A very detail article on Led lighting

[Harlequinmania]

Understanding Led Lighting and its use for reef aquariums

by Sanjay Joshi, Ph.D.

LED lighting technology for the marine aquarium hobby is constantly being updated and improved. In this article, Professor Joshi explains how LEDs function, assesses their suitability for coral-reef aquaria, and discusses the criteria for selecting an LED aquarium light

LEDs (light emitting diodes) are an exciting new technology that has the potential to change the way we light our aquariums. There is considerable interest in LEDs due to a large number of potential advantages: reduced power consumption, extremely long life (which could eliminate changing of bulbs), reduced heat input to the aquarium, selective dimming to simulate dawn/dusk/ tropical cloud cover, wide range of effective color temperatures to satisfy aquarist preferences, and more aesthetically pleasing displays that combine spotlights and uniform lighting.

It is this potential for addressing the aquarist's needs with a single lighting system that has generated such enthusiasm for LED lighting technology and its use in the aquarium. As with any new technology, there has been skepticism as to whether it will perform better than, or even as well as, existing technology. Understanding the technology and its implementation, how it functions, what aspects are relevant to reefkeeping, what the potential

pitfalls are, and how it compares with existing lighting are all important elements in allaying the fear and skepticism generated by this new and potentially game-changing approach to lighting.

WHAT ARE LEDS AND HOW DO THEY WORK?

LEDs were first developed in the 1960s, and initially they were used for indicator lights in electronic equipment, traffic lights, electrical signs, and simple displays (calculators, watches, etc.), where the light source is easily seen. The 5-mm LEDs that are commonly available were designed for these applications and have been in use for a long time and in large numbers, resulting in low manufacturing costs and retail prices.

LEDs are not really designed for illumination. Illumination requires that the light source provide enough light to permit the viewing of other objects, much like a typical light bulb or fluorescent light. Here the objects are made visible by the reflection of the incident light emanating from the light source. Modern high-output, high-power LEDs, which deliver meaningful levels of light for illumination, started to become commercially available in the late 1990s. Since their introduction there has been steady development in LEDs designed for illumination, focusing on providing the highest output per watt consumed and creating white light, with the ultimate purpose of replacing existing lighting technology for all household and commercial applications. For aquarium purposes, LEDs must not only illuminate but also stimulate and maintain photosynthesis at desired levels.

In principle, LEDs are similar to the simple silicon p-n junction diode. Diodes are electrical devices that allow electricity to flow in one direction, similar to a check valve in plumbing. The diode is formed by two slightly different semiconductor materials that create a positive and negative (p-n) junction. On one side of the junction, the p side, there is an excess of positive charges (holes), and the n side has an excess of negatively charged electrons. In a state of equilibrium, the electrons and holes stay on their respective sides, separated by a small “depletion zone” where the holes and electrons have combined and achieved equilibrium. To get the electrons and holes to move toward the junction, energy is supplied in the form of electrical voltage and current by connecting to a power source, such as a battery or DC power supply. The amount of voltage and current required for electroluminescence to occur is called the forward voltage and forward current. When the electrons and holes combine at the junction, the result is a release of energy, which, in an LED, is released as light (radiative recombination) and heat (non-radiative recombination).

A simple diagram depicting this process is shown in Figure 1 (previous page). The LED chip is typically packaged into a functional unit. Figure 2 is a photograph of an LED package.

Figure 3 shows the typical construction and packaging of a high-output LED. There are several features incorporated into the package design. There is a direct thermal path from the chip to the printed circuit board on which it is mounted. As we will see, heat management is a vital part of high-power LED use. The chip is encapsulated in a polymer/epoxy to increase the extraction efficiency of the light and provide protection against unwanted mechanical shock, humidity, and chemicals. The polymer/epoxy encapsulant also stabilizes the LED chip, bonding wires, and cathode and anode leads. Due to the softness of the polymer encapsulant, it is covered with a plastic cover that also serves as a lens. The chip is mounted on a silicone submount with built-in electrostatic discharge protection. Visible light is the energy released within the 400 to 700 nm wavelength range. While we may refer to light by its color, it is more accurately classified based on the wavelength or energy of the released photons of light. Energy released as photons in the 400 to 450 nm wavelength range appears to us as blue light, while photons at 650 to 700 nm appear as red/orange. The various colors of the rainbow (VIBGYOR) are arranged along the wavelength range from 400 to 700 nm. See Figure 4.

