an exact distance between the lamp source and the sensor was measured, the method used to produce this graph involved using the rated lumens rating of the lamp as a reference relative to the readings measured by the ambient light sensor.
|A graph of the output (lumens) of each lamp versus the power level setting in the lighting controller.|
Right away there are a couple of curves that stand out for cutting across the others: that of the GE Cool Daylight and the Feit Amber lamp. I've included a halogen incandescent and Cree Exceptional Light Quality LED lamp in this stage as controls to provide some reference lines for what are otherwise lamps selected as commodities and not because they are highly rated for their compatibility with dimmer circuits (as is the case with the Cree). Some manipulation of the data by changing the scale to logarithmic and reversing the order of the data amplifies what is happening.
|The same data with the vertical scale made logarithmic and both vertical and horizontal scales inverted.|
The halogen lamp reaches the highest point in this version of the graph, demonstrating that it has the widest range of lighting levels. However, this may not be entirely useful since there may be little to no value gained from being able to control light output at such low levels as single-digit lumens. In contrast, the Feit Amber has the lowest starting point on the graph indicating a short range from minimum to maximum brightness.
The Cree and EcoSmart lamps have the widest graphs demonstrating that they make maximum use of the power output range of the dimming circuit relative to their supported brightness ranges. The narrower graphs of the GE and Feit lamps suggests that they would provide less granular dimming control relative to the other lamps.
Of course, a big factor to consider here is the design of the dimming circuit. Though certainly worthy of a dedicated post, suffice it to say that the power control method used by this particular controller - known as leading edge dimming - is actually not of the type best suited to controlling LED lamps - that would be trailing edge dimming. As such, a circuit more optimized for controlling LED lamps might demonstrate wider control ranges for the Feit or GE lamps, and taller output light ranges for sources like the Cree.
Ultimately the EcoSmart lamps have to be congratulated for performing very comparably to the highly-rated Cree in this particular measure. The GE lamp performance is somewhat disappointing given the prominence of the GE lighting brand, whereas the limited range of the Feit Amber shouldn't be too surprising or discouraging for what is very much a specialty lamp likely not intended for general illumination.
As one of the target applications of this control system, we've already used this collected data to create what could be termed the ultimate dim to warm lighting range. Dim to warm LEDs like the Warm Glow series from Philips lower the output color temperature as the light is dimmed to simulate a similar result which occurs with incandescent lamp dimming. However, this color temperature shift is usually small, moving a 2700K lamp to 2200K when dimmed.
Using the dimming data above and the full spectrum of the lamps targeted for this prototype it was possible to create a scenario where the lamp output at full brightness possessed a cool 6500K temperature and, while dimmed, took the lighting color temperature down the very warm (and perceptibly red) 2000K temperature of the Feit Amber lamp before finally shutting off. This can be challenging to properly capture...Read more »
As our testing progresses we've been exceptionally fortunate in our ability to get all of the components working and operational in a timely fashion. What may be missing for some, however, is a view of our solution which appears like anything that would be labeled a light fixture rather than science fair. This entry will review the original design concept, current prototype, and discuss the possible applications and options we have considered to date.
Let's begin with a review of the original project schematic. As demonstrated below the schematic began with a primary controller. This controller would be responsible for managing the communication path between the sensors responsible for measuring light levels and color components. Integrating these results into actionable output commands would also be directed from the primary controller to a dedicated lighting controller. The lighting controller's primary function is to interpret lighting commands into AC power line changes to the associated dimmable LED lamps (up to four).
|Original project schematic documenting the primary controller in communication with multiple sensors as well as a dedicated lighting controller controlling up to four (4) lamps using traditional AC power controls (in red).|
Implementing this design as a prototype has proven straightforward. As documented below the current prototype board retained a standalone lighting controller with connectivity to four lamps. The primary controller is currently deployed as a standard arduino board. Given the availability of numerous I2C-based sensors we've actually experimented with more sensors than simply the spectrometer and ambient light sensor documented in the original schematic. Although the spectrometer has provided insight into the distinctions between each lamps color spectrum, we've also had good luck with taking color temperature measurements using an RGB sensor board.
|The current prototype system with lamps, lighting circuit controller, arduino controller, and three (3) sensors for lighting spectrum analysis, lux readings, and color temperature calculations using an RGB sensor. And yes, we did use a shipping box as an integral part of the prototype!|
What might be rightly questioned at this point is how this turns into a usable lighting fixture? The good news is that this part of the design is incredibly flexible. Realistically, the only component which would have to be incorporated into a lighting fixture would be the lighting controller element. These electronics could be housed within or adjacent to most any existing 2- to 4- light fixture to provide the necessary local modulation of AC power. The primary controller and sensor network could also be integral to the fixture, or it could be wired remotely or even done wirelessly. Part of our goal in designating a standalone lighting controllers was facilitating that kind of separation. The primary controller and sensors are incredibly simple elements which are much more complicated in prototype form than they would need to be if implemented on a single PCB as a remote device (and even in their current form they're still incredibly compact!).
