The smaller is the distance between individual LEDs, and the better it will be able to overcome the color shade effect. The distance between individual LEDs, however, cannot be smaller than the size of secondary optics. Fortunately, compact and efficient LED lenses are available today.
The best effect is achieved through the use of hybrid optics, combining a TIR lens and a special light diffusing material. Modern LEDs are quite efficient at the conversion of electric energy into light. However, even the most efficient LEDs today waste about half of the consumed energy in the form of heat. In fact, it is about 30 times as large as the heat density through the soleplate of a household iron! Heat removal from LEDs is very important since their lifespan and performance depend on the operating temperature.
This drop of efficiency is even more pronounced in cheap LEDs of Asian make. Unfortunately, commonly known technologies today do not permit mounting the LED crystal directly on the heatsink. The thermal resistance of the best individual LEDs is about 2. Let us consider the influence of thermal resistance on crystal temperature. Suppose we have two 3W LEDs of the latest-generation. When operating at mA, the respective voltage drop will be, approximately, 3.
In the first case, the LED package will add 2. We need to supply current to the LED and the conductors should be somehow isolated from the heatsink. Their structure is shown in Fig This will add another This will add 4. Once the heat from the LED has reached the heatsink, it cannot be instantly transferred to the environment.
A heatsink is usually selected in such a way that its average temperature would not exceed 50C when the surrounding air temperature is 25C. Manufacturers are constantly working to overcome the first obstacle and the best LEDs on the market are using packages with reduced thermal resistance.
This approach results in a significant reduction of thermal resistance, and the more LEDs there are on the MCPCB surface, the more pronounced is the effect. With this approach we shall be able to adjust the resulting spectrum as we find appropriate. Below, we shall consider the general rules which we believe are important when designing a LED assembly, pointing out the problems and solutions that we had found in the process.
First of all we need to stress that, even for the same LED type, the efficiency may differ significantly, and all serious manufacturers sort out the product into several efficiency bins which, inevitably, also differ by the price.
We believe that only the best most efficient bins are suitable for making the LED assembly: the price margin will pay off several times throughout the many years of operation. If not specified otherwise, we always use the most efficient LED bins that are available in commercial quantities at the time of manufacture.
Some explanations are required regarding this commercial availability, however. In the datasheets LED manufacturers usually specify a list of bins, including some bins which they are not yet capable of manufacturing in sufficient numbers at the time the document is created.
We have pointed out the importance of violet spectrum multiple times in our previous article. Beside the high photosynthetic activity, this spectrum provides strong fluorescence and, at the same time, it is very poorly visible to the human eye. We have also shown that, in natural sunlight, the amount of radiation in this part of the spectrum is significant.
Therefore we install on our assembly several true violet LEDs with different wavelengths, covering the to nm range. Spectral differences between these LEDs are hardly visible to the eye, and therefore we combine them all in one chain, without the possibility of individually controlling each sub-range. The next important component of the assembly are white LEDs. The situation is much better here — white LEDs are manufactured in great quantities for general illumination needs.
Usually they are based on royal blue LEDs with special phosphors applied on top, to convert some of the blue light into longer wavelengths. This technology is highly polished and is being perfected constantly.
White LEDs are readily available for a moderate cost, even when the most efficient bins are requested. Note that in a cool-white emitter a larger portion of the blue light about nm emitted by the crystal penetrates through the phosphor, whereas a larger percentage of this light is converted to longer wavelengths in the warm-white emitter.
The efficiency of this conversion is not particularly high, and therefore the efficiency of warm white LEDs is usually less compared with similar cool white LEDs. A cooler white, if required, can be obtained easily by supplementing the radiation from blue and violet LEDs on the assembly. Using this approach, we can use the most efficient cool white LEDs on the assembly and, at the same time, will have an efficient tool for increasing the overall CCT when required.
Looking at the spectral plots of white LEDs, it is easy to notice a marked drop in the nm range. To achieve a more even spectral distribution in this range, a blue LED with corresponding wavelength shall be supplemented to the assembly. The next color that is worth having, in order to better fill the gap in white LEDs spectrum, is cyan, or turquois, with wavelengths around nm.
This is a rather bright bluish-green color and is visually quite pleasant. We believe that this is an unnecessary option. First of all, the true green spectrum, in the nm range, is already present in sufficient amounts in the spectrum of white LED, and therefore a separate green LED is not required on the assembly.
Another disadvantage of using separate green LEDs is that so far the efficiency of commercially available green LEDs is extremely low and, to make their addition meaningful, a significant number would be required even though the human eye is most sensitive to green spectrum , and this will affect negatively the overall heat generation by the assembly. We believe that adding just a few green LEDs to the assembly, like most manufacturers do, is a tribute to fashion and does not have any noticeable impact on the aggregated spectrum of the fixture.
Separate green LEDs in a fixture can only prove useful for a hobbyist willing to experiment with very unusual spectral combinations like, for example, illuminating the tank with purely green light, or its combination with some other colors to achieve a certain design effect and thus making a narrow-band adjustments across the whole spectrum.
Anyway, such brave souls will require a completely different setup than your usual LED fixture, eliminating the white and any other wide-spectrum LEDs completely and substituting them with a multitude of different narrow-band LEDs. Unfortunately, 25c die temperatures are not very practical outside of the lab, so Cree, and a few other manufacturers like Philips Lumileds started to use 85C as the benchmark point.
This is a lot closer to real world conditions, and a much better predictor of performance. Not bad. Electrically, things improve again. Cree is gradually forcing the forward voltage down on their LEDs, which helps to increase efficiency, reducing driver requirements, or letting existing drivers run more LEDs in a single string.
While most incremental LED updates bring the forward voltage down a tenth of a volt or two, Cree has managed to get the forward voltage of the XT-E down 0. The current XP-E has an average forward voltage of 3. Keep in mind that these are average voltages and your experience may vary. As a result, this makes for a And you know what that means; less heat. The last thing to note about this LED is something that a lot of people overlook; thermal resistance from junction to solder point.
Close Call Email. Shop By Categories. Home Cree Led. Cree Led. Category Lighting Showing items 1 to Notify Me When In-Stock. Out of Stock. How does this work? I had the same problem and the fix is simple. Wire them in series. The positive from one fan goes to the positive of the 24V power to the buckpucks. The negative of that fan goes to the positive of the other.
That last negative goes back to the negative of the same buckpuck circuit. The power supplies get warm, as well. I don't know if they absolutely need cooling, but I happened to have two extra fans. I added them to the box that I mounted the power supplies in. Optics: I am fully aware that optics will increase the output of LEDs. However, I decided not to use them because they cut down on the spread of the light.
In order to use the optics, successfully, you will need to keep the LED arrays closer together and , most likely, need more LEDs to get the desired spread. That's really about it. After 11 years or so, the LEDs will lose some of their intensity.
But, that's 11 years down the road. Consider the cost of replacing bulbs in standard MH or PC fixtures over the span of 11 years. This type of lighting pays for itself over time. Your electric bill will thank you, as well. As always, your mileage will vary. You can make whatever you want and have it look like whatever you want.
The important things here are the wiring and cooling. Other than that, the canvas is yours to paint.
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