T8 LED Tube 18W with CRI of 90, 150lumen/watt
Not that there is anything wrong with the old sodium-discharge lamps. Producing practically all their light at a wavelength near the peak sensitivity of the human eye gave them a theoretical luminous efficacy of 200 lumens per watt of electricity consumed, though their real-world performance is around two-thirds that. The new white LEDs used in his street are rated at around 100 lumens per watt. Compact fluorescent lights (CFLs) used in the home produce 60 lumens per watt, while traditional incandescent bulbs emit a miserly 15 or so. In terms of energy savings, then, LED streetlights are contenders but not champs. But they have other advantages. One is longevity: they simply do not burn out, but gradually lose their brightness over the years.
The usual measure of an LED lamp’s economic life is its L70 rating—the time taken for its brightness to fall to 70% of its original value. Municipalities and commercial users expect to get ten years or more from LED lamps before that happens. Low-pressure sodium lamps, by contrast, last typically three or four years before starting to consume lots more power. Add to this the cost of changing an LED streetlamp—around 60, versus over 200 for a sodium lamp—and the lower maintenance costs alone can make the switch worthwhile. There are performance benefits, too. Unlike sodium lamps, LEDs turn on immediately, rather than taking minutes to warm up. They are also better at displaying colours. And recent research has shown that white light provides better visual perception than yellowish-orange sodium light. This stems from the way different photoreceptor cells in the eye, called rods and cones, perform their different tasks.
In daylight, vision is handled largely by cone-shaped cells packed around the centre of the retina. Apart from processing colour, cone cells help the eye perceive detail and rapid changes in surroundings. In the dark, by contrast, perception is handled almost exclusively by the more sensitive rod-like cells towards the retina’s periphery. For people driving at night, however, artificial lighting means conditions are usually neither pitch-dark nor light enough to see using cones alone.
Moreover, as a vehicle moves through patches of lighter and darker illumination, both rods and cones are required, and the demands on them change constantly. When it is briefly lighter, the eye is more sensitive to greenish-yellow light. When darker, it responds best to light in the greenish-blue part of the spectrum. It turns out that white LED lamps do a better job of meeting these conflicting requirements than sodium lamps can manage—and they do it at lower power levels, to boot. There could therefore be some energy savings from switching to LED streetlights after all.
Lighting accounts for roughly 17% of the electricity used in the United States. Reducing waste—through the introduction of solid-state technologies that convert a lot more of the energy into light—would cut that figure. LEDs convert more than 75% of the energy they consume into light; incandescents no more than 5%, the rest being turned into heat. An American law passed in 2007 required that light bulbs consuming 100 watts or more should use 27% less electricity by 2012. Bulbs rated at 40 watts and up would have to do the same by 2014.
Although some of the latest incandescent bulbs could more than meet the new efficiency standard, traditional tungsten-filament bulbs did not stand a chance. Meanwhile, CFLs had begun to acquire a poor reputation. Few lived up to their claim of longer life, especially when they were switched on and off repeatedly, or used in recessed ceiling lights where heat could build up and fry their circuitry. And though LED bulbs have a total-cost-of-ownership significantly less than that of sodium or other forms of industrial lighting, and have proved their worth to municipalities and businesses, they have yet to do the same for residential users.
That is hardly surprising, given an upfront cost of anything from 25 to 60 (before possible rebates from electricity suppliers) for an LED bulb with the luminous flux of a 100-watt incandescent light. Your columnist plans to upgrade his original 4.2 kilowatts’ worth of domestic indoor and outdoor lighting with LED equivalents, one lamp at a time, as the existing bulbs (mostly compact fluorescents) burn out. Replacements for 100-watt floodlights tend to cost around 40 a piece. But the popular 60-watt-equivalent can now be had for as little as 14.
Getting cheaper and smarter
Not all LED lamps, however, are created equal. Some of the cheaper ones have a chilly blue cast. None has quite the full complement of colours of the profligate tungsten-filament bulb, with its wide palette of wavelengths in the visible spectrum. The usual measure of a light source’s ability to reproduce colours faithfully is the Colour Rendering Index (CRI). By convention, incandescent bulbs are assigned a CRI of 100. Cheap LEDs can have a CRI of 50 or less. A really good one—such as the 45 xLedLighting T8 LED Tube 18W has a CRI of 90. Most LED Lights of Yuchi Industrial have CRI of 80 to 85.