In constructing our prototype we intended to remain abstract rather than choose a specific fixture design since we've imagined this concept having applications in multiple areas. From a simple overhead light, to a bathroom vanity, to an even larger fixture appropriate for commercial installations the fundamental concept was focused around color temperature control of commodity dimmable lamps. After putting in the hard work to construct their CalEarth building, I don't imagine anyone choosing the fixture design of our prototype as a centerpiece to their new home. However, I would like to imagine that whatever lighting fixtures are selected could incorporate some of the control elements we've evaluated.
One of the stated goals of the project was to speed implementation through the use of retail boards rather than scratch-built circuits. Of course, this brings its own unique sets of challenges and rewards, a couple of which I'd like to highlight here.
The primary controller and sensor elements have proven very easy to address and program. In large part this is thanks to the exhaustive and easy-to-follow documentation Sparkfun has constructed around the devices we sourced from them. Getting started guides have proven a valuable part of the experience above and beyond the usual starting points of datasheets or sample code.
However, while the lighting controller board was also easy to start addressing using the code samples provided (evidenced by our quick success at blinking a light) perhaps one of the most valuable pieces of documentation was sourced not from the manufacturer but rather from an Amazon reviewer(!). That review clarified the performance of the board's dimming range. The range is addressable from 100 (full off) to 0 (full on), but values above 66 produce either no output or full output (the latter being the most unexpected/undesirable result). We have been able to validate the same performance quirks with the controller used in our design, and have adjusted our code to limit itself to the functional range of the device. At this time it is unclear whether the issue is related to a hardware or firmware element within the device, but I'm sure further examination of the board's design would be interesting (if we didn't have more pressing matters at this time).
|Lighting Controller - Top View||Lighting Controller - Bottom|
In addition to the control circuits and sensors integral to our prototype, the idiosyncrasies of the circuits within the LED lamps themselves also require consideration. As part of the initial testing we were clearly able to record the different ways in which different light sources perform with a given electrical output level. The lamps pictured below, from left to right, are the GE Cool Daylight, the EcoSmart Daylight, a high-efficiency incandescent, and a Cree LED lamp (the latter two being part of our initial testing rather than components of the prototype). What you can clearly see the Cree (rather brightly) and the incandescent (albeit remarkably dimly) coming on from even the lowest output levels available from the lighting controller. As levels are increased the EcoSmart and then the GE light both activate.
|Varying responses to increasing output power clearly visible from (L-R) the |
GE Cool Daylight, EcoSmart Daylight, incandescent, and CREE LED lamps.
While we plan to incorporate quantitative measures of this response as part of our project, this speaks to not only the value provided by the sensors we've selected for this prototype but perhaps their necessity. While a design with integrated LED light sources could make assumptions based upon datasheet values, recording and utilizing lighting responses to power input could prove useful for not only the overall flexibility of the system but also for the value provided by abstracting the lighting generation layer from the control layer.
More to come soon!
The team recently added a link on the project page to the GitHub Project Dashboard. This should allow everyone to follow our progress during the final month of the competition.
The prototyping board has been constructed and can be seen below. The primary purpose of this setup is to create a safe space for working with AC voltages. As you can see, traditional lamp fixture bases were used to hold the lamps. The AC dimmer controller board is held up off the prototype board using bolts and nuts as a standoff system. The lamp cords were routed underneath the board. All of this is intended to minimize exposure to the AC side of the circuit while working on top of the prototype board with the low-voltage sensors. This configuration should also keep the light sources close together and, as such, within the field of view of the sensors associated with the project.
|Sensor-eye view of the lamps||Lighting controller close-up||Underside with AC cables|
You may see that the lamp cords in the underside view are separated in some places. This was done to facilitate later non-contact current measurements with a clamp meter. Voltage measurements should be possible using the terminal contacts directly on the lighting controller.
This is the first arduino project for everyone involved on the team, and we can already say that we've done our own variant of everyone's first arduino project: rather than blinking a low-voltage LED we've managed to blink an AC LED lamp!
