Optical Filters: Absorption vs. Interference

Several light sources produce light across various wavelengths. While this wavelength spectrum can be useful in lighting applications, other applications require light with a restricted visible spectrum. Limiting light wavelengths can be readily accomplished using specialized light filters.

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These filters, known commonly as optical filters, comprise materials such as glass, gelatin, or dyed plastic. They allow light to be transmitted selectively across a range of different wavelengths. Filters are often made with treated, transparent glass or plastic to enable the transmission of some wavelengths while selectively reflecting or absorbing other wavelengths. Because optical filters work by either reflecting or absorbing undesirable light, they are classified into absorption and interference filters.

Therefore, it is key to be able to tell the difference between absorption and interference filters. In this article, we take a look at each of these filters.

Features of Absorption Filters

Undesirable light wavelengths are blocked or absorbed by an absorptive filter while selectively transmitting the light of desired wavelengths or color range. These filters are typically made of dyed glass, pigmented gelatin resins, or synthetic plastics. Rare earth transition metals and colloidal dyes are also found in glass and plastic filters to boost the material's absorption capabilities. As a result of the materials used to produce these filters, they also generate fluorescence.

The capacity of absorption filters to transmit desired wavelengths is predicated on the amount of pigmentation or dye the filter contains and its thickness. The quality of the glass or polymer from which the filter is made also plays an important role, as it should be able to deliver uniformity of color and density across the entire optical surface of the filter.

Absorption filters are advantageous in applications that do not necessitate precise transmission wavelengths. They can also be used to isolate broad bands of wavelengths and for applications that require the blockage of short wavelengths while transmitting longer ones.

Image Credit: Shanghai Optics

Some typical application uses of absorption filters include:

• Creation of special effects in the cinema industry.
• Fluorescence microscopy.
• Use on-camera lenses for photography applications.
• Use indicator signals or lights on airplanes, motorbikes, and boats.
• Use on traffic signs.

Features of Interference Filters

Unlike a porous filter, an interference filter transmits some wavelengths while dismissing others through reflection or destructive interference. Interference filters are also known as dichroic filters. The name dichroic derives from the Greek word dichros which translates as &#;two colors&#;. This name is given due to the fact these filters adopt a two-tone effect as under transmitted light the filter appears as one color and another color when exposed to reflected light. The colors are usually on opposite sides of the color wheel since the wavebands are mutually exclusive.

Dichroic filters comprise thin, multilayered film coatings deposited on optical-grade glass. Advanced interference filters have consecutive dielectric layers deposited on the optical glass or polymer surface. When light hits the filter&#;s coatings, the film layers magnify and transmit some desirable wavelengths while reflecting the undesired ones.

The transmitted and reflected wavelengths depend on the filter&#;s bandpass, which is influenced by the nature of the layered surface and the number of layers on the glass surface. The reflective cavities between the film layers allow interference filters to achieve their precise filtering goals.

Dichroic filters are typically categorized into the following classifications:

1. Longpass &#; Filters that pass long wavelengths.
2. Shortpass &#; Filters that pass short wavelengths.
3. Bandpass &#; A bandpass filter will pass broad bands across multiple wavelengths.
4. Notch filters &#; Filters that have a narrow band notch effect.

In contrast to absorption filters, dichroic filters are better suited to applications that demand precise transmission wavelengths. The applications of dichronic filters include:

Image Credit: Shanghai Optics

• Calorimetry
• Color separation in TV cameras
• Disease diagnosis
• Optical microscopy
• Specialized filtration for photography
• Spectral radiometry

Pros and Cons of Absorption Filters

The main benefit of absorption filters is that they are comparably inexpensive. Other advantages include:

• Chemically resistant with the ability to resist abrasions and scratches since they have dyes and absorb chemicals impregnated into the filter material.
• Easy to clean.
• High stability makes them suitable for various operating conditions and climates.
• Provide uniform spectral characteristics.

