1. What is meant by "quality of light"? The quality of light can be just as critical as energy-efficiency and light output in settings where merchandise must look appealing, such as a retail application.The color-rendering index (CRI) is a scale from 0 to 100 that measures a lamp's ability to make colors look natural. Generally the higher the CRI, the better colors appear. This standard is based on burning Tungsten and using Planck’s blackbody radiator formula to determine the light’s spectrum.Color temperature refers to the color appearance of the light and is measured in degrees Kelvin or in lighting lingo, "K". A low color temperature corresponds to "warm" or a red-yellow appearance like incandescent lamps (2700K). "Cool" or white light comes from fluorescent lamps operating at 4100K, with 3500K considered neutral.In summary, the best lighting strategy combines the necessary features of light quality and cost-effectiveness. Consequently, the most energy-efficient lamps may not always provide the optimal lighting solution. Neglecting the color quality of light may affect product display effectiveness, consumer spending habits, or employee productivity.
2. Why are optics important for LEDs?
For all intents and purposes, LEDs are what we would refer to as a point source. Most conventional lighting emitters are broad source in nature. Another difference is that most conventional light sources are omni directional while LEDs emit light from a flat surface. This allows LEDs to be used with primary optics to increase their extraction efficiency which basically makes them more efficient. Secondary optics may be used to expand and steer the light in more useful ways such as spotlighting or interfacing to tertiary optical systems such as fiber optics or holographic diffusors. Due to the small nature of the LED emitter, practical optics can be fabricated to achieve this objective whereas conventional broad source emitters must rely on refectors or very large optics to accomplish this task.
Without properly designed and manufactured optics, the LED based lamp would not perform very well. High quality optics must maximize the extraction efficiency and minimize aborations affecting the light path while steering and shaping the light for the particular task. This is why the optics are so important in LED based lighting.
3. How are LEDs powered?
LEDs are basically diodes, which mean that they conduct electricity in one direction. Being semiconductor devices, they operate on the molecular and quantum level and exhibit very particular properties depending on their composite construction. The operation of these devices depends on a current biasing on the diode to a point where it will emit a steady stream of photons. In this setting, a very slight change in the applied voltage will result in a very large change in the current going through the diode. Electrically, this means that the diode does not have a linear voltage relationship to its output as one expects with an incandescent filament. In fact, this device is current controlled in that the photonic light output of the LED responds to changes in current and not voltage. This means that a special current controlled supply must be employed in order to power the LED safely without damage and for high efficiency operation. We refer to this type of supply as constant current type. For a bit more clarity, the current is d.c. and not a.c. to the actual LED device. For simple operation, a resistor may be added in series with the device to act as a basic constant current supply, albeit a low efficiency one.
4. What are other differences of LED based lamps?
Since LEDs are semiconductors they posses a property known as an intrinsic resistor. This "resistor" has some interesting properties in that it is inversely affected by temperature, which is not necessarily a good thing as will be explained. Secondly, the LED is really current controlled and not voltage controlled meaning that a slight change in voltage causes a large change in the current flow. As the LED is being powered, heat is generated in the device which lowers the resistance value of the intrinsic resistor, causing more power to be dissipated, causing more heat. To avoid this runaway effect, a large enough heatsink must be employed to cool the LED, perhaps even combined with temperature control electronics.
Conventional incandescent bulbs produce mostly heat and some light by heating up a coil of tungsten wire which exhibits a normal positive temperature resistance. They don't require any heat sink and are not electrically or phontonically efficient.
CFLs generate a plasma by electrically exciting mercury atoms to a state where they emit UV photons which then secondarily excite phosphor atoms to produce white light. This is more efficient than incandescent lighting.
5. How are LEDs measured?
Light sources in general are measured at a minimum using an integrating sphere. This is a technique that places the light source in the center of a sphere where the inside of the sphere is reflective. By measuring the total light bouncing around inside this sphere a good approximation of the total lumen output can be calculated. This method is useful for typcial light sources such as standard CFL or incandescent lamps since they radiate in a spherical pattern; close to 360 degress.
