Features

There are some key, fundamental considerations when manufacturing optical components: designing molded optical parts, designing and building the molds to produce them, injection molding, and handling components that manage light. Some are obvious, others less so. But one thing is certain. It is not for the faint of heart, and is, thus, best left to the experts. Complexities, including geometry, surface specifications, and material selection, are vitally important to avoid potential pitfalls along the way to designing, and ultimately producing, a flawless optical part.

Fig. 1 – Assay test trays and vials used in pharma and pathology photo spectroscopy analysis.
Optical part design is complicated—not only the outside dimensions and surface finish, but the internal dimensions, structure, and type of clear material selected. Those factors will ultimately direct and manipulate light for the part’s intended purpose. And, unlike other injectionmolded parts, light energy is moving around inside and outside the part. Most people are familiar with concave and convex lenses, (like a magnifying glass) usually round, disc-shaped looking objects. Often these lenses are internal to microscopes and telescopes. But in reality, “plastic” optical parts, including testing trays, vials, light pipes, and guides, are infinitely more complex and can be manufactured in a way that ground, polished glass or crystal could never be.

Polymer optical components can be configured into complex irregular shapes that each perform a specific function. They allow light (usually LED or laser source, but even fluorescent or incandescent) to move from one location to another, or even multiple points. Examples of these parts are light pipes behind dash displays, radio, AC/heater controls found in cars, to medical test vials and trays.

Like the biology teacher you had in high school, the slides and petri dishes you most likely used in class have largely been replaced now. In their place are 3" × 5" 96-well assay test trays, typically used in pathology labs and by pharmaceutical companies. (See Figure 1)

Manufacturing these parts out of glass, with internal prism features that have surface finishes to angstroms (millionths of an inch), would be next to impossible.

Other optical components direct, amplify, reflect, focus, diffuse, or otherwise “manage” light.

Manufacturing parts with Fresnel, micro prism structures like those found on headlights and taillights in cars for example, to pillow lens optical features— and most everything in between—can be individually problematic, each with its own challenges, but it can be done. However, it is necessary to design the part, design the mold, and mold the part to highly disciplined levels.

Component Design

Of primary consideration is: how will the part be used? Secondly, what are the outside and inside space constraints? Next, in what physical environment will the part be used? Will it be exposed to extreme temperatures, chemicals, moisture? Will it need to withstand vibration or impact? How will the part be used?

Fig. 2 – A light pipe, prism. and a test vial.
Usually, light will need a source (in the solar arena, it’s the sun, naturally) and an exit point or points. Hard, sharp edges, corners, and rough or textured surfaces, become exit, or light-loss, spots. Like electricity, light energy has a travel distance somewhat dependent upon the amount of energy, or in this case lumens or wattage going into it (vs. volts or amps).

Obviously, the further from the source, the brighter the light source will need to be. However, there are exceptions and contradictions. For example, if a magnifying lens feature were somehow incorporated into the part design, it might be possible to reduce the amount of light input, because it would now be amplifying light. Most optical design software programs take many of these factors into account.

It is critical to understand that surface finish, or “polish” per se, is not as important as the absolute dimension. This is particularly true with micro Fresnel structures. Light inside the part is not only going to exit at predesigned points in the geometry, but it will also reflect back into the cross-section of the part, and create a totally new light source that needs to be considered. (See Figure 2)

In some cases, this light may need to be dammed, or blackened out, to contain or manage it. So, while from one perspective, light design management may seem simple, it can be very complex, because it is necessary to shield the stray, reflective internal light, while also preventing external, extraneous light from randomly entering into the equation. Such is the case when the sun coming in from behind can wash out digital displays.

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