^ I personally don't use them on any of my tanks. LEDs are very powerful, and even on my 40B with the fixture 12" off the water I'm not using any. Here's a pretty good, but short article that explains optics!
Seeing the light: LED optics explained
Posted on January 27th, 2010 by Brian Blank 9 Comments
LED lighting is beginning to take hold in the aquarium hobby and it can be a very complex subject to master when trying to make an informed purchase decision as a consumer. We touched on the topic of underdriving or overdriving LED’s with coralSky’s Jeff Littlejohn earlier and were fortunate enough to get a hold of some great information on LED optics by Jeff from his forum post at coralSky to share with our informed readers.
Typically, when we think of LED optics, what comes to mind in a plastic lens of some sort used over the LED itself to focus or disperse the light. We need to take a step back and realize the LED itself has a protective casing called the primary optic. The primary optic serves two purposes: to protect and to shape the light output of the diode.
Manufacturers will use the spatial distribution (or shape of the light) when describing the LED, referring to the spread of the light from the central axis of the unit. In the aquarium setting, LED’s are mounted facing down and into the tank and if we imagine a line running vertically from the center of the LED, the spatial distribution of the light is measured in degrees from this central axis. For example, a 120-degree LED will extend the beam 60-degrees to either side.
“This is one of the reasons why LED lighting is more efficient than metal halide or fluorescent light sources, which emit light in a nearly spherical pattern,” says Littlejohn. “This means that most of the emitted light is directed away from the intended subject, and a reflector must be used to redirect this light to a useful direction. Since there is no such thing as a perfectly reflective surface, this results in a loss of efficiency.”
But just because an LED is rated at 120 degrees, it doesn’t mean you get the lighting punch of the LED across the entire spectrum. Just like any other point of light source, it’s going to be stronger the closer you travel to the center. Along the central axis the LED emits 100 percent of its relative luminous intensity and will lose intensity the farther you move away from the central axis.
For simplicity’s sake, if a 100 lumen will produce 100 lumens of light at the center and a measurement taken 25 degrees from its central axis, the output of the LED will appear to drop to only 80 lumens. Continuing on the path away from the center axis a measurement taken 45 degrees off axis will yield only 40 lumens, and so on, until at 60 degrees, only 10 lumens or so are emitted.
Now that we have a better understanding of the primary optic, let’s delve into secondary optics. The secondary optic are separate components that are usually made from optical-grade acrylic or polycarbonate mounted over the primary to help further shape the beam of light. The purpose of the secondary optic is to increase the relative luminous intensity. An example Littlejohn uses is the Fraen 8 degree optic that can actually increase the intensity of the LED 27 times.
“2,700 lumens out of a 100 lumen LED sounds great, right?,” notes Littlejohn. “Not so fast. All of this extra intensity is still only achieved directly under the central axis of the LED. In the case of a narrow optic, there is a SEVERE drop off in light intensity only a few degrees from the central axis. A narrow optic creates a “pencil” of high intensity light, and almost no light is emitted outside of this very narrow beam.”
The numbers sound incredible but in your typical aquarium lighting application the light would have to be mounted so high above the tank to get any usable spread off the LEDs making them unusable from a practical standpoint. There are some more practical secondary optics for use in the hobby, Fraen wide beam or the Ledil Rocket W for example, that do increase the light output of the LED but at a much smaller scale. The Fraen wide optic *increases output 2.4 times and directs it into a 58-degree come.
“Besides the obvious advantage of achieving more light from the same number of LEDs, for deeper tanks, the addition of a secondary optic will greatly benefit the light penetration through the water column,” he adds. “In my opinion, for tanks up to 24-inches deep, LEDs do not require a secondary optic to reach the bottom with a significant amount of light energy intact. For tanks over 24-inces deep, the extra light penetration provided by the secondary optic overrides enough of their disadvantages to warrant their use.”
So why aren’t optics used everywhere then? According to Littlejohn, there are disadvantages of secondary optics with some of them being pretty significant:
They can be expensive. They typically add between 25%-50% to the cost of each LED.
With few exceptions, they create a harsh transition from bright to dark. In other words, the “edges” of the cone-shaped light pattern can be very distinct. This creates a “flashlight effect”, where everything within the light beam is very bright, and everything just outside the beam is very dark.
They hinder color blending. Since most of us like the actinic effect of all blue lighting, and since most of us prefer a cooler color than the coolest white LED available, we must use a combination of royal blue and white LEDs in our systems. When secondary optics are added, you may see obvious white and blue spots in your tank. This effect can be reduced by reducing the center to center spacing of each LED, so that the cones effectively overlap, but it may still be very apparent as our corals grow towards the top of the tank. Also, our rock formations and corals cast shadows, which may be distractingly blue or white under secondary optics.
So what is the bottom line from all of this? For Littlejohn, there are two significant takeaways from his post.
“On shallower tanks, I’m a proponent of using straight LEDs without secondary optics,” he states. “I’ve found that achieving a sufficient amount of light for high-light corals is possible, there will be no flashlight effect, no spotting, and more uniformly colored shadows.”
What exactly is a sufficient amount of LED lights for light-intensive corals you may ask? Littlejohn recommends a LED density of around 24 LEDs per square foot for 10- to 12-inch deep tanks, 36 per square foot or 12- to 18-inch tanks and 48 per square foot for tanks 18 to 24 inches deep using a nominal drive current he outlined in the previous post here at Reef Builders.
“On deeper tanks over 24 inches, I like to add a mix of secondary optics to the LED array. I’ve observed that a relatively small number of secondary optics can provide enough extra light penetration while minimizing spotting and colored shadows,” he says. “It may take some trial and error, but I believe a good starting point is to cover 15 to 25 percent of the LEDs with a good secondary optic, and LED densities of 54 or even 60 LEDs a square foot may be required, at a nominal drive current.”
We see great potential in the hobby over the upcoming years with LEDs and other advanced lighting and we will continue to bring you more quality information on the advances in this promising technology.
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