LEDs typically radiate <180 degrees and may exhibit more complicated radiation patterns. For this type of radiation, no matter what the source, an intergrating sphere may not provide an accurate approximation of the light output. In this case, a device called a goniometer or goniophotometer is used. This device is a type of robotic measurement device that creates a virtual sphere by moving a phometric receiver around the light source so that discrete measurements are made along a spherical pattern. The end resultant data is used to not only calculate the total lumen output, but also the graphical representation of the output pattern.
Companies that need to supply this data will typically send the lamp out to a third party measurement company which charges to perform this measurement. At DMS Lighting, we have our own measurement laboratory and provide goniometer data of the actual lamp assembly. This is important as LED emitters tend to change frequently and also measurements should be made with the optical system installed. It is common for standard emitter data to be presented, such as lumen output, of the emitter only from the LED manufacturer,s data sheet which will be misleading. Many LED makers are either basic assemblers or importers without any engineering, design or measurement experience.
6. How do I compare LEDs to other light sources in terms of useful output?
Of course the holy grail of lighting is to create a light source that produces only good quality light without any heat. The excitement of LED lighting stems from this quest since it appears that over time, just as with computing power, LEDs will be one of the most energy efficient light sources on earth. In the last few years, LEDs have kicked out quite a few standard light sources in terms of lm/W efficacies. Of course the cost of the lamp still needs to be justified as we all know, but even that is less prolematic when one also considers the lifetime expectations of properly designed LED lighting systems, whether light bulb replacements or large lighting infrastructure projects.
One of the reasons a direct comparison of the total lumen metric is confusing or in many cases misleading is due to the apple and oranges nature of older lighting technologies versus new LED technologies. As we mentioned before, by nature, LEDs are not omni-directional light sources. In order to design and LED lamp as an omni-directional source, we have to either use optics or multiple LEDs in some type of array. Once either of this designs is executed, a more direct apple to apple comparison can be made. The point is, you must really compare apples to apples in order to make sense of the lm/W metric and total lumens. Another example is spotlamps such as MR16s. Since these lamps emit a conical pattern, we should only compare, lm/W / total lumens between these types of lamps with the same beam angles to get a true comparison.
A third thought to be considered is the validity of the measurement data, or even the truthfulness of the data. For instance, all lamp makers use the total lumens (we already know the wattage when we buy the lamp) metric as one of the main figures of merit to compare the performance of a lamp. This is easy to understand as it equates to brightness versus power draw to the user. This is where the problem comes about, not only for LED lamps, but sadly more so for LEDs. Some data presented is misleading where a comparison between the conventional sized lamp in terms of wattage is made. Some Web posted data claims a 1 Watt LED lamp will replace a 60 Watt standard bulb. Going through the numbers and giving the maximum benefit of doubt to the LED marketer here, and assuming the lamp is apple to apple, we could assign the following to the LED lamp: emitter at 60 lm/watt, optical efficiency at 85%, therefore, 51 lm/W = 51 total lumens. By comparison, a poor incandescent 60 watt lamp might operate at 12 lm/W x 60 Watt = 720 total lumens.
As you can see in the above example, one would need 720/51 or 14 of these 1 watt LED lamps to be equal to the lumen output of the 60 watt bulb even though the LED lamp is rated at 51 lm/W vs. 12 lm/W for the older type bulb. The lesson here is to make sure you are comparing similar type products and only buy from quality companies with experience in lighting and LED technology.
7. In simple terms, when do I choose an LED product today?
Let's start by recapping some basics.
LEDs and CFLs are far superior to incandescent lamps in terms of both operations lifetime, efficiency and return on investment.
LEDs last much longer than CFLs and are becoming more efficient as well and come on instantly without any warmup period.
LED lamps are much more efficient at producing mono colored light and can provide color mixing in some configurations such as with RGB and RGBA.
