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.
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:
- Longpass – Filters that pass long wavelengths.
- Shortpass – Filters that pass short wavelengths.
- Bandpass – A bandpass filter will pass broad bands across multiple wavelengths.
- 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.