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The Wonder Light
So now let's look at what a high brightness white LED has to offer and why we will be seeing a lot more of them in the future. Some of the following might be a bit technical, so I suggest you just skip the parts you don't understand. There is enough general information that most people will find it useful and interesting.
The technology used in the white LED starts with a blue LED combined with some very special phosphorescent materials. The blue light excites the phosphorescent materials and generates the reds, yellows and greens needed to produce white light. The light has a color temperature of 6500 degrees, which is similar to noonday sun. If you look at the spectrum through a prism, you will get a mostly complete and well-proportioned rainbow. This is why the LED has a very good color-rendering index of 85. Many other white light sources, such as a fluorescent light have just a few widely spaced spectral lines and as a result have a poor color-rendering index.
The first and greatest contribution of the new white LEDs is their reliability. These LED bulbs are solid state Light Emitting Diodes and have an expected lifetime of around 100,000 hours. That's over 11 years of continuous operation.
If you look at one of these bulbs closely you will see there are some wires and a little block of material completely encased in clear plastic. The little block of material is the part that is generating all the light. It is almost impossible for the LED bulb to fail because of physical abuse unless you smash the plastic or break the wire leading to it. Putting the LED on a cement floor and hitting it with a hammer is one way to break it. So realistically, as long as you can get power to the bulbs, you will never be in the dark.
The way these bulbs fail under normal conditions is to slowly generate less light. You can expect the light output to drop about 3% for every 1000 hours of operation at full power (25 degrees C, 20mA). At reduced power settings, the percentage drop per 1000 hours gets very small indeed.
Like all solid state devices, the failure rate goes up with the junction temperature. Increased ambient temperatures and increased current will drive up the junction temperatures. Putting a large number of LEDs into an array has the same affect as raising the ambient temperature so extra caution must be used for large arrays. But since there is no easy way for you to measure the junction temperature, be sure to stick to the manufacturer's published limitations or reliability will suffer.
How Much Light Do I Get?
OK, so these things are never going to burn out. But how much light do they generate and how bright are they? To answer those two questions you must understand the difference between light output and luminous intensity. Light output is the total amount of light the little block of material generates. Luminous intensity is how brightly a surface is illuminated. Think of it this way: the light output is the same as a measure of water and the luminous intensity is how deep the water is if you pour it into a container with straight walls. If the bottom of the container is large, the water will be shallow. But if the bottom of the container is small, the water will be deep.
Light works the same way. If you spread the same amount of light over twice the area, the luminous intensity will be half. And if you move the light source twice as far away from the surface it will illuminate four times the area. But I still have not answered the question about how bright these LEDs are. The answer is they are plenty bright -- if you use more than one. You will need an array of12 LEDs for smaller passages or light colored passages, what I will call the typical caving condition. You will need an array of 25 LEDs for darker or larger passages. 50 LEDs are only needed for very dark passageways, such as large lava tubes. These numbers assume you are using the best quality devices at normal drive currents.
The above numbers also assume the LEDs you are using have a half-angle of about 15 degrees (i.e., the brightness has dropped to half by 15 degrees off the center axis) and produce an optimum beam pattern. Which leads us into the optical properties of LEDs. Each LED has its own lens. For most flashlight and headlamp applications, a beam of light is desired. However, for optimum usefulness, the beam intensity should fall off smoothly from the center of the beam to the edge of the beam. Such a beam will provide a smoothly lit path from your feet to where you are looking if the beam is aimed about 5 paces in front of you. You will find that the beam patterns change from one manufacturer to the next and so careful selection is needed for best results. You may have to combine LEDs with different optical properties to get the desired beam pattern.
The beam pattern from an incandescent light with parabolic reflector is a narrow15 degree beam. From the center of the beam to just outside of the beam is contrast ratio of 6 f-stops - a very harsh contrast ratio indeed. However, the beam pattern of an LED array using superior optics has no discernable beam because of the smooth transition from the central bright area to the dimmer edge areas. A visual cone of over 45 degrees is well illuminated and we still have not reached the 6 f-stop contrast point. We have traded the brighter peak luminous intensity (almost 2 f-stops) with its harsh viewing conditions for a smoothly lit scene that is safer to more around in and more pleasant to look at. I think the following photographs will illustrate this point.
