A scientific comparison
All but a few LED manufacturers and sign companies have confused their clients when comparing their products to neon. In their advertisements, they've published incomplete and misleading information, using such slogans as, "Our lighting system consumes less than one-fourth of the energy of comparable neon." But in these comparisons, they failed to mention that their "comparable neon" will give 10 times more light output than their LEDs. To clarify the situation, as a physicist, I'll compare LEDs to neon for illuminating channel letters. But please note that the development in the semiconductor industry is so fast that the picture given here may change in a few months or years. Comparing apples and peas In the lighting industry, it's been difficult to compare values because a reliable international standard for brightness and luminous flux didn't exist before 1931. Luminous flux, measured in lumen, is the total light output of a light source into spherical space around the source, independent of the source's directivity. Simply put, lumen represent the total amount of light generated. Contrary to this is the parameter brightness. A luminous surface can be very bright but small. But a tiny, bright surface usually doesn't emit a huge amount of light because it's concentrated in a small spot. Many different types of lighting units are historically based and still used in marketing, and these units can't be easily compared. Therefore, it's difficult to obtain comparable numbers for products not directly related to lighting -- especially from LED manufacturers. Overall efficiency of a lightsource is what's really important because it states how many lumen will result from one watt of electrical input. Now, the first experimental results on LED light output, measured according to lighting standards, are available. Thus, we can accurately compare the luminous efficiency of LEDs to other types of lighting for sign applications. For this comparison, remember that, by definition, the unit "lumen" is weighted by the human-eye response curve. Comparisons of colored light provide reasonable data only when comparing light sources of similar spectral distribution. Lifetime considerations Some store chains that adopted LED illumination for channel letters have reverted to neon. Having operated approximately 1 1/2 years outdoors, their signs' LED light output has reportedly degraded to a degree that's inadequate. It has been proven (see ST, August 2001, p.85) that environmental operating temperatures over 150 degrees F. even for a short duration, can permanently damage the LED, leading to a light-output drop of as much as 70% in a few weeks.
The intensity of thie diffusion-based effect depends on many factors, of which all are not exactly known today. However, it can be stated assuredly that the frequently advertised lifetime of LEDs in outdoor applications -- 100,000 hours -- can't be reached. A realistic value may be 5,000 to 10,000 hours, depending on environmental conditions.
Ninety years of experience (yes, 2001 was the 90th anniversary of the invention of the neon tube by George Claude) hae shown that an average neon tube will prvode approximately 20,000 to 30,000 hours of useful life, but tubes with more than 80,000 hours are also common.
(Marcus does a sample calculation in red LEDs and green LEDs vs. 15mm-clear, red neon for 2-ft.-tall channel letters for a motel sign. Please see ST's November 2001 issue, page, 40.)
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