The type of semiconducting material used to build the p-n junction determines the color (or the energy distribution of the released photons). Red, yellow, and orange LEDs are produced using aluminum gallium indium phosphide (AlGaInP), and indium gallium nitride (INGaN) alloys are used for green and blue lights. Changes in the composition of the materials used change the color of the light emitted. LEDs typically produce monochromatic light (light of a single predominant color), which means that the light distribution has a narrow spectral width. White light, on the contrary, has a wide spectrum comprising a wide range of colors. Creating white light that matches with human perception is a challenge.

White light is produced by LEDs mainly through the use of phosphors activated via short-wavelength light (UV or blue)—in much the same way that fluorescent lights produce light. New phosphors are continually being developed to improve the quality of white light. Another way of making white light is by mixing different-colored LEDs to create a spectrum that appears white. White light can be created by using red, blue, and green LEDs in close proximity to ensure proper mixing of the output. The table at top left compares the different technologies used for white light. Blue LEDs using phosphors are currently used for most aquarium lighting. Figure 5 compares the two phosphorbased white light LEDs to natural sunlight.

When reading about white LEDs, you will often encounter the terms luminous efficacy, color temperature, and color-rendition index.

Luminous efficacy is the term used to describe the energy efficiency of a white light source—how well it converts the energy input in watts to light output as perceived by the human eye. Luminous efficacy is expressed in lumens/watt (lumens are the unit used to measure how light is perceived by the human eye). Typical incandescent lamps have a luminous efficacy of about 15 lumens/watt. Compact fluorescent and metal halide lights have luminous efficacy ranging from 90-110 lumens/watt. Commercially available LEDs have reached up to 110 lumens/watt, while LEDs in research labs have reached 231 lumens/watt. Higher efficiency is better for users because it reduces energy needs and fewer LEDs are required to achieve the same light levels.

Color temperature is the term used to describe the color of the light produced by comparing the color to that of a standard “black body” (an idealized physical body that absorbs all electromagnetic radiation) at a certain temperature. Color temperature is expressed in Kelvin (K). For example, a color temperature of 5,000K corresponds to the color of light produced by a black body when heated to a temperature of 5,000K. As a frame of reference, the color temperature of natural daylight ranges from 5,000K to 6,500K.

The color rendition index (CRI), the most misunderstood of all lighting metrics, is used to assess the impact of the light source on the perceived color of an object. It measures how accurately the light source of a given color temperature creates the colors of the object being lit when compared to a light source of the same color temperature generated by a black body. A set of eight standard color samples is illuminated by the light source and by a black body matched to the color temperature of the light source. If none of the samples change color appearance, the light source is given a CRI rating of 100. The CRI decreases as the average change in color appearance of the samples increases. A CRI rating of 80 or above is considered good. It is important to remember that CRI is calculated for light sources of a specific color temperature, so it is not valid to compare, say, a 2,700K 82 CRI light source to one of 3,500K 85 CRI.

THE OPERATING CHARACTERISTICS OF LEDS AND WHAT AFFECTS THEM

Given that the main purpose of LEDs for our application is the generation of light, it is important to understand what factors impact the light output and the life of LEDs, and how. We know that the light output of currently used light sources, such as fluorescent and metal halides, degrades over time, affecting both the amount of light produced and the spectrum of the light that is output. Typical lamp replacement times are 6-9 months for fluorescents and 9-12 months for metal halides. LEDs, on the other hand, promise a lifetime of 50,000 hours, which is 11.4 years if used 12 hours per day.

Proper operation of LEDs requires an understanding of the relationship between forward voltage and current and its impact on light output, as well as on the life of the LED. The amount of forward voltage and current required by an LED is determined by the semiconductors used and is readily available from data sheets provided by the manufacturers. The amount of light emitted by an LED is proportional to the amount of current passing through the device in the forward direction. As the current is varied, the light output will likewise vary, with higher current producing more light.

