Collimate Light from an LED | Thorlabs Insights 

@ronaldnonnenmacher6768 Choosing a lower-NA lens can be (and is) used to increase the fraction of optical power delivered from a light source to a distant target area. However, a positive result is not guaranteed, since lower-NA lenses often collect less light from the source. This is a particular concern in the case of highly divergent sources like LEDs. The basic approach to determining whether a lower-NA lens will be beneficial starts by estimating the minimum optical intensity required at the photosensor. Sensor parameters like active area and responsivity must be considered. Then the availability of lower-NA lenses must be considered, and their abilities to collect acceptable amounts of source light determined. The amount of light a lens collects depends on parameters including the source’s viewing angle (i.e. beam spread angle) and the lens’ NA. If a lens is found capable of collecting enough light, the divergence equation included in the video can be used to estimate the beam intensity at the detector. That intensity is then compared with the minimum intensity required by the photodetector to see if there is benefit. Note that the beam divergence after the lens depends on the size of the source, as well as whether the source is incoherent (e.g. an LED) or coherent (e.g. a laser).

@user-cf5sm9cn6v The bi-convex lens shape can collimate light from an LED but the collimated beam will include more aberrations (particularly spherical) compared to other lens shapes. Whether this collimated beam is suitable depends on your particular application. Typically bi-convex lenses are used for imaging applications where both surfaces are used to focus the light to a point in the image plane.

@ParTaban The collimated beam with the higher 0.76 NA lens collected more power and provided a larger irradiance at the measurement plane in the video compared to the collimated beam provided by the lower 0.24 NA lens, as you pointed out in the measurements. However, the collimated beam output from the higher NA lens also has a larger divergence, so the irradiance will reduce at a quicker rate as the measurement plane moves away from the collimating lens. This means that while the lower NA lens has a lower overall irradiance near the collimating lens, the lower divergence provides a more consistent irradiance as the collimated beam travels away from the lens.

Yes, the typical goal of collimating light is to provide an output beam with all the rays traveling parallel to one another. In an ideally collimated beam, the image of the light source would only be visible infinitely far away from the lens. In between the lens and infinity no image would be visible, because in the ideally collimated beam, rays from all points on the light source would overlap one another. However, in real world applications, it is typically not possible to move the image infinitely far away. This is because divergence separates the collimated ray bundles originating from different points on the light source. Due to this, one approach to collimating light from a relatively large source is to adjust the lens until the image is as far away as possible. When the viewable distance is too short to confirm that the image has been moved to the maximum possible distance, adjusting the lens to move the image past the farthest viewable distance is one way to push the expected image closer to its maximum possible distance.

@Tferdz Thanks for your suggestion! The setup and alignment of an optical tweezer system is a great topic. Do you use back-focal-plane interferometry when aligning with the scattered light? Are there certain aspects of your approach that you think are the most interesting?

@@thorlabs we use this setup 10.1007/978-1-4939-6421-5_7. We add some complexities by sending rotating linear polarized light so we can collect torque measurements. Besides that it's forward scattering with collection by a PSD for the XY and a photo detector for Z. The reason we separate is that we put an iris to block light for the z detector since it makes it more z-sensitive by removing xy cross talk. But it would be cool to have a video where we can show people at least some basic of optical tweezers, which is not that complicated.

@Tferdz This is really helpful, thanks! It is great to have a better understanding of the type of video that you would find useful to share, and we enjoyed learning more about your alignment approach!

@user-gz3sf7gy6z We are sorry that you had trouble understanding. Are there one or more topics that you would like us to explain in more detail in an upcoming video?

@@thorlabs I thank your efforts but I understood the subject.

@ahmedmostafa-tf2vo Yes, lenses like the aspheric condensers used in this demonstration are regularly used to collimate light from a variety of different light sources, including filament lamps. Achromatic lenses are another option, since they can provide better results for broad-spectrum sources, but it can be difficult to find high-NA achromatic lenses. However, in all cases, the divergence of the collimated beam will increase as the size of the emitter increases.

@@thorlabs what is the difference between aspheric and achromat? i know aspheres help with reduced spherical aberration and i believe achromat help with chromatic aberration, i am using a lens from my binocular made of two elements or doublet i think now is this an achromat and if so is it good for both aberration correction?

@ARCSTREAMS You are correct that aspheric lenses are designed to reduce spherical aberrations, which can be seen in light collimated from a point source on the optical axis, and that achromatic lenses reduce chromatic aberrations, which can be seen when trying to focus two different colors to the same point along the optical axis. The terms “aspheric” and “spherical” describe lens shape, while the term “achromatic” describes lens performance. Achromatic lenses are typically created by combining two different glasses, forming a doublet lens. It is possible for an achromatic lens to use aspheric lens shapes to help reduce both spherical and chromatic aberrations but spherical achromatic lenses are much more common. Spherical achromatic doublet lenses have been shown to have better spherical and chromatic aberration performance than spherical singlet lenses, as well as provide better broadband and off-axis performance than aspheric singlet lenses: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=120&tabname=Application . As for the difference between spherical and aspherical lenses - spherical lenses always have sides that are spherically shaped (i.e. have a constant radius of curvature), while aspheres have sides whose radius of curvature varies across the side of the lens. Spherical lenses are very common, since they are easier to grind and polish into shape. Aspheres are typically specifically designed to perform optimally in a particular system. If an asphere designed for one system is used in a different system, it may provide light with significant aberrations. If the aspheric lens has been designed to perform well when used with light of one wavelength, it may provide light with significant chromatic aberrations when used with light containing a range of wavelengths. Because of this, it is important to check the specifications of an aspheric lens before using it in a system.

@@thorlabs thank you for the info, so if i understood correctly aspheres are not typically made to correct chromatic aberration is this right? because a yt poster made this long range flashlight using a white led and an aspheric lens instead of using an achromat or doublet which makes more sense to correct for chromatic aberration rather than spherical aberration thus help improve his collimation and long throw better i would think since it would be the chromatic errors that would be significant at play here for his goal,, as for me i built one using the objective lens of my binocular which i believe is an achromat or doublet

@ARCSTREAMS You are correct in thinking that aspheric lenses are typically not designed to correct chromatic aberration, although chromatic aspheric doublet lenses do exist. A benefit of the RU-vidr’s choice to use a high-NA aspheric condenser lens in a long-range flashlight includes maximizing the amount of light collected from the LED. In addition, these lenses are usually relatively inexpensive. When a lens is not an achromat, the effect of the chromatic aberrations are often visible as a rainbow along the beam's perimeter.

@JulianJXhori Thanks Julian! Good luck in your upcoming physics courses!