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.
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.