Collimated Light Source

Collimated light sources emit light in a parallel beam which can be used in many areas of spectroscopy. A well-known example of collimated light is a laser, which produces collimated light of a single wavelength.

Producing a white collimated light source presents an interesting engineering challenge. By design lasers emit collimated, directed light, but most broadband light sources emit in all directions. To create a high quality collimated white light source requires the right combination of collimating optics, light sources and geometry.  

What Is Collimated Light?

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Collimated light requires all light to travel in a parallel beam, rather than in all directions as light normally does. Collimated light has minimal beam divergence, so the light will not deviate from the beam path over reasonably large distances.

Many techniques rely on collimated light such as ellipsometry, laser spectroscopy and reflectometry.

Creating a perfectly collimated beam of light i.e. one where the beam has a very low divergence can be tricky. This is particularly true for broadband (or white) light sources where a broad range of wavelengths are required.

Lasers

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Lasers inherently create collimated, monochromatic light with extremely low divergence. This makes them one of the most versatile tools used in modern technology.

Laser light is generated inside a small cavity, then emitted as a parallel beam. The fact that the beam only contains a single wavelength means it all follows the same path when passing through lenses, prisms and windows.

Collimated White Light

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This is not the case for broadband or multiwavelength sources (such as supercontinuum lasers), where wavelength dependent refraction effects in optics can result in separation of the component colours and can change the beam divergence.

Broadband white light can be generated using various light sources, such as a glowing filament or a phosphor-coated LED. The light that is emitted from these extended sources tends to radiate uniformly in all directions and does not travel in a parallel beam. To create a parallel beam, we need to collimate it. This is usually achieved using a collimating lens or a system of lenses.

However, the finite size of these light sources makes it difficult to collect all rays and make them travel in the same direction without a complex sequence of optical components.

These collimated white light sources are necessary for many applications:

• Microscopy applications: where additional optics are used to focus light.
• Machine vision applications: where parallel rays of light are used for inspection.
• To illuminate objects for image analysis software..

Collimation Optics For White Light

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Lenses

The simplest way to produce a collimated beam of white light is to use a point-like source and a traditional convex lens. If the source is placed at the focal point of the lens, all the rays that emerge will be parallel and collimated. While this might sound simple, it is difficult to create a true point-like source. Real light sources like glowing filaments and LEDs tend to be quite large and hence far from point like. This tends to result in beams that are divergent and far from collimated.

One approach to account for this is to scale the size of the lens so that its focal length is large and the filament or LED appears more point like in the optical system. However, this approach is impractical because it requires large lenses in comparison to the size of the light source to successfully collect all the emitted light. This is also difficult as a lens will have a slightly different focal lengths for each wavelength. Placing the point-like source at all these different focal points is clearly not possible.

Alternatively, you can shrink the size of a light source (through using an ultrabright LED) and use conventional (normal sized) lenses. This also has associated difficulties because the amount of light emitted scales with the size (or surface area) of the light source. So, while this approach results in better collimated light, the resultant beam tends to have a weaker intensity.

Creating a collimated LED light source is a balance of increasing light intensity with an acceptable degree of collimation and a reasonably sized lens. This is possible, but there will still be some beam divergence.

Parabolic Mirror/Spherical Mirror

Another way to create collimated light sources is by using a point light source and a reflector. The point source is placed at the focal point of a parabolic mirror. This indirectly produces a collimated light beam along the optical axis as the parabolic mirror reflects incoming light.

This method can be useful if you are using a harsh light source, and will produce light without introducing any spherical aberrations.

However, this often requires a larger set up, producing a large diameter beam. It is also not the most efficient way of producing collimated light. Moreover, these systems are quite sensitive and difficult to align so if the point source strays at all from this focal point, the collimation is poor. Finally, any surface roughness or defects on the mirror surface can cause scattering affecting the collimation. Therefore, this is not the most commonly used approach in most optics set up. However, it can be used within larger light sources to increase spatial uniformity.

Types of Light Sources

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To create a white-light collimated beam of a reasonable size for use in laboratory experiments, the light source needs to:

• Have or create a broad spectral emission in order to produce white light
• Ideally, have a high enough intensity to reduce lens size

Arc Lamp Sources

Xenon and other arc lamp sources create a smooth broadband output by sending a current through a plasma within a bulb. These produce a broad spectrum of white light that is well characterized and have a reasonable intensity. However, they are delicate bulbs which require frequent replacement and can be expensive.

Laser Pumped Light Sources

One light source used in collimated light sources are laser excited phosphors such as inorganic luminophore compounds based upon Yttrium Aluminium Garnet (YAG) and its derivatives. When these materials are excited with ultraviolet (UV) or blue light they fluoresce strongly, emitting light over a broad range of wavelengths.

For example, Cerium doped YAG (Ce:YAG) based phosphors emit a yellowish green light across the 475-700 nm wavelength range. When this is mixed with the blue light that is used to excite the phosphor a cold white light is obtained. For example, a blue emitting LED can be coated with a Ce: YAG based phosphor to create an intense white light.

These light sources emit over a broad range of wavelength with good spatial consistency and good temporal stability. The diagram below shows the consistent spectral output of this light source over a period of 180 minutes.

Combination LED Light Sources

LEDs produce light at a single wavelength, but they are very efficient and relatively cheap to produce. LEDs also have the benefit of having a long lifetime, require lower maintenance and have lower cost that other light sources. By combining multiple LEDs with mixing optics, you can create a collimated white light source.

However, this doesn’t produce as broad a spectrum as Xenon and these light sources are difficult to collimate as they cannot act like a true point source.

Supercontinuum Lasers

In supercontinuum lasers, a laser is pumped into a photonic crystal fiber which significantly broadens its spectral output. The crystal structure broadens the spectral output significantly. These lasers are often pulsed system, creating pulses of white light rather than a continuous wave.

These sources are excellent for producing white collimated light with very high intensity. However, these are very expensive, and many applications do not need this intensity. Furthermore, as Class 4 laser systems, supercontinuum lasers require strict laser safety protocols.

Further Resources

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