For example, if one eye is turned outward, a base-in prism may be used to move the image to accommodate for the position of that eye. If one eye is higher or sees an image higher than the other eye, a base-down prism may be used to accommodate for that eye and move the image up.

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.

In normal vision, the two eyes work in tandem by pointing at the same image, simultaneously. Each eye collects distinct visual information to be sent to the brain for processing. The brain then combines the visual input from each eye to produce a single image for you to see clearly.

Since double vision can be a sign of a serious health condition, it is important to see your doctor as soon as possible to determine the cause.

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.

Schedule an appointment with an eye doctor for a comprehensive eye exam, and to discuss any questions you may have about prism glasses or vision therapy.

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.

So why aren’t optics used everywhere then? According to Littlejohn, there are disadvantages of secondary optics with some of them being pretty significant:

Very often, eye doctors will prescribe more than one pair of prism glasses if varying prism powers are required for different activities. This is usually necessary if the two eyes are only misaligned intermittently or for certain distances, or when the eye turn occurs as a result of increased visual demand or fatigue.

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.

If you experience eye strain or double vision, contact an eye doctor near you, who can diagnose and treat any underlying visual problem.

Double vision occurs when the two eyes are not aligned properly and are unable to work together to point in the same direction, at the same time.  As a result, the brain is unable to accurately process the distinct visual input from the two eyes to form a clear, single image.

Moreover, since prisms change the way light enters the eye and brain, they can have far reaching effects on the entire nervous system— affecting breathing, heart rate, muscle tonicity, and even attitude.

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.

“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.”

Vision therapy retrains your eye-brain connections to improve your visual skills for clearer, single vision, with or without your eyeglasses.

“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.”

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.

“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.”

“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.”

Not only do prisms displace images to improve vision, they also facilitate changes in perception, as the base of the prism slows down the rate at which light enters the eye, causing objects to appear further away.

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.

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.

“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.”

While prism lenses will help to relieve your visual symptoms while wearing your  glasses, vision therapy is a long term solution for your vision problems.

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.

More recently, people are hearing more and more about prism glasses for a variety of vision problems. Prism glasses are typically prescribed for people who are experiencing double vision, as these lenses help to merge the two images into one clear image.