Sep 142014

One small issue with LEDs is they have a different quality of light. We have measurements that worked well with Incandescent, HIDs, and Fluorescent lamps. CRI (Color Rendering Index) which gave us a relative measure of quality and CCT which gave us a lamp’s color temperature. But these measurements were based on factors that are no longer in play in the world of solid state lighting.

In the July-August 2014 issue of Architectural Lighting, Alice Liao writes about the issues facing the consumer of quality lighting and the issues facing the professional user of lighting.

“Both CRI and CCT are derived through rote mathematic simulation rather than through empirical measurement. CRI testing is calculated on a computing device using a source’s spectral power distribution (SPD), a diagram that depicts the radiant energy a source emits at different wavelengths of visible light—wavelengths of 380 to 780 nanometers—and the spectral reflectance of each color chip. CCT is also computed from the source’s SPD.”

After Lias discusses the short comings of these measurements, we’re introduced to the Color Quality Scale (CQS). Developed by the National Institute of Standards and Technology. The CQS tests with a broader range of colors, higher chromas and deeper saturation.

“CQS also factors in extreme color temperature, which impairs a source’s ability to render color, and takes a root-mean-square of the color shifts of all 15 test colors rather than an average. This ensures that poor performance on a few samples is given proper weight.”

I am hopeful the IES Color Metrics Task Group will come up with a simple scale for consumers who are already confused about LEDs. They should also develop a set of ratings for Professionals, whose requirements for light require more data not less. For those interested in this important topic, I highly recommend this article.

The CIE chormaticity diagrams map perceived color.

The CIE chormaticity diagrams map perceived color. Lightness, the third dimension of the color space, is not shown in these two-dimensional graphs. The CIE created the 1960 Uniform Chromaticity Scale (UCS) to reduce the limitations of the 1931 system; it has since been updated by the 1976 UCS. The Planckian, or black body, locus—shown by the curved lines within the filled areas—indicates the color that a black body radiator emits within each chromaticity diagram as it is heated up.
Credit: U.S. Department of Energy

Jan 102012

In the current issue of Electrical Wholesaling, Jim Lucy, the chief Editor, comments on a side-by-side comparison of Fluorescent and LEDs he saw at Lowes. I’m not surprised. LEDs often have an odd color cast. LEDs are now as efficient as Fluorescents but they are still more money. LEDs do have their uses. Often they are ideal in low wattage situations like decorative lighting. I’m sure they will get better and cheap as time goes on.

But what really galled me is that the LED on display was emitting a ghoulish blue-white light that could only appeal to someone trying to duplicate the lighting in a 1950s-era Soviet interrogation room. By any measure, this display did much more harm than good to the admirable cause of LED lighting, particularly in a setting where homeowners and other potential retail customers are probably getting their first look at LEDs.

Apr 222011

Bright Energy Solutions (Missouri River Energy Services/Moorhead Public Service) has added custom incentives for qualified indoor LED lighting installed in commercial and industrial facilities. “In order to be considered for a custom incentive, all LED products must be either Energy Star® qualified or approved by the Design Lights (TM) Consortium. Check out the latest Design Lights Qualified Products List and Energy Star Products. If you are considering the use of indoor LED lighting in commercial and industrial facilities please contact your local utility prior to the ordering of materials. Preapproval is required for all items to be considered in the Custom Incentive Program.”

In addition to these custom rebates Bright Energy offers specific incentives for Exit lights, reach in refrigerated case lighting, Energy Star qualified recessed downlights, and Energy Star qualified recessed downlight retrofit kits. These specific incentives are listed the Lighting Retrofit and Lighting New Construction application forms.

Nov 302009

Jim Benya Looks At Basic LED Circuits in the Fall 2009 LED edition of Architectural Lighting. Benya points out LEDs are fussy about power and heat. LEDs operate on low voltage DC power and require a Transformer, Voltage Regulator, and Driver in order to function in our 120/277 volt world. The Transformer converts line power to 24 volt DC current. The Voltage Regulator keeps the Voltage constant and the Driver keeps the current constant. Any changes in the current or voltage can cause the LEDs to fail.

Once converted to low-voltage DC, power then passes through the LED to generate light. But as with a fluorescent lamp, there must first be a circuit that regulates the amount of energy—or the lamp will blow up. For a fluorescent lamp this circuit is called a ballast; for solid-state lighting it is called a driver. The typical contemporary white LED is designed to operate at either 0.35A, 0.70A, or 1A depending on watts, brand, and model. It is the driver’s job to regulate the DC power for the specific LED. An added complication is that seldom in lighting is one LED enough, and it is common to have several LEDs mounted and wired together. For example, a basic downlight might employ 20 or more 0.25W or 0.5W LEDs.

Drivers vary from simple resistors to integrated circuits that precisely control the diode itself. Add Dimming to fixture and the circuit becomes even more complex. Voltage can not be cut to the LED so the circuit has to vary the ratio of on/off cycles per second to achieve the effect of dimming. If you wish to control the color of your LED lighting system you add either RGB (Red, Green, Blue) LEDs to your dimming driver or RGBA (Red, Green, Blue, Amber) LEDs to your dimming driver and balance the color by varying the dimming to each LED.

Benya points out, the industry has no standards for Drivers, “We need standards in solid-state lighting that go well beyond the lumen measurement, efficacy, and life testing that have dominated solid-state lighting discussions to date. For instance, due to their poor 70 percent efficiency, drivers are often avoided when the energy efficiency of solid-state lighting is discussed.. As it is, LEDs have already developed a reputation for failing to meet rated performance and life.”

The good news is the industry went through similar troubles with the introduction of T8 fluorescent lamps and electronic ballasts. It is just a question of time before the issues of LEDs get sorted out.

Contant-Current Circuit

120v AC --> 24V DC Transformer --> Driver --> LED - LED - LED -->

Constant-Voltage Circuit

120v AC --> 24V DC Transformer --> Voltage Regulator --> Driver --> LED
                                                     --> Driver --> LED
                                                     --> Driver --> LED
Oct 092009

In the August 2009 issue of IEEE Spectrum, Richard Stevenson writes about a problem specific to LEDs known as droop.

Blue LEDs are those used to produce white light. Once the diode’s power level reaches an output sufficient for general lighting applications, the efficiency of the LED declines significantly- requiring more power to maintain a usable light level. Until the scientists and engineers understand this problem it may be difficult to overcome- at least in the near term.

The LED’s Dark Secret has a detailed explanation on LED Droop. I found the graphs helpful and the article also helps explain how LEDs work.