The power supplied to LEDs is regulated by LED drivers (the electronics needed to control the voltage and current). The drive circuits for LEDs must provide suf- ficient voltage to overcome the forward voltage drop at the diode junction (where photons are released), while regulating the current to the correct value for the specific device. The fact that the same LED can be driven at different current levels (350, 700, or 1000 mA) allows for the creation of 1- to 3-watt LEDs using the same LED chip.

One factor that significantly affects the life span and spectral characteristics of an LED is heat. Both light and heat are produced at the junction of the diode that makes up the LED device. Any input energy that isn't converted to light is released as heat. Heat is produced by all lighting technologies. The biggest difference between the heat produced by LEDs and that produced by metal halide and fluorescent lighting is that the latter two have a large component of infrared radiation, whereas an LED does not produce infrared radiation. The heat produced by LEDs takes the form of conductive and convective heat. Figure 5 (page 43) shows the typical distribution of heat for different light sources.

Although LEDs do not heat aquarium water directly in the manner of a conventional light bulb, heat does have a significant impact on the performance characteristics and life span of LEDs. The temperature at the junction is a key metric for evaluating LED product quality and life. The junction temperature is affected primarily by the drive current, the thermal path, and the ambient temperature. An increase in temperature leads to a change in the forward voltage, which results in a proportionately larger increase in forward current, which further heats the junction. The increase can be controlled by driving LEDs with constant current drivers. The light output of LEDs increases with increasing drive current, which is why the same LED can be driven at varying power. However, this comes at the cost of efficiency. The change in lumens/watt at different levels of current does not result in a linear change in output. Doubling the current does not double the light output. Furthermore, increasing the current also increases the heat generated and the heat-related effects, which in turn necessitates increased heat dissipation efforts.

The data provided by manufacturers in most advertising materials often reflects a junction temperature of 25deg;C. In practice, most LEDs operate at higher junction temperatures. Higher temperatures result in lower light output, changes in the light spectrum due to changes in dominant wavelength (ranging from 0-030.13 nm/C), and, most important, a reduction in the life of LEDs. Figures 6-7 (page 43) show the impact of temperature and current on the light output and life of LEDs. As seen in Figure 6, as the junction temperature increases, light output decreases. This loss is generally higher for white LEDs than for blue LEDs. As Figure 7 shows, increasing the drive current increases the light output, double the light output, but also reduces the efficacy. The best compromise of light output and efficacy occurs where these two curves intersect. Air temperature plays a role in thermal transfer between the LED and the surrounding air. The junction temperature and drive current both impact the LED's predicted lifetime.

The huge impact that heat has on the LEDs requires that this heat be managed in an effective manner. Heat must be moved away from the die (the small block of semiconductor wafer where light is emitted) in order to maintain expected light output, life, and color. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings. Heat must be conducted away from the LED in an efficient manner, and then removed from the area by convection. The final removal of heat can be accomplished passively, for example using a finned aluminum heat sink with a large surface area, but higherpower arrays may require active convection via forced-air cooling (fans) or even water cooling.

Heat transport is a critical element in the design of an LED fixture. The need to use materials that are highly conductive, which can be expensive, leads to trade-offs between cost, performance, manufacturability, and other factors.

LED DRIVERS

LEDs are low-voltage light sources that require direct current (DC) to operate. The LED driver performs several critical functions—converting the incoming alternating current (AC) power to the proper DC power, regulating the current that flows through the LED during operation, and protecting against line voltage fluctuations— and is analogous to the ballast unit in a fluorescent or High Intensity Discharge (HID) lighting system. Drivers can also perform other control functions. They can dim the light by reducing the forward voltage or using pulse width modulation (PWM) via digital control; PWM allows dimming with minimal color shift in the LED output.

There are basically two approaches to driving LEDs: constant voltage or constant current. A constant voltage driver is most commonly implemented via a voltage source significantly higher than the diode forward voltage drop and a resistor in series. There are two main drawbacks to this design. Since the diode current depends exponentially on the voltage, a small variation in voltage results in a large change in current. Further, the forward voltage decreases with temperature and this results in a significant increase in current, which increases the junction temperature even more and can create a runaway current. The resistor in series is used to limit the current and control the impact of the shifts in forward voltage. The resistor also provides the ability to compensate for a decrease in light output due to higher temperature. However, this reduces the efficiency due to increased power consumption at the resistor.

