How Does A Dichroic Filter Work?

Berry Mathew

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How Does A Dichroic Filter Work?

Numerous optical applications rely on dichroic filters for their function. For example, chromatography—the separation of colors—is a crucial concept in many industrial processes, helping in the manufacture of some pharmaceutical products. Chromatography relies heavily on using dichroic filters. This is just one of many uses for these color filters. 

How They Work

Dichroic filters, at least some forms of them, have been in existence for centuries, although most of their current applications were developed in the recent past. Their development is heavily grounded in numerous principles of physics. It’s crucial to understand some key terms for a better appreciation of how these color filters work. These include:

i)Refraction: This is the change in the speed or direction of light as it crosses from one medium to another. For example, light passing from the air through water will experience a redirection.

  1. ii) Refractive index: Also known as the index of refraction, this is an arbitrary figure that represents the extent to which a light ray bends during refraction. The speed of light in a vacuum is used as a reference point for determining the refractive index.

iii) Thermal resistance: This is a measure of the ease with which heat can pass through a material. 

  1. iv) Sublimation: This is when a substance changes directly from a solid into a gas.
  2. v) Melting point: This is the specific temperature at which a substance changes from solid to liquid. 
  3. vi) Wavelength: A wave is a disturbance. Wavelength is the distance between the high points (crests) of two successive waves.

vii) Incident ray: An incoming light ray that strikes a surface at a specific angle. 

It helps to think of dichroic filters as selective mirrors that allow only light of a specific wavelength to go through while keeping away the unwanted sections of the light spectrum. Dichroic filters work by creating tiny layers of varying refractive indexes. 

The refractive indexes are responsible for changing the speed of light when they go from one medium to another. A good example is when water and oil mix. Both of them have different refractive indexes. When the light goes through each of them, they bend the rays at slightly different angles, resulting in the characteristic rainbow effect.

Manufacturing a dichroic filter requires certain transparent materials. Such materials should also be able to reflect light at specific wavelengths. Thin layers of glass are joined with micro-layers of oxides of certain metals like titanium. The formed coating is applied onto a glass substrate. 

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How They Work

Interference with specific light wavelengths is a hallmark of dichroic filters. The micro layers that form its surface help to facilitate this interference.

The different micro layers coated onto the glass substrate to form a dichroic filter have different refractive indexes. 

When light goes through the dichroic filter at a specific angle, each of the micro layers that are coated on its surface will reflect some of that light. Each light ray reflected from the micro layers has a different path length. The differences in path length enable the reflected rays to interfere with the incident ray that led to the reflection in the first place. As a result of this, only specific wavelengths are reflected. The rest are transmitted. 

Angles of incident rays on a dichroic filter are crucial because the precise path length difference relies on it. This means that the effectiveness of dichroic filters depends on this. When the angle of incident rays is increased, the dichroic filter moves toward shorter wavelengths. This has implications for applications like spectral filtering—a process for eliminating data from images using their wavelength properties.

Manufacture Of Dichroic Filters

There are several methods of manufacturing dichroic filters. These include:

i)Plasma Deposition

  1. ii) Ion-assisted vacuum deposition

iii) Thermal Evaporation

  1. iv) Ion beam sputtering

  2. v) Ion plating

Ion beam sputtering, also known as ion beam deposition (IBD) involves using an ion beam to apply one material onto another. In the case of dichroic filters, this means the application of micro layers to a glass substrate. This process involves a low production rate, giving top-grade dichroic filters. This is why many manufacturers prefer it to other processes. 

A dichroic filter made using the ion beam sputtering method can produce unusually polished surfaces, being able to offer a reflectivity of over 99 percent. All this is achieved with almost negligible absorption and scatter losses. 

Common applications of dichroic filters include multispectral imaging, fluorescent microscopy, LCD projectors, and laser harmonic separators. For ultraviolet (UV) water purification, dichroic filters are also crucial. 

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