Optical Management

An LED lights optical characteristics can describe how the LED bulbs might appear to humans and what functionally it might serve in an LED home lighting environment. A good understanding of basic optical characteristics associated with LED lamps, as well as associated terms will play a critical role during the initial developmental stages of a custom LED design project.

LED LumensEvery light emitting diode produces light within a specific range of angular degrees, between 0 and 360, commonly referred to as the beam angle. Most LED lights contain a beam angle somewhere between 10-degrees and 180-degrees. To further ones understanding of the LED optics, consider the following familiar lighting devices. A light bulb found in common household application, such as a standard 60-watt light bulb, would probably contain a beam angle close to 350 degrees. This is because it emits light in almost all directions with the exception of a narrow segment where the electrical socket is located. A cylindrical fluorescent bulb would have a 360-degree beam angle because it emits light from all sides and in all directions. In contrast to these examples, a keychain laser pointer might have a beam angle as narrow as 1 degree.

LED Optics

The foundation unit for visible light measurements is the candela. An older unit, candlepower, is equal to approximately one candela. Candela is the LED luminous intensity in a specified direction. Datasheets abbreviate candela as cd, or more commonly as mcd (milli-candela) when referring to a fraction of one candela. One thousand mcd is equal to one candela. LED lights datasheets typically publish luminous intensity at a typical or test condition. Normally 20 mA for low power LED lights, the test condition is a compromise between LED intensity and efficiency. Publishing too high of a value would impair LED performance. Publishing too low of a value would obviously not encourage potential designers to incorporate the light emitting diode into their custom LED lighting designs. Most datasheets also offer an intensity rating at the maximum drive current. Increasing ambient temperatures can adversity affect luminous intensity. Temperature fluctuations can originate from environmental factors or thermal energy produced by the LED. Older LED technologies, such as 5mm and 3mm through-hole LEDs, tend to produce a narrow beam of intense light. Corresponding datasheets will publish the luminous intensity (candela rating), as opposed to a luminous flux rating.

Beam Angle and Luminous Intensity

An important concept to understand is the relationship between beam angle and luminous intensity. Beam angle and luminous intensity share a negative correlation. As the beam angle increases, luminous intensity decreases. On the other hand, as luminous intensity increases, the beam angle must decrease. This is assuming two LEDs that produce an equal amount of total light, or luminous flux. LED lights with a narrow beam angle will appear more intense to spectators due to a concentration of light over a smaller area. On the other hand, an LED light with a wider beam angle will appear less intense, due to the dispersion of light over a broader area. The principal concept is that the amount of total light, or luminous flux, remains constant in either case. Some people tend to believe that luminous intensity relates directly to the total amount of light produced by the LED. This is a very common misconception in the LED lighting industry today.

Luminous Flux / LED Lumens

Luminous Flux defined is energy radiated from a light source, per unit time, over wavelengths visible to the human eye. LED Lumens, abbreviated as "lm", pertain to the associated unit of measurement. Some people like to think of luminous flux as the overall visible light produced by LED lights. Most datasheets publish luminous flux at a pre-selected condition referred to as the typical or test condition. This condition is a fair compromise between maximum efficiency and maximum luminous flux, since efficiency is lost as power increases. Datasheets will also provide a luminous flux rating at the absolute maximum allowable current setting. Ambient temperature (Ta) is one factor that can adversely affect luminous flux. The LED luminous flux will diminish as temperature increases, as a result from the surrounding environment or thermal energy produced by the LED light itself. Modern LED lights are capable of producing an increased amount of total light over a broader area. The corresponding LED optics datasheet will publish a luminous flux (lumen rating), rather than a luminous intensity or candela rating.

White LED Lumen Maintenance

As the LED ages, the luminous flux will continually diminish. White lumen maintenance refers to an LED's ability to produce light based on its total number of accumulative hours of operation. LED luminous flux expressed as a percentage, should always coincide with the standard temperature and typical drive current. A device rated for 75% white LED lumen maintenance at 40,000 hours should have decreased in luminous flux by 25% after 40,000 hours of accumulative operation. If the white LED's original luminous flux rating were 60 lumens, it should produce approximately 45 lumens after 40,000 hours of accumulative operation. The point that the luminous flux reaches 50% determines a white LED's usable life. For this example, consider an LED rated for 50% white LED lumen maintenance at 50,000 hours. In this case, the usable LED lamp life is limited to 50,000 hours of accumulative operation. However, many light emitting diode manufactures like to publish their LED lifespan as 100,000 hours or greater. They fail to mention that lumen maintenance is well below 50% at 100,000 hours. Technically, the true LED lifespan may be only half of the manufacture's published value.

Optical Wavelength

Optical wavelength pertains to an LED's electromagnetic emissions, usually within the visible range of the electromagnetic spectrum. Optical wavelength determines the color visible to the human eye. All colors correspond with a specific wavelength, measured in nano-meters (nm). The LED optical wavelength is ultimately determined during the LED manufacturing process. Each color or optical wavelength corresponds with a set of elemental materials that have been predetermined to produce colors within a specific range of the electromagnetic spectrum. Many red, yellow, amber, and orange LEDs utilize the AlInGaP technology (Aluminum, Indium, Gallium, and Phosphide). Many white, blue, aqua, and green LED lamps utilize the InGaN technology (Indium, Gallium, and Nitride). LEDs that produce visible light normally fall within the range of 380 nm to 770 nm. Emissions below or beyond this range are classified as ultraviolet or infrared, respectively. Ultra-violet (UV) emissions from 400 nm can be especially dangerous to the unprotected human eye. In contrast, the human eye is most sensitive to green light at 555 nm. Numerous factors can affect LED optic wavelengths including, drive currents, ambient temperatures, and manufacturing tolerances. Datasheets may state a manufacturing tolerance such as +/- 2 nm. In most cases, the appropriate optical wavelength for a specific application is obvious. However, applications engineers at Lunar Accents Design Corporation undergo intensive training to consider all the fundamental optical characteristics required for developing new LED lighting applications.

 

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