The preferred approach is the constant current source, which provides the ability to maintain a steady forward current through the LED as the voltage drop across the LED junction varies. The constant current source also allows for variations in the power source of the LED circuit, without affecting LED forward drive current. As a result, the LED lights will provide a continuous luminous output during operation.

LED BINNING

Due to the variability inherent in their manufacture, and the fact that each silicon wafer in an LED has 50-100K chips that may vary in color and intensity, LEDs are tested and sorted into “bins,” categories based on several measured parameters such as luminous intensity, forward voltage, and dominant wavelength. This is done to provide color consistency and homogeneity within a given product.

LED LIFE SPAN

Manufacturers often provide LED life span data. Two types of data are often cited. The traditional source lifetime is calculated according to the B50 standard, which provides data on the number of operating hours after which 50 percent of the population will fail. However, LEDs typically do not burn out like incandescent, fluorescent, and metal halide lamps, hence this data is not useful or easily available. Instead, the decline in light output is calculated. The L70 is based on lumen depreciation (or lumen maintenance) or hours of use before an LED drops to 70 percent of its initial lumens.

DIRECTING LED LIGHT

Because LEDs are built using surfacemount technology on planar substrates, they are surface emitters of light and generally exhibit half-space emission, unlike metal halide/HID lamps and fluorescent lamps, which emit light in a 360-degree pattern. This is an advantage for lighting an aquarium, since the light is required mainly in the half-space below the lighting fixture. Traditional lighting technologies require the use of reflectors to direct the light downward into the aquarium. Typical LEDs have a light spread angle of around 120°, depending on the design of the LED die, and, as with any other source of light, the light intensity is highest at the central axis and reduces toward the periphery. The distance of the surface to be illuminated from the light source and the light-spread angle determine the intensity of the light falling on the surface. As the light source moves further away from the surface to be lit, the light falls on a much larger area and is reduced in intensity. Secondary optics are often used to further focus the light so that higher intensity can be maintained at the expense of the spread of light.

LED optics can be either lenses or reflectors. Lenses tend to be more efficient than reflectors at shaping the light beam. Collimators are lenses that can be used to change the beam angle and the divergence and shape of the beam. They are typically designed for specific LEDs and must be used in conjunction with those LEDs. A typical design is a collimator lens that spreads the light on the surface in a specific angle. Collimator lenses are available in varying angles, and they can be made of glass or optical plastic (such as acrylic or polycarbonate). The choice has an impact on the manufacturing (and subsequent retail) cost and quality. Tertiary optics are used to further vary the distribution of light and may protect the elements of the LED system.

LED optics are particularly helpful in increasing the light's intensity and ability to penetrate deep into the water column. But since light spread is sacrificed, it may be necessary to add more LEDs to a fixture. Adding secondary and/or tertiary optics to an LED can easily increase its cost by 25-50 percent.

Choosing too narrow an optic can create further problems with blending of light. Fixtures for reef aquariums often use a mix of blue and white LEDs. Using secondary optics with narrow distribution can result in poor mixing, resulting in spots of white and blue light. This can be mitigated to some extent by the proper design and spacing of LEDs, and is less apparent further from the light source.

INTEGRATION OF LED LIGHTING SYSTEMS

A typical LED system comprises several major components, each of which has been discussed earlier. The costs, reliability, and performance of a system depend on the components chosen and how they are integrated. Hence, rather than focusing on LED efficacy and life alone, in practice it is necessary to have an overview of all the components and the impact of efficiency losses (optical, thermal, driver, etc.) throughout the system. These losses may combine to lower LED efficiency by as much as 40-60 percent.

