All Light Emitting Diodes (LEDs) are not created equal, and it's important for the consumer to distinguish quality lamps from sub-par product. At a very minimum, consumer need to understand basic terms. The following LED related terms are a good start:
Efficacy- With high end and commercial grade products, efficacy will often be listed on packaging or on a cut sheet (information sheet). Efficacy is defined as lumens per watt. The higher a lamp's efficacy, the more efficient it is. Look for numbers higher than 80 in PARs and A19s and higher than 90 (or higher than 100 in the case of new technology) in LED tubes and panel lights (wall packs, security lights etc.). A higher efficacy equates to doing more with less, greater lumen output with fewer watts.
Kelvin- Kelvin temperature, or Kelvin, actually refers to the color a lamp provides. More specifically Kelvin refers to the color emitted by the diodes inside the LED lamp. Lower numbers indicate a warmer (more yellow) color, while higher numbers indicate a cooler (more blue) color. Daylight white tends to be 5000-5500K. Spaces lit with incandescent lamps tend to range between 2400 and 2700K. In my experience, clients who have been working under fluorescent or incandescent lamps are initially most comfortable with 3200-3400k. That being said, color does have an effect on productivity. 4100-4500K tends to be most pleasant and to have an energizing effect.
CRI- Color Rendering Index or CRI indicates how closely a light source mimics natural sunlight. A higher CRI typically indicates a higher quality product. CRI should not be confused with Kelvin. A high CRI does not mean a lamp is putting out a blue-white light. Warmer colors can have a high CRI as well. Think about the sunset: If I were to walk outside with a color meter and take a reading at sunset, I might find a Kelvin rating of 2600. The question of CRI is raised based on how I perceive colors and clarity at that 2600K under natural sunlight versus how I perceive color and clarity under artificial light. CRI is about perception.
Heat Sink- A heat sink is the portion of the LED lamp that is often made of metal and will typically look like fins or folded metal. Heat sinks can be made of almost any material and their purpose is to dissipate heat. Consumers and even some "experts" will assume and express that LEDs do not heat up over time as they continue to function. That isn't true. The actual diode produces some negligible quantity of heat as compared to other types of lights (like an incandescent), but the driver and internal components of the LED lamp can get quite hot and even melt.
This is where a good heat sink comes in. A good heat sink pulls heat away from the driver and internal components of the LED lamp. The cooler the components are while functioning, the longer they will last. Heat kills lamps. A very good LED lamp will operate at ambient temperature with the driver components and heat sink dissipating heat to maintain a temperature of 75 to about 110 degrees Fahrenheit. Aircraft-grade aluminum has been the material of choice for heat sinks in high quality lamps. Currently, some lamps use alloys. Cheaper lamps will either have no heat sink at all, or will have a heat sink made of plastic. Size is also an issue. While larger heat sinks can dissipate more heat, this functionality is self-limiting. A PAR30, for example, should not have a heat sink so large it won't fit in a recessed can. Also, the heat sink tends to be the most expensive part of an LED lamp, so lamps with larger heat sinks are generally more expensive. At some point, this ratio becomes cost prohibitive.
Diode- The diode is the component that actually emits light. When the diodes are initially produced, they emit a very blue (perceived as harsh) light. The diode is then coated (usually with phosphor) to produce a warmer color. Each coating of phosphor reduces the K rating by some uniform (hopefully) measure. Simply put, the difference between a 2400K lamp and a 5500K lamp is the density of the coating. A different process is used to create color-changing or colored diodes. For the sake of simplicity I won't address it here, but I am happy to answer any specific questions about it. Quality diodes (such as those manufactured by Cree) have three primary characteristics:
- The diode wafer is cut from the center of a semiconductor sheet.
- The diodes for each lamp or board come from the same bin, so they share characteristics (this is also true for the coating process).
- Diodes are coated using an automated, consistent, regulated process that ensures as much as possible that K ratings are uniform and that there are no irregularities in coating densities.
The best material and a uniform production process are what differentiate high quality diodes from their less expensive counterparts.
Driver- In an LED system, the driver is what supplies and manages the energy to the board the LEDs sit on. Quality control is crucial here. A bad driver means a failed lamp. Toshiba and Philips both make a pretty good driver. I won't dig too far into this here because drivers can be a fairly complicated topic (dimmable versus non dimmable, types of dimming, 12V vs. variable volt vs. 120 volt, etc.). I welcome any specific questions you might have, though.
Lens and beam angle- These concepts are complicated. Beam angle basically means the light spread on the ground and the angle of light as it travels in its straight line (not to be confused with oscillation or spectral length) when emitted from the lamp or diode. Lenses control the beam angle when they are placed over the end of a diode or diode array. A narrower or tighter lens means a more concentrated beam of light. A wider lens means a more diffuse beam of light. Think spot vs. flood. Not understanding lensing can lead a consumer to misjudge the light output of an LED lamp. A 10W LED with a narrow 45-degree beam angle will appear to put out more light than a 10W LED with a wide 120-degree beam angle. The efficacy of both lamps as well as the lumen output at the diode and lamp face could be identical.
Lumen output- Lumens are the unit of measure for light output. The higher the lumens, the greater the light output (not to be confused with intensity- intensity can be affected by multiple factors, like beam angle).
As you can see, there is a tremendous amount of consideration that goes into choosing an LED lamp. When searching for the best residential LED lighting products, It is very important to choose wisely.
David X. Smith is a graduate of MIT and a freelance blogger & journalist. David covers technology independently & enjoys reviewing LED products, management systems, and generally all things tech. David's goal is to better educate the public so that the individual will become a more educated & conscientious consumer.
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