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| Beam pattern from a 24-LED array, 1.4W (left) and two-cell headlamp with 2.5V 500mA halogen bulb, 1.4W (right) |
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This scene is looking at a saguaro cactus about 30 feet (9 meters) away. The lamp is pointed about half way up the cactus. The left photo used a 24-LED array; the right photo used a two-cell headlamp with halogen bulb.
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This scene is looking at a streambed with the lamp pointed about 15 feet (4.5 meters) ahead. The left photo used a 24-LED array; the right photo used a two-cell headlamp with halogen bulb.
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I should also touch on the subject of how much light is needed to perform a task. For instance, when I said you needed an array of 12 LEDs for small to average size passage (a rather subjective term, I must admit), I am talking about providing enough light to clearly see interesting things on the floor, walls and ceiling. If all you want to do is traverse a passage without falling into a hole, a single LED will do fine. It's like the difference between hiking a beautiful mountain trail in daylight versus on a dark night -- following the trail is different from seeing all the scenery.
Efficiency
When you consider the over all performance of a lighting system, there are three major considerations in addition to the beam pattern. First, your eyes are able to adapt to a wide range of light levels with relatively small changes in visual acuity. As long as you can still see color, there is enough light for high visual acuity. Below this level, your visual acuity drops rapidly. Of course, fatigue, altitude and illness can have an adverse affect of your visual acuity and increase your need for light.
Second, your eyes respond to light in a non-linear (logarithmic) fashion. What this means is that the light level has to change dramatically for you to notice a slight change in illumination. If you change the light level by 20% you may not even notice the difference.
Finally, LEDs can efficiently produce a wide range of light output. Roughly, light output is proportional to power input. I'll talk more about this later, but for this discussion, you can assume a linear relationship.
Combining these characteristics allows us to build a lamp system that is super efficient because we can reduce the power consumption by allowing the eyes to adapt to a comfortable level of light. The important thing here is the ability to generate the correct amount of light needed for a task and to do so efficiently. And because of the way your eyes work, when we reduce the power we are reducing it to 25% at each reduction. Going the other way, we are increasing the power by 4 times for a significant increase in light output. This is the reason bright lights must consume so much power.
For best results, the different light settings should be spaced about 2 f-stops apart which corresponds to a power change of about 4:1. This keeps the total number of settings to 3 or 4 which has several advantages over continuously variable dimming. First, it makes it easy for the user to know exactly which setting is being used and hence keep track of power consumption. Second, it makes it easy to adapt to lower light levers because the next lower level is always within a comfortable range of your eyes. Finally, with a continuously variable setting, the user tends to make slight increases without realizing the total cumulative affect and thus tends to use more power.
So let's talk about the efficiency of these LEDs. The white LEDs have advanced to the point where they are now more efficient than the low power (less than 2 watt) incandescent light bulbs. They are just about as efficient as the higher power bulbs (3 to 5 watts) incandescent light bulbs. Within another year they will have passed the higher power bulbs in efficiency.
The efficiency of the white LEDs changes depending on how much power you are running through them. If you take 20mA as the starting point (100%), adding power reduces the efficiency while reducing power increases the efficiency. At the high power extreme, increasing power by 7.5 times only results in 3 times the light output for a staggering drop in efficiency. At a low power extreme, dropping the power by 80% still yields 30% of the light for a substantial increase in efficiency. The moral to this story is if you need more light, add more LEDs, don't increase the power per LED.
Driving the LEDs
LEDs have a very steep voltage/current curve. What this means is if you change the voltage by a small amount the current will change by a large amount. This also means that LEDs need to be tightly controlled in order to achieve consistent results. There are four basic ways to control the power in an LED: under power, resistance, constant voltage control and constant current control. The ideal situation is to use constant current control.
The simplest way to control LEDs is to connect them to a battery that does not have enough voltage to drive them to normal current levels. All small LED flashlights that use two alkaline batteries use this method. There is no inherent power loss but the LEDs are always dimly lit because they cannot draw full power.