Typical optical efficiency through secondary optics is between 85 and 90 percent. Depending on the placement of the secondary optics, there may also be light loss when some of the light is reflected back into the fixture, especially when reflectors are used Thermal loss relates to the decrease in light output as its design, passive versus active cooling for heat removal, and choice of pathways for heat transport will all play a role in the design of the system. Passive cooling relies on a large enough surface area and mass to spread the heat and on the difference between the ambient and heatsink- surface temperatures. Active cooling relies on the forced removal of heat via fans to increase air flow and hence the rate at which heat can be removed. A passively cooled LED fixture operates quietly, while an actively aircooled fixture creates fan noise. The fans themselves are another potential point of failure in the system. Heat also degrades other components, such as lenses and encapsulants, substrate connection materials, and onboard electronics.

LED driving electronics do not convert power with 100 percent efficiency. Electrical losses in the driver decrease the overall system efficiency by wasting input power. Electrical losses due to driver inefficiency can be anywhere between 10 and 20 percent, and high-efficiency drivers typically have much higher costs.

MULTI -CHIP AND RGB LEDS

Typical LED chips measure around 300 micrometers on each side and are packaged into individual LEDs that include the wire leads, epoxy encapsulant, and heat sink. A typical lighting fixture for aquarium use requires several hundred 1-watt LEDs or several dozen 3-watt LEDs, which must be assembled, wired, driven, and thermally managed.

New developments in LED technology have resulted in multi-chip LEDS that may incorporate several hundred chips in a single LED package. Multi-chip LEDs are available in power ratings from 10-100 watts, and can provide an alternative to using a large number of lowpower LEDs. Heat dissipation from large chips is focused on a relatively small area compared to heat dispersal from several separate lamps in a large lighting fixture. Due to the intensity of the heat, more heat must be dissipated. In addition, there is a drop-off in external quantum efficiency as chip size increases; a drop-off of more than 25 percent has been reported for chips with an area of 1 square millimeter.

There are several new LED fixtures for aquarium use that implement this multi-chip LED technology (e.g., Ecoxotic Photon Cannon in 50W and 100W versions). Another variation of the multi-chip design is one produced by DiCon, in which several individual high-output LEDs can be packed into a high-density array on metal substrates to improve heat transfer. Differently colored LEDs can be mixed in the array to create light with better mixing than that achieved using pegboard fixtures with alternating white and blue LEDs. Multi-chip designs also have a single large light optic, as opposed to having one for each individual LED.

Other developments in multi-chip LED technology include the development of multicolor chips—LEDs that incorporate differently colored chips (e.g., white, red, blue, green) in a single unit and provide separate driving controls for each color. These LEDs allow adjustment of each different color and fine-tuning of the appearance of the light. Using more sophisticated controls can provide the user with infinite possibilities for adjusting intensity and color.

LEDS AND THE AQUARIST

The aim of this discussion so far has been to provide a basic understanding of how LED systems operate and what impacts their operation and life span. Now we will focus on the practical elements of LED lighting to assist aquarists in choosing and using LEDs for their aquariums.

Efficiency. Up to 200 lumens/watt may be commercially available in the next few years, and LEDs will have the highest lumens/watt rating of any light source in the near future. Even current-generation LEDs are capable of providing more lumens/watt than fluorescent and metal halide systems. LEDs provide light comparable to that of a metal halide or fluorescent setup, but use less power, resulting in lower electricity bills and “greener” reef systems.

Design freedom. LEDs have a fairly small form factor or required design space, and hence offer a wide variety of sizes and shapes, permitting aesthetic designs that are hard to achieve with large metal halide reflectors and fluorescent lamps. LEDs can be packaged as spotlights, strip lights, or combinations of different colors of LEDs, providing the opportunity to be creative in lighting a reef aquarium.

Spotlights can be used to highlight certain corals or areas of the reef. Blue strip lights can replace the actinic fluorescents that are very common in reef aquariums. Mixing differently colored LEDs and using the dimming feature of the drivers creates various lighting effects, such as moonlight, dawn/dusk, and even cloud effects during the day, using a single luminaire fixture and exploiting the controllability possible with built-in or wireless controllers.

Selective dimming of differently colored LEDs allows the user to fine-tune the color and achieve a wide range of color temperatures. The picture at the top of page 47 shows the range of designs of various LED fixtures available from Ecoxotic.

No heat or UV. Both metal halide and fluorescent lighting generate infrared heat that warms the water column, often necessitating the use of a chiller to maintain temperatures below 80°F. The cost of chillers and the associated power usage can be a large component of the setup and operational costs of a reef system.