More updates soon...
As mentioned earlier, fluorescent lamps attempted to address their color spectrum limitations through the addition of newer and more complex phosphors targeted to produce additional color wavelengths. LEDs have another option given their naturally monochromatic nature through the addition of more LEDs engineered to output desired wavelengths1. However, this method has challenges associated with the efficacy of different LED sources to produce light as perceived by the human eye. This design comes under the header of color-mixing LEDs (where LED light emission is the only source), and hybrid-LED designs (where one or more color-mixing LEDs are combined with phosphor-coated LEDs). Though both designs merit further discussion to understand their place in the present and future of white light LEDs, the role of the human eye in color perception also deserves examination.
Earlier the low efficacy of filament light sources was demonstrated by highlighting their tendency towards producing energy in wavelengths invisible to the human eye (particularly as infrared heat energy). Wavelengths of visible light are not, however, all perceived with similar levels of sensitivity by the human eye nor are their means of production using semiconductor sources equally efficient. Just as CRI establishes the conformance of a light source of a given temperature to color rendering associated with an incandescent source, the luminous efficiency function (or eye sensitivity function) demonstrates that the eye has significantly different responses to each of the wavelengths of the visible spectrum. This sensitivity peaks at the green 555 nm wavelength. As expressed in luminous efficacy, it demonstrates that an ideal monochromatic source (excluding all other losses) could have a peak luminous efficacy of 683 lumens per watt. See the figure below for a chart of eye sensitivity.
Of course, a monochromatic source would score some 0 CRI and as such be unsuitable for general lighting. However, it is this curve which high efficiency sources are exploiting by combining peaks in the blue and red regions, where the eye is less sensitive, to combine and produce a light which both appears white while providing suitable efficiency. Early fluorescent lamps did just that, producing high luminous efficacy while outputting only two primary wavelengths. Thus, they also suffered from the limited CRI possible with such a combination. Balancing high efficacy with acceptable CRI is the primary challenge for lighting such as fluorescents and LEDs (whose emissions and/or phosphors often exhibit emission in defined wavelengths rather than as a continuous spectrum as with incandescent light sources).
LEDs seem an obvious candidate for mixing together different colors without suffering the energy losses associated with phosphors, but significant differences in efficiency exist today among LEDs operating at different wavelengths along the visible spectrum. The least efficient LED designs are those engineered to output at or near the 555nm peak eye sensitivity wavelength referenced earlier. This is commonly known as the green-gap (see the figure below) and is one of the reasons why phosphor-coated systems have prevailed over color-mixing only designs to date. While blue LEDs have achieved power conversion efficiencies of 60% and greater, red, green, and amber sources have been limited to 44%, 22%, and 8% respectively2.
The normalized eye sensitivity chart also expressed as luminous efficacy.1
LED efficiency as a function of wavelength.3 Contrast the trough in this curve with the peak in the chart to the left.
Collectively these factors help to explain the popularity of the dominant LED system design based upon blue light LEDs. They also could be seen to recommend the approach we've taken to incorporate commodity white lamps rather than build a color mixing solution one wavelength at a time. Of course, integrating a color spectrometer will enable some level of understanding...Read more »
Although the earlier logs regarding phosphors suggest that fluorescent and LED sources can be manipulated from relatively monochromatic emitters of photons into producers of white light, the quality of that white light must be quantified given the distinctive construction of its constituent color spectrum. Lacking the relatively smooth and continuous energy band of wavelengths associated with incandescent sources results in a performance difference as it relates to color rendering. A metric for this performance is the Color Rendering Index or CRI and ranges from 0 for a monochromatic source to 100 for incandescent sources (with typical values for LED and fluorescent light sources ranging from 80-90)1. The efficacy of a light source can be significantly benefited by limiting its energy output to a finite number of wavelengths, allowing the light to appear white while only generating a fraction of the wavelengths associated with the incandescent spectrum. However, the capacity of such a light to produce acceptable color rendering is a trade-off which has to also be taken into consideration.
As documented in the first project log about color temperature, the color temperature of the light source defines the peak of the energy curve of the associated light spectrum. Lower color temperature light sources accentuate lower energy light wavelengths like reds and yellows (such as the light from an incandescent filament bulb). Higher color temperature ranges transition from white to blues with 5000 Kelvin to 6500 Kelvin range sources being most closely associated with balanced white to slightly blue daylight. Note that in these cases white is meant to suggest a balance of light energy as perceived for each wavelength. Lower temperatures (2000-3000 K) are typically described as warm, temperatures around 5000 Kelvin are labeled as daylight, and even higher temperatures (≥6500 K) are labeled as cool.
