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Considerations for Using Light Pipes to Improve HMI Design

Jun 05, 2023

In general, HMIs will possess three characteristics: easy to view, easy to understand, and intuitive.

For these reasons, lighted indicators (Figure 1) are commonly used to provide operators with the information needed to safely and effectively use the equipment. One reliable and efficient way to transmit light to these indicators is by using light pipes.

While there are various ways to configure lighted indicators for HMIs, light pipes (both flexible and rigid) are a very popular choice among engineers. There are several advantages to using them, but these advantages are based on careful consideration of critical factors such as light pipe geometry, the material used, viewing angle, operating environment, and others. When accounted for early in the design, such factors will make all the difference in the effectiveness of a lighted indicator for an HMI.

Light pipes are clear plastic rods (either flexible or rigid) used to efficiently transmit light from an LED (usually on a PCB) to a user interface with minimal loss of light intensity. However, light pipes, are just one option for providing an indicator for HMIs.

Other options include:

LED panel mount indicators are a unit consisting of an LED contained within a housing mounted directly on the HMI panel.

Typically, LED displays are intended for small, lightweight devices that are typically portable and include both illuminated seven-segment displays and capacitive touch LED displays.

As the name implies, circuit board indicators are LEDs mounted directly on the circuit board and offer easy assembly and good design flexibility.

Before going into detail on light pipes, it is important to differentiate light pipes, light guides, and light diffusers.

With that clarified, let's look at the two basic types of light pipes: flexible and rigid.

Flexible light pipes, such as those shown in Figure 2, are made from polycarbonate fiber and can transmit light around tight spaces and corners, and they do so with minimal loss of light intensity.

They also securely attach to the LED light source and carry the light a considerable distance (although longer distances will lead to greater energy loss). In addition, flexible light pipes give design engineers much more freedom and good control over the optics.

Rigid light pipes, such as those shown in Figure 3, are polymer rods used to transmit light when the distance from the PCB LED light source to the control panel is relatively short.

In addition to transmitting light along a straight line, there are also custom geometries that will allow light to be transmitted at other angles. However, the light will only follow these non-linear rigid pipes to a certain point.

Benefits of rigid light pipes include a lower cost than their flexible counterparts; drawbacks are constraints on the length and less design flexibility.

Depending on the design of the device, the LED chosen, the material used for the light pipe, and its design (which will be discussed in the next section), light pipes can offer several advantages when used for HMI indicators, which include:

Light pipes can be used for an extremely wide range of HMI applications. They are commonly used as critical indicator lights on machinery where the viewing angle and color are vital to correct operation and the safety of operators. Critical design options for HMI light indicators include factors such as the LED color, color schemes, and viewing characteristics, just to name a few.

Light pipes are cost-efficient and, when designed correctly, they provide a reliable means of transmitting light to the user very effectively.

Light bleed (or light loss) occurs when light escapes somewhere other than the desired exit point. Light bleed can lead to several problems, such as a glow on the interface where there should not be, LED colors shining over each other or mixing, and (probably the most dangerous for HMIs) false readings.

A combination of the right LED, a well-designed light pipe, and an optimized distance between the LED and the indicator point can minimize the possibility of light bleed.

Carefully designed light pipes can also achieve highly effective signaling even in difficult environments when the most effective type of LED is combined with options for preventing light bleed and careful attention is given to the choice of light pipe and its material. This, in turn, leads to enhanced safety and improved operator response times because they can clearly see the indicator and recognize its meaning.

Finally, ease of installation can be achieved with certain options, such as a low-profile diffused light pipe with a press fit.

How is an effective light pipe designed that can take advantage of the potential benefits just discussed? There are several important aspects to consider when selecting a light pipe design, and the earlier these factors are addressed, the more effective the design will be.

What may seem like a simple choice, such as whether a narrow LED is chosen, can trigger other factors that can seriously constrain design choices or severely compromise the performance of the light pipe.

The type of light pipe, its geometry, and the material used is extremely important in the design of an effective HMI lighted indicator.

Refraction is a major factor in light pipe geometry, as are the concepts of reflection and the surface area available for the light to reflect off as it continues its journey through the light pipe. In some instances, the light pipe exterior surface may need to be painted in order to prevent light from escaping.

This is also part of the reason why the choice of light pipe material is extremely important: the material will affect how the light reflects and refracts.

Typical materials used include:

Note that the choice of material is also influenced by the operating environment.

The color scheme that will be used in the HMI is also equally important. Down below, Figure 4 shows a standard color scheme for industrial equipment, and similar schemes and icons exist for other types of equipment and interfaces.

For safety and effectiveness, questions must be asked about the viewing angle, starting with where operators or users will usually be located relative to the interface. For example, will they be sitting near the equipment, holding it in their hand, standing on a scaffolding, or perhaps lying on the ground?

The desired level of illumination intensity and the uniformity of illumination are factors for the effective use of a light pipe in an HMI, as illustrated in this case study for a medical device. And keep in mind that illumination does not depend on just the light pipe used but also on the type of LED selected for the indicator lights and its brightness.

There are distinct categories of PCB-mounted LEDs that can be used with light pipes. Surface mount (SMD) LEDs, as the name implies, are one of the most commonly used LED types for light pipes and can work with both flexible and rigid light pipe options. Through-hole LEDs are older mounting options and require the use of a wave solder machine for installation on the PCB board.

There are also right-angle options for both SMD and through-hole LEDs. As to which LED mounting type is best, the answer lies in the engineering design requirements.

The type of surface used with the light pipe also has a major impact on the usability of a light pipe design. Available surfaces include:

The environment where the light pipes will be used significantly impacts the design, often becoming the deciding factor for what type of material to use.

For example, environments that are outdoors or involve high moisture levels require a more ruggedized light pipe, such as those with IP (Ingress Protection) or NEMA (National Electrical Manufacturers Association) ratings that are in alignment with a designer's needs.

Environmental temperature is also important, including the storage and operating temperature, as it will become a major factor in the type of material used.

If the HMI is used in a high-vibration environment or the equipment it is on will be subject to impact loads, that must be taken into account early in the design phase. Such operating environment issues can be mitigated through mechanical solutions, light pipe accessories, or additional mounting hardware.

Early during the design phase, it is also important to establish the distance from the source LED to the display indicator and whether that distance is a straight line (and if it is not, what angle is involved).

Both factors will constrain the type of light pipe used as well as other characteristics. Keep in mind that the shorter and straighter the distance from the LED to the indicator position on the HMI, the easier it will be to achieve a highly efficient transfer of light.

On the other hand, longer distances may require the use of higher intensity LEDs or light pipe accessories to achieve good results. The angles involved also highly constrain the type of light pipe, its geometry, and the materials used.

Light pipes are a solution for HMI indicator lights, enhancing the ability to view, understand, and quickly interpret their meaning. There are a massive number of standard light pipe product offerings to meet most needs, and these choices and combinations can quickly become overwhelming.

VCC is an option that provides a range of HMI light indicator solutions. Because of the often-complicated nature of light pipe design, they offer support to navigate the many options and characteristics, even on a short timeline.

Some of the light pipe design services they include are:

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Figure 1 Light pipes Light diffusers Figure 2 Figure 3. Figure 4. Flat surfaces Domed surfaces Semi-domed surfaces Fresnel lenses