Why SSL Technology Produces Conflict with Old Standards
SSL University
The issue of standards conflicts raised by SSL integration in general illumination is a regular occurrence. This has happened before, specifically with the introduction and deployment of fluorescent lamp technology, and then again with HID. Each new light source technology brings with it characteristics that defy standards and assumption founded on an older, established technology.
Life Ratings - Is the first definition in conflict of new technologies. Incandescent lamps were pretty easy to test to failure, so using a life rating based on actual test results, and establishing a point at which 50% will have failed is straightforward. Early fluorescent testing took longer, but the standard worked - sort of. In reality, fluorescent lamp mortality is not as linear as incandescent lamp failure, and far more reactive to switching, thermal conditions, etc.., but the fit was close enough. Application of this test rating to HID was also a compromise, but a reasonable one. LEDs, on the other hand, have a significantly different service life characteristic. In theory, operating an LED at a low temperature, with proper current regulation will result in an indefinite lifetime of light output. Solid-state devices have no filaments or gas to fail, so have no "failure" mode in the conventional sense. Further, testing a product that can last thousands of hours, whose technology changes on a monthly basis makes the conventional standard inappropriate. The solution? Rate the LEDs to a point at which they have lost 30% or 50% of their rated initial lumens - the ASSIST recommendation for life rating, also termed L70 or L50 life. In fact, this has not yet been fully tested, as operating 24/7, 50,000 hours of life is 5.7 years - about 12 generations of product development down the road. To begin to address this, the test standard LM-80 collects information over a min. of 6,000 hours and describes lumen depreciation. Whether or not a protocol can be developed to extrapolate life of a product from this small data sampling is yet to be seen. it may be some years before we can truly define and understand the life of LED products. In the meantime, published service life of LED products are based on theoretic data.
Color Standards have evolved with each new technology. The first real work on this topic grew from the deployment of fluorescent lamps, whose phosphor conversion created visibly different light than incandescent lamps. Attempts to understand the color difference and define it in such a way as to create a uniform description eventually led to the development of the CCT (CIE 1931) , or correlated color temperature rating we use today. Yet, this was inadequate in describing why two sources of the same CCT produced different visual performance on illuminated surfaces. The CRI (CIE 1964), or color rendering index. Unfortunately, the color space used at the time, and the use of comparisons to idealized sources of a given CCT, do not accurately predict the color performance of sources with saturated light output, such as tri-phosphor fluorescent lamps and multi-metal HID sources, which do not have a complete spectral distributions, but are made up of spikes of saturated color bands. Solid-state lighting, specifically LEDs, create additional issues with the older CRI standard, as they have a fuller spectrum, with a pronounced hump shape peaking at some dominant wavelength, producing their CCT. These factors have generated a new efforts to redefine color accuracy, added terms such as color purity, and are causing a general re-evaluation of old methods and color sampling used in defintions. NIST has developed an updated protocol similar to CRI, named the CQS, or Color Quality Scale. This includes additional colors to capture saturated performance, and redefines the calculations used to determine performance of a test source.
These are just two of the most active areas under development today, casued by the emergence of SSL technology. Other areas include the consideration of blue light content, specifically surrounding the fact that white LEDs emit light at a wavelength aligned with melatonin suppression in humans, and is perceived as daylight by many species of animal. Flicker is another area of concern as AC LEDs emerge and become more common. This is a topic not well defined beyond academic study. Another area of standards development surrounds the extremely small size and high intensity of LED sources, and at what point do these become an eye hazard.
Standards for testing, light characteristics, electrical operation, and materials content are all in a constant state of flux as researchers discover new approaches and build understanding of light and human sight. While LEDs seem to be the focus of many of these efforts, they are but the most current source under scrutiny, in a process that has evolved over decades of continual improvement and adoption of many before it.
We'll look into other areas under study in future installments
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