As seen in the technical discussion, LEDs do not produce any infrared heat, so there is no additional transfer

of heat to the water and chillers are not required. On the other hand, using LEDs could necessitate the addition of a heater, depending on the ambient temperature. LEDs do not generate any UV light unless specific UV LEDs are used.

Long life and low maintenance. The life span typically claimed for LEDs (where they produce 70 percent of the initial light) is around 50,000 hours. While this may be true under ideal conditions, a typical LED fixture often has a shorter life, depending on the quality of the system components, the system design, and the manufacturing quality. Even if we assume a more conservative estimate of 60-70 percent of the figure claimed, this means that a well-designed LED system could be used for five to seven years without having to change bulbs. This would represent a significant cost savings, given that most reef aquarists change fluorescent lamps every 6 to 12 months and metal halides every 9 to 12 months.

While all these advantages are significant in their own right, they are beside the point if the LEDs fail to provide the light your corals need. The question is, how much light do corals actually need? Based on the experiences of many hobbyists over years of successful reef-keeping, we know that corals can adapt to and thrive in a wide range of lighting conditions. Specific guidelines for individual corals are hard to obtain, but on the basis of my experience measuring light levels in aquariums, the box on page 47 provides a reasonable guide.

The light requirements of corals are usually expressed as photosynthetic photon flux density (PPFD) and measured in micromoles/m2/sec. This unit of measurement is quite different from the lumen measurements used to specify light output based on human vision.

When measuring in lumens, light output is weighted differentially, with green given the highest weight and red and blue given much lower weights. This is expressed as a photopic curve and defines the luminous efficiency function. However, where photosynthesis is concerned, all photons of light are treated equally, hence this weighting is not necessary. Unfortunately, LED manufacturers do not provide data in terms of micromoles/mVsec, and it is not possible to convert from lumens to PPFD without explicit knowledge of the spectrum. There are special light meters, known as quantum meters, available that can measure light as micromoles/mVsec, which basically is a measure of the number of photons falling onto an area measuring 1 m2 in one second. Using a quantum meter, the light spread from LEDs can be measured and compared with the requirements of the corals to establish whether the LED light can provide adequate light for their growth. Empirical observations seem to indicate that if the light can generate coverage of the bottom of the aquarium at a light level of 100 micromoles/mVsec, there will be an adequate light gradient across the vertical cross-section of the tank where corals can be placed. Corals with a high light requirement will readily thrive in the upper third of the tank, with adequate light at the bottom for those with a lower requirement.

Detailed analysis and comparison of LED fixtures and their light output is available online and is not the focus here. Suffice it to say that the range of choices available to aquarists is expanding rapidly, with options for all budgets.

Every aquarist will have to decide for himself or herself what benefits LEDs provide compared to other, longer established lighting technologies—or even more radically new choices, such as plasma lighting—and whether they warrant the purchase of LED lighting now or in the future.

SanjayJoshi, Ph.D., professor of industrial and manufacturing engineering at Perm State University, University Park, Pennsylvania, will discuss plasma lighting in a future issue of CORAL.

[sunny03]

boss can post the link where you get this artical .. want to see those graph or pics.

[Harlequinmania]

This is extracted from the latest issue of the CORAL magazine;

http://mobileservices.texterity.com/coral/20110910/?folio=36&article_id=112905&lm=1317749465000#pg39

[Jameshong]

This is extracted from the latest issue of the CORAL magazine;

http://mobileservice...7749465000#pg39

Thanks for sharing... !

[sherman]

Very interesting article for anyone interested in LED as aquarium lighting.

[paul118]

This is extracted from the latest issue of the CORAL magazine;

http://mobileservices.texterity.com/coral/20110910/?folio=36&article_id=112905&lm=1317749465000#pg39

I am interested in True Lumens Pro LED strip light. Do u know if any shop in Singapore sells this? Thanks in advance..

http://www.current-usa.com/lighting/truelumen-pro-led-striplights

[Harlequinmania]

i dont think this is available here currently, but reefsystem do carry a similar products from ecoxotic. You can see the light in their showroom.