Understanding the Role of Optics in LED Lighting

How an optical system is designed is the key to understand its purpose and importance.

The technology behind LED lighting is impressive as it is. However, LED lighting is more than just a revolutionary means of lighting that saves energy.  You need to consider a lot of things including lenses and reflectors. These two magnify (both literally and figuratively) the importance of LED lighting in our daily lives. As a matter of fact, since time immemorial, optics have played a major role in how we have harnessed and used the power of light.

Lens

(Professional lens design for LightgogoKit, Lightgogo3, Lightgogo4 and Lightgogo5 solar lighting products)

What are LED Optics?

Let’s play a little game here, what goes into your mind when people say LED optics? Do you imagine this highly complex dome lens that helps in focusing or spreading the light? Well, technically you are right. However, that’s going a bit too far. In order to better understand LED optics, let us try to take a step back first.

Look for any basic LED light near you. Assuming you turned it off. you can see a small protective dome over the diode. That is what we call the primary optic. Its purpose is to protect the diode and to help shape the “output”. However, the primary optic can’t really focus the light and the end result is still pretty broad and may not be used for most applications. For starters, it lacks intensity over the distance which makes it useless for lighting large rooms. This is where the secondary optics (lenses, reflectors, TIR optics, etc.) come in. The job of the secondary optics is to collect all that light and magnify its intensity towards the target.

Sounds easy, right?

Well, we’re not there yet.

It’s easy to assume that creating reflectors and lenses for LED’s are quite easy given that manufacturers have been doing this for years. However, one must understand that LEDs emit light differently from older, more traditional forms of lighting. For example, incandescent lights illuminate 360 degrees. LEDs, on the other hand, only illuminate 180 degrees (also called directional lighting). Due to the basic design of an LED, it can only emit light at a maximum angle of 180 degrees.

LED lighting fixtures use at least one type of secondary optic to alter the beam of light coming from the LED source. There are two kinds of secondary LED optics: Lenses and Reflectors. Here is a quick rundown of each kind.

Lenses

The LED lenses come in various shapes and sizes. They can come in square, hexagonal, or round shapes. They are commonly made out of plastic and silicone in order to give them flexibility but still be rigid enough to stay in shape. 

Due to the versatility of lenses, they can be used with single or multiple LEDs. As such, they are often the choice for LED modules or strips. With the right type of lenses, they can come up with various effects. This is all thanks to the precise control one can get from using a lens. Add the fact that a lens can give LED bulbs a certain aesthetic and you have one complete package.

Reflectors

Another type of secondary LED optic is called reflectors. Reflectors work almost the same as lenses although the difference is that reflectors don’t have as much control as lenses. They are, however, a much cheaper solution compared to lenses.

Most reflectors are made out of plastic and they often have metal coatings. To achieve varying effects, reflectors often come smooth or multi-faceted inside and have different shapes. For further customization, reflectors often contain sub-lens  to diffuse or control the light a bit more.

Why Do We Need our LED Lights to Focus?

The LED lighting is taking the world by storm and there is no denying that it will be the standard for years to come. The technology is still improving and it is now being used in different applications. These include the following:

  • interior spot/down lighting
  • outdoor street lighting
  • garden lighting
  • architectural lighting
  • spotlighting

Note that on all the applications mentioned above they all need one thing – focus lighting. With just the primary optic and the diode, LED lights simply cannot perform for such purposes. Without secondary optics, LED lights won’t be able to deliver enough intensity to the target surface.

We won’t get too technical here but think of it this way. If you wanted to showcase the garden landscaping you have at home, would you want the light to just be thrown all over the place or focused on the parts that can really call the attention of the viewers?

Secondary optics work in such a way that they offer a controlled beam of light that you can point at a certain spot to get the full intensity. In a way, secondary optics are used to collimate the light rays. Collimated rays of light contain parallel rays and therefore do not spread as much. While physics will tell us that there is no way to line up the lights in a perfect parallel due to diffraction and limitations of the emitter, our eyes can barely see the difference.

To further understand how a secondary optic or lens can collimate a beam, let’s introduce a few terms. Now, understand that there are things such as viewing angle or full width half maximum (FWHM). What you only need to remember is that FWHM is the angular width of the beam when the intensity at the edge is half the intensity from that of the center of the beam. While this is the easiest way to classify optics, this still omits the major differences presented by different optical platforms (usually by different sized diodes). Basically, optics may have identical viewing angles but still different in intensity and quality due to the emitter’s optical design.

While the main purpose of the secondary optic is to collimate the beam of light, it’s not the only purpose it has. Secondary optics are also used to ensure color uniformity and even light distribution within the targeted area. Depending on the optic and lens you use, you will get varying results.

Leveraging the Use of Optics for Better Light Distribution

It will be hard to describe how good a specific type of lens performs. As a matter of fact, there’s no single metric for it. However, experts still find a way to quantify the performance of optics using several parameters and how they relate to each other. 

But let’s try to simplify it for our sake. 

There is no single metric that can fully describe the optical performance of a given lens. To specify an optimal lens for a given application, lighting designers need to understand several parameters and the relationships between them. 

When talking about luminous flux, you will be focusing on a lens’ optical efficiency.  For lenses made with high-quality materials, they can reach efficiency ratings of 90% or higher. While this looks like a good way to measure how “good” a specific lens is, it still won’t give us the big picture. There are still a lot of factors to consider such as Full-Width Half Maximum FWHM angle and the candela-per-lumen (cd/lm) figure. With all these factors considered, designers can come up with more realistic assessments of lighting effects and “how good they are” in terms of distribution and intensity.

Let’s consider a lens with perfect symmetry that delivers the maximum intensity of light at its center. Find the angle where the intensity of illumination is at 50% from the maximum value and you will get the FWHM. The FWHM can give us a good idea of how narrow the beam is. So this does not get too confusing, some suppliers provide the angle wherein the intensity is at 10% of the maximum. This is so designers and light engineers can have a good idea of how much light strays from the main beam. By comparing the difference in intensity between the FWHM and 10% angle, we can easily see how concentrated and narrow the produced beam of light.