These two metrics are important to consider in unison since CRI only measures the conformance of a source’s color rendering performance to that of an incandescent source of the same color temperature. However, the different color temperatures have naturally different impacts upon the way colors appear to the human eye. The example obvious to most would be the warm light visible when the sun is at the horizon (typically rated at 2300 K) versus the sun in the noon sky (5000 K). The strong presence of red wavelengths emanating from a 2300 Kelvin source would suggest that a white (or any other) object would naturally appear more reddish regardless of the rated CRI. The CRI can identify the possibility of unintended color shifts for any given source at a specified color temperature when compared to a source of a different type at the same rated color temperature. However, the CRI does not preclude the presence of color shifts which are a function of a given color temperature. At the same time, personal preferences and historically established expectations may ensure demand for lighting options at every color temperature range.
If CRI is a primary distinction to be overcome between efficient lighting sources and the incandescent sources they replace what other drivers exist to maximize color performance? Well that will be a great subject to tackle in the next project log!
1 Color Rendering Index (CRI) Explained.” Full Spectrum Solutions, Full Spectrum Solutions, Inc., 2 Aug. 2017, www.fullspectrumsolutions.com/cri_explained.htm.
As with fluorescent lamps, LEDs are light sources which generate their output through luminescence. Diodes are a fundamental semiconductor device which allow current to pass in one direction through its terminals while preventing flow in the other direction1. This flow in what is termed the forward direction will only occur at a minimum voltage threshold defined for a given diode based upon its design characteristics. The defining material characteristics are somewhat beyond the scope of a simple project log, but suffice it to say that an LED is a diode constructed with selected semiconductor materials and specific impurities such that as current flows through an LED electrons will cross a positive-negative or p-n junction where they will fall to a lower energy level (this property is common to all diodes). The difference in the energy level of the electron as it crosses the p-n junction is released, in part, in the form of heat. However, in the case of LEDs this energy level difference is specifically engineered such that the release can also include photons of a specific wavelength. The result is that LEDs are a source of nearly monochromatic light. LEDs experience losses to heat as a side-effect of the flow of current through the device, but this is a far cry from incandescent which were in an earlier log shown to more efficiently radiate heat as infrared than any other part of their visible spectrum. The exact LED output wavelength can be tailored based upon material selection2. Rather than being limited to a couple of ultraviolet emission bands as defined by mercury plasma emissions in a fluorescent, an individual LED can be engineered for an output wavelength along the light spectrum from infrared to ultraviolet and points in between all based upon the selection and combination of appropriate component materials.
Although LED light sources can be tuned to produce light at lower energy levels within the visible spectrum, the most common white light LED design incorporates a blue LED and an associated yellow phosphor. The yellow phosphor is designed to emit a broad spectrum of visible light including reds, greens, and yellows. Some of the source blue LED light is intentionally allowed through the phosphor material resulting in a white light with color rendering performance often meeting or exceeding earlier fluorescent bulbs. The solution is simply termed a phosphor-coated LED. Despite both being monochromatic sources with a phosphor-expanded spectrum, note the differences between fluorescent and LED source outputs below.
|Normalized visible light spectrum energy output of a typical fluorescent.5||Normalized visible light spectrum energy output of a typical LED.5|
The most encouraging news regarding phosphors is that decades of research has made sure that the quantum efficiencies of current phosphors on the market can be more than 90% (but not yet exceeding unity in the visible spectrum)3. However, the conversion of a photon of energy from a higher wavelength to a lower wavelength results in a loss of energy which can significantly limit the maximum total efficacy of the system. In the case of fluorescent lamps the maximum system efficiency assuming no other losses could be roughly estimated by the ratio of the emitted wavelength (254 nm) to that of mean wavelength of visible light (550 nm)4. Based upon an ideal white light source as referenced in an earlier log (250 lumens per watt), fluorescent lights sources would be capped to around 115 lumens per watt. In fact, the highest efficacy noted for any fluorescent system is indeed around 100 lumens per watt. Unfortunately, compact fluorescent lamps compromised this peak efficacy through design modifications such as smaller tube diameters which generally reduced their efficacy to no more than 70 lumens per watt....Read more »
This entry will differentiate the light output of fluorescent sources from incandescent lights. Highlighting these distinctions is a valuable step even for a project built upon LED lighting. Although primary light generation in an LED light source is completely distinct from that of either incandescents or fluorescents, the similarities LED light sources share with fluorescents are strong enough to justify an examination of the unique properties of fluorescent lamps. Let's review!