Disadvantages of absorption filters include:

• A limited selection of glass available for filter applications
• Low peak transmittance values
• Longpass filter glasses are characterized by high autofluorescence
• Not well-suited to high-power applications
• Poor slope performance
• Performance is dependent on the filter thickness and the optical density of the filter material
• Sensitivity to heat, meaning the filter will heat up and deform if the light is too intense

Pros and Cons of Interference Filters

Interference filters can be distinguished from absorptive filters that transmit over large wavelengths, as they can narrow to specific wavelength bands. Other main benefits of these filters are outlined below:

• Do not produce fluorescence
• The filter's hard coatings make the color more durable, meaning that bleaching does not occur with prolonged use.
• They have low heat sensitivity compared to absorption filters, as they reflect rather than absorb light, which means they can be used with intense light sources.
• Suitable for high-power applications

Despite their advantages, dichroic filters do have a few disadvantages, as noted below:

• Angle-dependent as the thin-film coatings are sensitive to the illumination incident angle
• Expensive
• Humidity and thermal cycling often cause the coatings of these filters to separate from the glass
• Not as durable as absorptive filters, meaning care must be taken when cleaning and handling filters
• Produce polarized light at high incident angles

What Is the Best Option?

When deciding between absorptive and interference filters, the decision should be made based on the application of the filter. Other factors that should be taken into account when choosing an optical filter include:

• Cut-on and cut-off properties
• Operating environment
• The angle of incidence of incoming light
• The energy level of the incoming light
• The surface quality of the filter is typically expressed in terms of dig number and scratch
• Wavelength of interest

This information has been sourced, reviewed and adapted from materials provided by Shanghai Optics.

For more information on this source, please visit Shanghai Optics.

Light sources tend to produce light covering a range of different wavelengths, and while a wide wavelength spectrum may be beneficial in certain lighting applications, other applications necessitate the use of light with a restricted visible spectrum.

Specialized light filters are one of the main tools in restricting these unwanted light wavelengths.

These filters are widely known as optical filters. They are manufactured from a number of different materials, including glass, gelatin, or dyed plastic, and are designed to allow light to be selectively transmitted in a selection of specific wavelengths.

Filters are often made using treated, transparent glass or plastic in order to permit transmission of certain wavelengths while selectively absorbing or reflecting others. Optical filters are classified as either absorption or interference filters because these filters work by either absorbing or reflecting (interfering with) unwanted light.

This article outlines the key differences between absorption and interference filters.

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Features of Absorption Filters

An absorption filter blocks or absorbs unwanted wavelengths while simultaneously selectively transmitting light of a desired color range or wavelengths.

These filters are typically composed of pigmented gelatin resins, dyed glass, or synthetic plastics. Rare earth transition metals and colloidal dyes are occasionally employed in glass and plastic filters in order to improve the material&#;s absorption ability.

These filters produce fluorescence due to the materials used in their construction.

Absorption filters&#; capacity to transmit an application&#;s specific wavelengths is impacted by factors such as the amount of dye or pigmentation on the filter, its thickness, and the quality of glass or polymer used in its production. Material quality is especially important because it must be able to provide uniformity of color and density across the filter&#;s entire optical surface.

Absorption filters are valuable in scenarios where exact transmission wavelengths are not critical. They are employed to isolate wide bands of wavelengths and are particularly useful in applications that necessitate blocking shorter wavelengths while allowing longer wavelengths to pass through.

Image Credit: Shanghai Optics

Common uses of absorption filters include:

• Camera lenses in photography settings
• Special effects in the movie industry
• Fluorescence microscopy
• Traffic signs
• Lights or indicator signals on motorbikes, boats, and planes

Features of Interference Filters

Unlike an absorptive filter, an interference filter works by leveraging reflection or destructive interference to transmit certain wavelengths and reject others.

Interference filters are also referred to as dichroic filters. The name &#;dichroic&#; is derived from the Greek word &#;dichros,&#; meaning two colors.

Dichroic filters take on a two-tone effect, hence their name. These filters appear as one color under transmitted light and a different color under reflected light, with colors typically found opposite on the color wheel because the transmitted and reflected wavebands are mutually exclusive.

