Absorption vs. Interference Filters

Many light sources produce light that covers a wide range of wavelengths. While this wide wavelength spectrum may be useful for some lighting applications, some applications require light that has a restricted visible spectrum. Restriction of unwanted light wavelengths can easily be accomplished by the use of specialized light filters.

These filters, commonly referred to as optical filters, are made of materials such as glass, gelatin, or dyed plastic. They allow light to be selectively transmitted in a range of wavelengths. Filters are often constructed with treated, transparent glass or plastic to allow the transmission of some wavelengths while selectively reflecting or absorbing others.Since these filters work by either reflecting or absorbing unwanted light this is why optical filters are classified into absorption and interference filters.

Wondering what’s the difference between absorption and interference filters? Worry no more, as below is a detailed look at each of these filters.

Features of Absorption Filters

An absorptive filter blocks or absorbs unwanted wavelengths while selectively transmitting light of desired wavelengths or color range. These filters are often made of dyed glass, pigmented gelatin resins, or synthetic plastics. In addition, colloidal dyes and rare earth transition metals are also used in glass and plastic filters to enhance the absorption ability of the material.  Due to the materials used to manufacture these filters, they produce fluorescence.

The ability of absorption filters to transmit desired wavelengths is based on the amount of pigmentation or dye on the filter and also on the filter’s thickness. The quality of the glass or polymer used to produce the filter is also important, as it should provide uniformity of color and density on the entire optical surface of the filter.

Absorption filters are useful in applications that don’t need precise transmission wavelengths. They are also used to isolate broad bands of wavelengths and for applications that require the blockage of short wavelengths while transmitting longer ones.

Some common uses of absorption filters include:

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

Features of Interference Filters

An interference filter, unlike an absorptive filter, works by transmitting some wavelengths while rejecting others through reflection or destructive interference. Interference filters are also referred to as dichroic filters. The name dichroic comes from the Greek word dichros which means two colors. This is because these filters take on a two-tone effect as the filter appears as one color under transmitted light and another color under reflected light. The colors are often opposite of one another on the color wheel since the wavebands are mutually exclusive.

Dichroic filters are made of thin multilayered film coatings deposited on optical-grade glass. Modern interference filters have successive dielectric layers that are deposited on the optical glass or polymer surface. When light strikes the coatings on the filter, the film layers will magnify and transmit some of the wavelengths and reflect the undesired ones. The wavelengths transmitted or reflected are dependent on the filter’s bandpass, which is determined by the nature of the layered surface as well as the number of layers on the glass surface. Interference filters achieve their precise filtering due to the reflective cavities between the layers of the film.

Dichroic filters are often classified into the following categories

1. Longpass – These filters pass long wavelengths.
2. Shortpass– They pass short wavelengths.
3. Bandpass– A bandpass filter will pass broad bands of multiple wavelengths.
4. Notch filters– These filters have a narrow band notch effect.

Compared to absorption filters, dichroic filters are more suitable for applications that require precise transmission wavelengths. The applications of these filters include:

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

Pros and Cons of Absorption Filters

The primary advantage of absorption filters is that they are low-cost. Other advantages include:

• High stability makes them suitable for a wide range of operating conditions and climates.
• Are chemically resistant and able to withstand abrasions and scratches since they have dyes and absorb chemicals impregnated into the filter material.
• Provide uniform spectral characteristics.
• Easy to clean.

Disadvantages of absorption filters include:

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

Pros and Cons of Interference Filters

Interference filters, unlike absorptive filters that transmit over large wavelengths, can significantly narrow to specific wavelength bands. Other notable advantages of these filters are explained below:

• Suitable for high-power applications;
• Filters have hard coatings that make the color more durable, meaning that it can’t be bleached over time;
• Not as heat sensitive as absorption filters as they reflect, not absorb light which means they can be used with intense light sources;
• Do not produce fluorescence.

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

• Expensive;
• Care must be taken when cleaning these filters as they aren’t as durable as absorptive filters;
• Angle-dependent as the thin-film coatings are sensitive to the illumination incident angle;
• Produce polarized light at high incident angles;
• Humidity and thermal cycling often cause the coatings of these filters to separate from the glass.

What Is the Best Option?

When choosing between absorptive and interference filters, selection should be made based on the application in which the filter will be used. Other factors that one should consider when choosing an optical filter include:

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

Hopefully, now you’re well informed on the difference between absorption and interference filters. If you’re looking for a reliable supplier of absorption or interference filters, contact us. We are an optical manufacturing company that offers quality and cost-effective optical filters that are customized to your needs.