Fluorescent light sources generate radiation through a completely different mechanism from incandescent filament bulbs and produce a radiation output spectrum which is also distinct. Fluorescent lamps are a gas discharge light source in which an electrical voltage excites mercury vapor in a low-pressure tube. This excitation results in the promotion of electrons within the mercury atoms temporarily to higher levels of energy. When those electrons return to their normal state energy is released in the form of photons at specific energy levels or wavelengths. These are notable in the case of mercury for being in the ultraviolet spectrum. Rather than a continuous spectrum of wavelengths, fluorescent lamps instead exhibit an output radiation spectrum with peaks at a few wavelengths in the invisible UV range1 (see the figures below). This process, distinct from the emission of light through heating which defines incandescence, is known as luminescence.
|Normalized visible light spectrum energy output of a typical halogen incandescent source.4||Normalized (internal) output spectrum of a mercury vapor fluorescent (all peaks in UV).5|
At this point fluorescent sources have been defined by the method through which they convert electrons into photons. However, what has been reserved until now is the ultimate method by which these sources produce white light given the limited spectra output relative to the continuous band emitted by sources like the sun. For fluorescent sources the simple answer is phosphors. Through the process of photoluminescence substances known as phosphors, typically composed of various inorganic compounds in combination with rare earth metals, are used to absorb energy output by LED and fluorescent sources and convert them into lower energy wavelengths2. For fluorescent light sources this is critically important since the lamp must minimize the amount of UV radiation emitted to keep UV output within safe limits. Early fluorescent phosphors admitted UV photons and radiated visible light in the blue and orange parts of the visible spectrum. This light output (figure below) effectively produces a white light source but its associated color spectrum differs significantly from that of incandescent sources and drives a worthwhile discussion on the color rendering performance of the light (reserved for a later post). More modern fluorescent phosphor formulations known as triphosphors emit visible light in the blue, green, and red areas of the spectrum with an associated improvement in color rendering (usually requiring an increase in cost and complexity of manufacture)3.
|Normalized visible light spectrum energy output of a typical fluorescent.4|
LEDs present a slightly different opportunity since their light source is materially different and, as such, are worth a separate post!
1 Schubert, Fred. Light Emitting Diodes. 2nd ed.
2 Jones, Eric D. “Light Emitting Diodes (LEDS) for General Illumination: a OIDA Technology Roadmap.” Mar. 2001.
3 “How Is White Light Made with LEDs? | LED Lighting Systems | Lighting Answers | NLPIP.” Lighting Research Center, Rensselaer Polytechnic Institute, 2003, www.lrc.rpi.edu/programs/nlpip/lightinganswers/led/whiteLight.asp.
4 “Efficiency of LEDs: The Highest Luminous Efficacy of a White LED.” DIAL, DIAL GmbH, 4 Aug. 2017, www.dial.de/en/blog/article/efficiency-of-ledsthe-highest-luminous-efficacy-of-a-white-led/.
5...Read more »
A key concept at the heart of this project is that of color temperature. As such, allow for this short introduction to the history of the concept. Future entries may further elaborate on the complexities associated with this metric, but the following should provide at least a basic introduction to how this terminology has become a core component of the discussion when talking about light sources.
Electric light sources had existed in different forms prior to the commercialization of the tungsten filament incandescent light bulb around the start of the 20th century. However, the continuing presence of incandescent bulbs over a century after their initial popularization underscores the importance of understanding their basic operation and critical drawbacks. The basic construction of traditional light bulbs incorporates a glass enclosure which contains a filament wire connected to electrical contacts in a base. The glass enclosure is evacuated and typically filled with an inert gas. The filament materials are selected and constructed to provide a targeted amount of resistance when connected to a source of electricity. The most common material associated with filament bulbs is tungsten, but early models used carbon filaments. The result is that when connected to an electrical source current passes through the filament such that it is heated to a target temperature. Once heated the filament will glow, or incandesce, per a relationship between an object’s associated temperature and the total energy of radiation emitted as well as the color of visible light emitted. At lower temperatures, a source emits relatively low levels of total energy with a peak of the energy being in the infrared and red parts of the spectrum1. This is generally perceived as glowing red hot. However, as the object is heated up more energy overall is emitted and the spectrum includes more components of the visible spectrum (generally composed of wavelengths of light between 400 nm to 700 nm). An object reaching these higher temperatures is generally perceived as glowing white hot. Color temperature defines the wavelength at which the peak energy is emitted and as such is directly associated with the relative balances of energy for each of the wavelengths of visible light. Note in the figure below how only a small section of the wavelengths (marked on the left) include the visible light spectrum.
