Back to Articles
Light Pipes & TIR: The Combo for Optimum Performance
As you’re likely painfully aware, light pipes seem simple, but in all actuality, they are pretty complex. So many decisions that can impact how well (or not so well) a light pipe performs — from the brightness and viewing angle of the LED to the material and shape of the light pipe itself.
Why is it so difficult to shine light through a tube and have it exit exactly where you want it?
To get the light to behave precisely how you need it to, we first have to dive into some fun phenomena in physics that complicate the tasks at hand.
Challenge #1 – Understanding How Different Surfaces Impacts Your Illumination
Ever notice how lights shine differently through different materials?
When designing a light pipe, it will become obvious that a ray of light shifts in direction when shone from one surface to another, like through water to air. This phenomenon called refraction is the shift in direction of a wave of light. Refraction can take place whether it travels from one medium to another or if the original medium changes at a gradual level.
To ensure light goes through your light pipe’s surface in the appropriate direction and doesn’t appear bent or broken, the Refractive Index will need to be accounted for in your light pipe design.
The formula goes like this:
N = C/V
C is the speed of the light
V is the velocity of light in that medium (e.g., water, air, vacuum, acrylic, polycarbonate)
The Refractive Index demonstrates how much of the light is bent, or refracted when it hits your surface.
In a perfect world (aka a vacuum), the light breaks at a rate of 1.0. In the real world, the rates are slightly different:
Air = 1.003
Water = 1.33
Acrylic = ~ 1.49
Once you’ve calculated how the light will break or distort when shining through your medium, it’s time to figure out how to ensure the most light will end up at the exit point of your light pipe. Only there can your end-user view the light and react to whatever status it may be signaling.
To determine the best approach for making that happen, you’ll need to understand another phenomenon in physics…reflection.
Challenge #2: Sending light around corners… TIR to the rescue!
Reflecting light is essential when you want it to bend around corners in a light pipe. And the sweet spot for making that happen is called total internal reflection (TIR). Total internal reflection happens when the angle at which light is reflected when it shines through a surface is greater than a certain limiting angle known as the critical angle.
The maximum angle of refraction is 90 degrees. After that, the entire beam of light would be reflected rather than a portion of it being refracted.
TIR can only occur when the angle of incidence is larger than the critical angle. And the critical angle will vary depending on the media the light shines through.
Because many devices that use light pipes have rounded corners where the light needs to travel to the HMI, understanding how the critical angle and TIR work together is crucial for optimal light pipe design.
In the diagram above, you can see how refraction affects the ray of light as it travels from a water surface to an air surface. You can also see how leveraging TIR can help ensure the ray bounces back to the exit surface.
Most light pipes are made of either acrylic or polycarbonate. And of course, each material has its own critical angle.
Clear polycarbonate = 39
Acrylic = 42
These small degree shifts can add up to big differences in how your light pipe performs, so be sure to account for the specific properties of the light pipe material you’re using in your design.
The right-angle light pipe has a 45-degree reflection prism surface. Without proper calculations and preparations to achieve TIR, much of the light will be lost due to refraction instead of going around the curve to its final destination.
One way to prevent light loss is to use a light pipe that is curved to accommodate the critical angle, so all of the light reflects off the curve and shines exactly where it’s supposed to. The thickness and bend radius of the light pipe also come into play. VCC’s team of experts can help you calculate the precise angles and specifications based on your light pipe’s properties.
Reflection and TIR are just a few factors that go into effective light pipe design.