The most common way to control LEDs is to add a resistor in series with the LED. The resistor acts to limit the current flow by causing a voltage drop across the resistor. You can use the internal resistance of the batteries themselves and this what is done for all the small flashlights that use 3 alkaline batteries. However, even with well-chosen values, the tolerances are sloppy. The solution is very inexpensive but has the problem of wasting a lot of power in the resistor. Power losses of 15% to 25% are typical.
A more efficient way to control power in LEDs is a constant voltage power supply. Using modern electronics, these power supplies are small, light weight, reliable and highly efficient. The drawback to using constant voltage power supplies is that the supply voltage must be carefully matched to the LEDs.
The ideal way to control power in LEDs is to use a constant current power supply. These power supplies are more complicated and expensive when compared to the constant voltage power supplies, but it eliminates all the matching. In addition, the power supply automatically compensates for temperature and LED variations.
Lot Variations
Speaking of LED variations, it is probably worth taking a few moments to talk about the differences between individual LEDs. Years ago when I learned how semiconductors were made, I was amazed to learn just how random the results were. If you make 50 identical parts on the same wafer, all 50 parts will be different. In fact, you have to test each part to figure out what the part is. In the case of white LEDs, the voltage curve, the efficiency and the color all vary from one LED to the next. As a result vendors sort the parts into separate bins and sell them at different prices. The best LEDs are the most expensive, as you would expect.
Now you might have been wondering why not combine red, green and blue LEDs in order to get white. Surely it would be more efficient then using a blue LED with the phosphorescent materials. And indeed it is more efficient. But given the LED to LED variations, it is almost impossible to produce a consistent white light. Each color of LED has its own family of characteristics. It is a very difficult engineering problem to compensate for the 6 major variables and eliminate the color fringing.
Purchasing white LEDs from a single bin that have been tested to close tolerances is a simpler way to run a production line. In the end, the product will be less expensive and perhaps just as efficient.
Everything Has Its Price
High quality white LEDs are still expensive. The manufacturers are tending to keep the price constant while improving the efficiency. In the last two years there has been a significant increase in efficiency and color consistency while the price has remained unchanged.
The high efficiency parts with close color tolerances are going for $8 each in single quantity. Larger quantities are available for $3 each. At the low end, the low efficiency LEDs that cannot make the color specification are selling for as little as $1 each in single quantity.
Make no mistake about it -- you get what you pay for. If you want the good parts you will have to pay for them. I have tested samples from many manufacturers but only one manufacturer has been able to consistently provide top-of-the-line parts with high efficiency, consistent color and appropriate optics.
I have been tracking the market for white LED flashlights and headlamps and have noticed an interesting trend. If you plot the list price of each lamp against the number of LEDs in that lamp, all lamps on the market fall in the range of $12 to $24 per LED. At the low end is the Photon Micro-Light® II with a single LED for $22 and at the high end is the Action LightTM with 25 LEDs at $299 ($12 per LED).
Given the relatively high cost of LED lamps, the issue of an LED flashlight or headlamp quickly becomes one of utility. People will purchase a 1 to 3 LED flashlight for $20 to $45 and accept the fact that it will produce a useful if limited amount of light. If damaged, it's not a great expense. But such a unit will never be a main source of light. On the other hand, to purchase something that will be a primary source of light is going to be an investment, so you want the lamp to last a lifetime. To last a lifetime, the lamp must be well built, rugged and tolerant of water, mud and dust. And while we're at it, it should be very reliable, have multiple brightness settings and be efficient.
Don't expect to see a significant drop in price for LED lamps any time soon. Over the next couple of years the LED unit price may drop by 20% to 30% but this will only make a slight change in the over all lamp cost -- under 10%. As always, look for quality in whatever lamp you buy.
Copyright © 1998-1999 by HDS Systems, Inc. All rights reserved.
Updated September 1999
www.hdssystems.com
HDS Systems Inc.
P.O. Box 42767
Tucson, Arizona 85733 USA
520-325-3004
Toll free: 877-437-7978
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