LED lamps will be easier to work with dimmers than CFLs.
The decision to use an LED lamp should be based on the following concepts today:
A need to lower cost of maintenance for the lamps such as office, hotels, casinos, etc.
A need to lower the heat load of the installations.
A need to permanently install lighting in hard to access areas.
A need to add colors and special effects or mood lighting.
A need to lower the operation costs while making a better presentation.
8. What makes up a basic LED light system?
From the points we mentioned before, there must be a quality LED chip or die; there has to be a primary optical system, which is generally supplied by the chip supplier as part of the emitter's construction; there will generally be a secondary or tertiary optical system to manage the light output; there must be a current controlled high efficiency power supply; there must be a heatsink assembly; and finally there must be some sort of connection system.
Beyond these basic components, other systems can be added such as dimming controls and digital controls such as DMX. DMX is particularly useful for RGB and RGBA systems for color mixing.
9. What is DMX control? DMX or DMX-512 as it is also refered to, is a protocol based on RS-485 that is used to control both theatrical stage lighting and effects as well as LED lighting. The DMX 512 was developed in 1986 and revised in 1990. It is a standard developed by the Engineering Commission of USITT. Today the standard is also known as E1.11 or DMX 512-A.
DMX in concept is vey simple in that it is a dimming system developed around 8 bits (computer bits) of data for its resolution. Each of these bits is assigned a channel address (1 of 512) which in effect allows 512 addresses or devices to be dimmed with the 8 bits of data. Today, DMX in this simple form can be used to control mechanical parts of the lighting as well as the dimming control making it very useful in designing lighting that can be computer controlled and therefore co-ordinated with other parts of a total program. Those of you that figured out it takes 9 bits to get 512 can move to the head of the class. This feature was added to accomodate more channels for electro-mechanical functions. This means today we have 256 steps of control resolution per channel and 512 channels to work with in one group.
For LEDs, the 8 bit dimming allows for R (Red), G (Green), B (Blue) LEDs to be dimmed in a fashion that provides for 24 bits of total color mixing information. Combine this with a DMX compatible LED driver, DMX computer software and your have a very handy way to produce great color effects. DMX systems can also be daisy chained so that many units can be connected onto a single control line. Technically, this is defined as 32 units per DMX cable in the 1990 specification release.
DMX has the ability due partly to its simple address structure, to be expanded into groups of channels, or universes as thay are refered to, which can allow for very large systems to be controlled based on this simple protocol. This is one reason that DMX has been around for so long since it is still very useful for its intended pupose.
10. How does DMX work in practice with LED lamps?
DMX is based on the ability to reach a particular address in order to instruct the dimming level of the device. In the case of LED base lamps, typically there would be two cases; an addressable fixture; or an addressable piece of hardware that will drive the lamps. Setting the address on either of these typically will be done manually with binary DIP switches, that is to say, ON or OFF.
The addressing will be in binary increments such as 1,2,4,8,16,32....256. In order to have an address of 9, in that case you would switch on 1+8 with all other switches turned off. Next your device would be connected to a DMX transmitter via a DMX cable. This may be a device connected to a PC that runs DMX software and has a USB port connection to the transmitter or it may be a console. In between these devices, you may have a DMX splitter to create more universes or simply to electrically isolate your signal in a complex system.
Once the address is logged into the system, you can start to control the device using this setup. Most of the time, a set of devices will be connected and synchronized to match other parts in some type of program such as a stage presentation or entertainment event. For instance, the lights on a modern ferris wheel are typically controlled by a DMX system to provide interesting patterns as the wheel rotates.
Using a DMX LED driver as a processor can usually provide some internal stand alone programming. Also some DMX black box type units can download DMX from a PC or from a DMX source and play it back under its own power without the original program connection.
DMX can also be run remotely via ethernet when either a LAN or WAN to DMX converter box is placed in front of the signal chain.