|The energy curve for each temperature demonstrates that invisible infrared wavelengths (> 700 nm) represent most emitted radiation for incandescent sources. Visible light wavelentghs are highlighted by the color rainbow on the left. The peak energy wavelength decreases as the temperature of the source increases. Diagram excerpted from Iacopo Giangrandi's "Black Body Radiation and Color Temperature."1|
The primary advantage of such a light source is that it is comparable to the light emitted from the sun. Although not seemingly related, both an incandescent light bulb2 and the sun generate light though heating3. The spectrum of light produced includes the continuous spectrum of light wavelengths across the entire visible spectrum. The significant disadvantage of such sources is that radiation generated typically includes a significant component of wavelengths outside of the visible light spectrum. In the case of incandescent bulbs, although essentially 100% of input power translates to output energy only 6% of that energy is within the visible spectrum4. A disproportionate amount of power input is converted into invisible infrared energy which is perceived as heat radiating from the bulb. The efficacy of a given source is expressed in the luminous flux, a measure of the power of light radiation as perceived by the human eye, per watt. An ideal source defined based upon the 5800-degree Kelvin color temperature spectrum associated with the midday sun, but restricted to only the visible light spectrum, would possess an efficacy of 250 lumens per watt....Read more »
One of the stated goals of the project is to create a color temperature controlled light source using commodity LED lamps. With such a wide range of possibilities available in the market today two primary constraints were used to guide the team on lamp selection:
Fortunately dimmable LED lamps are indeed available from every imaginable retailer. For this project we opted to source our LED lamps from big-box home improvement stores primarily due to the wide selection available in that setting.
Selecting lamp color temperature deserves a modicum of explanation as the sources in question (at this stage) are all still fundamentally white light sources. Retail buyers will be familiar with correlated color temperature (CCT) labels on lamps by either their numerical value, expressed in degrees Kelvin and typically in a range between 2000 K and 6500 K, or by their marketing terms such as warm white (temperatures in the 2000 K range) , soft white (often used for temperatures in the 3000 K range) to cool white and daylight (referencing temperatures from 5000 K and above). Future project entries can elaborate upon the some of the meaning behind these terms.
Nonetheless, even with the constraints above the options available remained numerous so a few other considerations were selected as options which might facilitate early testing. Note that these are choices which could later be intentionally ignored as we learn more about the capacity of the system to provide a color-controlled lighting experience and also as we consider opportunities for expanded functionality from the device.
The first choice meant that in theory the system would, at a minimum, allow direct switching between two or more color temperatures at an equivalent level of output. This functionality is typically limited to smart lighting or special purpose lamps/fixtures. Another functionality this should automatically enable would be a dim-to-warm experience where lower lighting levels could be tied to lower CCT values, emulating the dimming performance of older incandescent lamps. Again, this is a feature that is sometimes integrated into premium, manufacturer-specific LED lamps but is otherwise not a normal functionality associated with LED dimming.
The second choice is more complicated but was also a consideration as part of the testing phase. LED light sources are highly directional and, as such, LED lamps use all manner of methods to position LEDs and route their light output to ensure appropriate distribution of light. Typically, LED lamps designed as incandescent-replacements attempt to match the pattern of light distribution appropriate to the lamp shape (omni-directional for classic A-lamps, to highly directional for reflectors). To prevent differences between models from creating measurable differences with the testing platform one additional refinement was made to initially opt for LED filament-style lamps of comparable shape. This should hopefully minimize any differences at the sensors which might occur because of significant differences in beam shape and LED placement (though other considerations will also be made regarding sensor placement to further minimize this issue).
Given all of this, the following two lamps were selected as the primary range of light sources for our initial testing:
|Soft White (2700 K)||Daylight (5000 K)|
These lamps are relatively inexpensive if not cheap by today's standards at under $6 per individual lamp. The shape is a straight tubular design popular for vintage/retro- applications known as ST19. Note how both lamps have their LED filaments oriented in the same vertical fashion. The lamps are both rated to output 800 lumens which, though referenced as a 75 watt replacement for vintage ST19...Read more »
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