​Collimation and Divergence, in Fiber Optics and Collimated LEDs

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Collimating the light of an optical fiber or LED is crucial for efficient light delivery and also improves the uniformity of the light when it hits the target. In a previous blog post we discussed the importance of matching the numerical aperture (NA) of a collimating lens to the NA of the optical fiber. This blog post will discuss the relationship between the size of the lens and the divergence angle of the collimated light.

When it comes to LEDs and large core multi-mode fiberoptics, close-to-perfect collimation is not possible. There will always be divergence. The angle of divergence depends on the ratio of the emitter size and the size of the lens. The smaller the emitter relative to the size of the lens, the lower the divergence will be. In other words, collimation can be improved by either using a larger lens or a smaller emitter.

A lens that is double the size has a doubled beam diameter but only half the divergence. This can have surprising outcomes. When the divergence angle is reduced by 50% the spot size is reduced to just ¼ of the original size. As a result a larger beam has a smaller and more intense spot of light than the smaller beam when used at a distance.

Regarding Fiberoptic Collimators:

So when choosing a collimator for your optical fiber, it is worth thinking about the working distance, and the divergence angles of the collimators in order to find the best one for the application. Sometime the larger collimator will be the best choice to get the smaller spot. And when you want to cover a larger area, the smaller collimator might be the best choice.

Prizmatix offers 2 sizes of collimator, half inch and one inch. The half inch collimator has NA=0.5 and the one inch collimator comes in NA=0.5 and 0.6.

The core diameter of the fiber also needs to be considered. A larger core fiber will increase the total power, but also increase the divergence. It is important to maximize the intensity of the light, which can not be increased by using a larger core. This is explained in my blog post about NA.

For more information about Prizmatix fiberoptic collimators and and explanation about how to calculate the divergence angles see https://www.prizmatix.com/optics/Collimator.htm

Regarding LEDs:

Prizmatix Mic-LEDs and UHP-T LEDs have adjustable collimating lenses installed. The level of collimation will depend on the size of the LED chip. Higher power LEDs tend to be larger and the result is a more divergent beam. This is ideal when covering a large area, such as with UHP-T-MP for microplate illumination. But when you need intense light in a small area, for example when fiber coupling or with a microscope, a smaller LED with lower total power but more intensity (mW/mm^2) is often a better choice. You can read about the different model and their various emitter sizes in this blog post, or you can contact us to help determine which model is best for your application.

For applications where lower divergence and high power is needed, our UHP-T2 models is a great choice. This 2 inch model has double the lens size, so the divergence angle is half of the regular UHP-T model, and the spot size is ¼. That means you have 4x more intense light than the UHP-T model at a distance.

Another new option is our new HB-LS05 high intensity source (not available online). With an extremely small 300um emitter, the light source offer extremely low divergence of just 13mRad. We also offer a non-collimated version that can be used as a point source for application such a Schlieren Imaging and shadowgraphy.

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