Dichroic filters consist of thin, multilayered film coatings applied to optical-grade glass. These modern interference filters feature successive dielectric layers deposited on surfaces of optical glass or polymers. When light hits these coatings, the film layers enhance and transmit certain wavelengths while reflecting unwanted ones.

The specific wavelengths that are transmitted or reflected depend on the filter&#;s bandpass, which is influenced by the characteristics of the layered surface and the number of layers present. The precise filtering capability of interference filters results from the reflective cavities formed between the film layers.

 Dichroic filters are typically classified into four categories:

• Longpass filters: These filters pass long wavelengths.
• Shortpass filters: These filters pass short wavelengths.
• Bandpass filters: These filters pass broad bands of multiple wavelengths.
• Notch filters: These filters exhibit a narrow band notch effect.

Dichroic filters are better suited to applications requiring precise transmission wavelengths, especially when compared to absorption filters. Common uses for dichroic filters include

Image Credit: Shanghai Optics

• Optical microscopy
• Color separation in film cameras
• Specialized filtration for photography
• Diagnosis of diseases
• Spectral radiometry
• Calorimetry

Advantages and Disadvantages of Absorption Filters

The key advantage of employing absorption filters is that they are extremely cost-effective. Other benefits include:

• High stability, meaning that these filters can be reliably employed in a wide range of climates and operating conditions.
• Chemical, abrasions, and scratch resistance, with the filters&#; dyes able to absorb chemicals impregnated into the filter material.
• Spectral characteristics remain uniform throughout the filter&#;s operation.
• Simple to clean, meaning these filters can be maintained with ease.

Disadvantages of absorption filters include:

• Sensitivity to heat - if the light is too intense, the filter&#;s temperature will increase and it will deform.
• The small selection of glass available limits filter application.
• These filters are unsuitable for high-power applications.
• It is only possible to achieve low peak transmittance values.
• High autofluorescence is a common issue with longpass filter glasses.
• The filters&#; slope performance is poor.
• Performance is highly dependent on filter thickness and the filter material&#;s optical density.

Advantages and Disadvantages of Interference Filters

While absorption filters transmit over large wavelengths, interference filters are able to narrow to highly specific wavelength bands. Other noteworthy benefits of these filters include:

• These filters can be successfully employed in high-power applications.
• Hard coatings on the filter make the color more durable, meaning that it will not become bleached over time.
• These filters are not as heat-sensitive as absorption filters because they reflect light rather than absorbing it. This means they can be used with intense light sources.
• These filters do not produce fluorescence.

Despite their many advantages, there are some disadvantages inherent to dichroic filters. These include:

• These filters are not as durable as absorption filters, meaning that extra care must be taken when cleaning or maintaining them.
• Dichroic filters are expensive.
• Thin-film coatings are sensitive to the illumination incident angle, making these filters highly angle-dependent.
• There is the potential for dichroic filters to produce polarized light at high incident angles.
• Humidity and thermal cycling are likely to lead to the coatings of these filters separating from the glass.

Choosing the Most Appropriate Filter Type

The choice between absorptive and interference filters should, first and foremost, be based on the application in which the filter will be employed. There are also a number of other pertinent factors when selecting an optical filter, including:

• The filter&#;s cut-on and cut-off properties
• The wavelength of interest
• The angle of incidence of incoming light
• The filter&#;s potential operating environment
• The energy level of any incoming light
• The filter&#;s surface quality, which is usually expressed in terms of dig number and scratch

Summary

This article explored the difference between absorption and interference filters, listing a number of common applications of each and investigating their respective advantages and disadvantages.

Shanghai Optics is a reliable supplier of both absorption or interference filters, specializing in quality and cost-effective optical filters customized to meet the needs of customers&#; applications.

Acknowledgments

Produced from materials originally authored by Shanghai Optics Inc.

This information has been sourced, reviewed and adapted from materials provided by Shanghai Optics.

For more information on this source, please visit Shanghai Optics.

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