Creating Lighting Effects With a Light Guide Bundle
The flexible multi-leg light guide bundle is an excellent way to create a variety of lighting effects. These guides are incredibly versatile and withstand the weight of multiple light sources.
A Light Guide Bundle consists of single or compact fiber bundles surrounding by sheathing which is mounted and glued in ferrules at either end. SCHOTT offers a wide range of sheathings and heavy duty end surface terminations to fit your specific application.
1. Optical value
A Light Guide Bundle is a bundle of optical fibers that are designed to collect and transmit light. This type of device is available in standard or custom configurations. It is usually terminated with a ferrule. Its performance specifications include wavelength, acceptance angle, bend radius, and numerical aperture.
The numerical aperture (NA) is a calculated, optical value that indicates how well the device collects light over a range of input angles. It equals the sine of the acceptance angle and depends on the cladding and core index of refraction.
Optical bundles come in many forms, from rigid plastic or glass to liquid and hybrid designs. SCHOTT Lighting and Imaging offers a wide variety of fiber types, sheathing materials, and bundle sizes to meet your application’s needs.
While the aforementioned optical bundles have their place, some engineers are opting for lighter, more flexible alternatives. One such alternative is a Liquid Guide Bundle, which combines the best of both worlds: high-quality light transmission and flexibility.
Another option is a Fused Fiber Bundle, which may include multiple layers of fusing to form an effective optical channel. The resulting bundle has an enhanced optical capacity and is especially useful when collecting light from a source that produces low-brightness radiation.
The best way to measure the optical value of a Light Guide Bundle is to consider all of its components and their performance. The optimum design uses each component to its maximum advantage. In addition, it uses the best possible material selection to maximize light collection and performance. The most important factor is the quality of the fiber used. It should be carefully chosen for its refraction index, and it should be engineered to have the correct bend and curvature for optimal performance.
2. Optical geometry
The optical geometry of a light guide is defined by the geometric configuration of entrance and output windows and by the shape of the channel’s cross section. Depending on the ray’s direction of entry and its angle of incidence, the channel’s cross section can have many curvatures ranging from straight to non-linear.
This radii of curvature of individual channels can also vary due to the entrance and output window locations, channel torsion and even natural or engineered channel randomisation which results in very large differences in the orientation of the channel’s cross section relative to its center. The resulting variations of the channel’s cross sectional shape can result in significant deviations of the light channel’s total internal reflection character, thus causing a loss of light transmission.
In order to maintain a high level of total internal reflection, the entrance angle imax of all rays should be at least equal to the critical angle P/2. The critical angle can be calculated by using the material index of refraction. For most plastics and glass this is approximately 42 degrees.
To preserve the total internal reflection character of a channel the entrance Light Guide Bundle angle imax is usually larger than this value. The effect of the larger entrance angle is that the ray is reflected by a large surface area of the channel’s cladding material.
Moreover, the larger entrance angle leads to an increase in the numerical aperture (NA) of the light channel. The larger NA of the light guide means that it can collect a large amount of light rays, thereby increasing the efficiency of the light guide.
When a light channel has a relatively high entrance angle, this causes the Fresnel reflection losses to rapidly increase at the input window of the light guide. This occurs because the entrance angle of a ray is inversely proportional to the bending radius of the corresponding cladding material and to the ray’s axis of rotation.
This results in an oblique and often steeper ray trajectory of the incoming rays. Hence, a large number of rays will enter the light guide in a narrowly focused direction which is parallel to the entrance window’s normal.
3. Optical sheathing
Optical sheathing is used to provide a high degree of uniformity and controllability when it comes to light transmission, especially for fibers with different indices of refraction. Sheathing is also useful for improving the thermal stability of optical fibers.
The core of an optical fiber may be made of quartz or a glass material, while the cladding may be made of a polymer material. This combination can have an index of refraction lower than the core, which improves the light acceptance angle and reduces thermal losses.
As an alternative to a core-cladding arrangement, a light guide bundle may be designed as a single, fused, or pressed component. This can have a variety of advantages, including higher strength, reduced weight and costs, and easier interconnection with other components.
In a preferred embodiment, the cores of individual fibers are fused or pressed into a solid bundle upon which additional cladding material may be applied. The intimate core contact section of the bundle avoids the disadvantages of prior art light guides that include spaces or voids between the cores of both cladding materials and cores of voids.
This leads to a better light guide fiber core contact, which is necessary for efficient optical performance. As an added benefit, the cladding material does not have to be as densely packed as the core, which can lead to further reductions in total losses due to absorption and scattering in the cladding materials.
Moreover, a better heat conductance between the cores in a cladded area is possible, which can further improve the temperature stability of the whole bundle. This can make the use of higher power levels and/or longer durations of operation much more feasible than in the case of a prior art light guide fiber bundle that has a densely packed cladding area, which leads to thermal stresses.
For this reason, a sleeve is provided with an end, which is inserted into the bundle of light guide fibers. The sleeve is filled with adhesive and the numeric aperture NA of the sleeve is greater than the numeric aperture NA of the light guide fiber bundle, thereby providing an optical termination that has only a small amount of loss.
4. Optical termination
When preparing a Light Guide Bundle, it is important to consider the optical termination. This is a critical aspect of the overall design and must be considered carefully because it will impact the efficiency and reliability of the system.
Optical terminations may vary significantly in their characteristics because of a number of factors. Some of these factors include the angular alignment and centricity of the fiber to the laser focus; the diameter of the laser focal axis; the shape of the input face of the fiber; and the distribution of rays from the light source.
These factors can make it difficult to create a termination that Light Guide Bundle is consistent in space and time. For example, the asymmetric distribution of the energy in a laser beam can make it difficult to accurately distinguish between desirable and unwanted energy.
Therefore, a proper optical termination must be designed to separate the different energy populations. It is particularly important to create a design that produces a demarcation between a high order population of rays and a low order population of rays, which can be achieved through the use of various techniques.
One of the most efficient ways to do this is to create a light guide bundle with a low interstitial area. This will allow a high acceptance angle and reduce the amount of reflection.
Another advantage of creating a Light Guide Bundle with a low interstitial area is that it will be more efficient at collecting light from lower-brightness sources. This is because of the smaller area that the rays are forced to travel through before they reach the output window.
In addition to reducing the amount of internal reflection, the low interstitial area also makes it possible to reduce the overall power loss at the light output window. This is because the total internal reflection margin angle g can be calculated using the equation b x c, where c is the critical angle of the ray that it is accepted by the fiber for internal reflection and g is the average internal reflection angle for all the rays in the light guide.
