Introduction
Optical filters are indispensable core components in modern optical systems, selectively allowing light within a specific wavelength range to pass through while blocking unwanted spectral portions. Whether in microscopy, spectroscopy, chemical analysis, or machine vision, optical filters play a crucial role. MOK Optics offers a variety of optical filters with different filter types and precision levels to meet diverse needs from basic research to industrial applications. This article provides an in-depth look at the manufacturing technology, key specification definitions, and characteristics and applications of various optical filters.
Key Optical Filter Terminology Explained
To effectively select the right optical filter for your application, it’s essential to understand the following technical terms. These specifications will help you accurately evaluate filter performance and ensure a perfect match with system requirements.
Center Wavelength (CWL)
The center wavelength is primarily used to define bandpass filters, describing the midpoint of the spectral bandwidth where the filter achieves maximum transmission efficiency. Traditional coated optical filters typically exhibit peak transmission near the center wavelength, while filters using hard coating technology display a flatter transmission curve across the entire spectral bandwidth, providing a more uniform spectral response.
Bandwidth and Full Width at Half Maximum (FWHM)
Bandwidth refers to the range of wavelengths through which incident energy passes through a filter, usually expressed as Full Width at Half Maximum (FWHM). FWHM is calculated by defining the upper and lower limits of the bandwidth between the two wavelengths where the filter reaches 50% of its maximum transmittance. For example, if the filter’s maximum transmittance is 90%, the wavelength at which 45% transmittance is reached is used as the boundary of the FWHM.
Optical filters have different bandwidth requirements for different applications
10nm or below (narrow band): Suitable for applications requiring precise wavelength selection, such as laser purification and chemical detection.
25-50nm (mid frequency): Widely used in machine vision systems.
Above 50nm (wide band): Commonly used in scenarios requiring a wider spectral range, such as fluorescence microscopy.
Blocking Range and Optical Density (OD): The blocking range describes the spectral region where the filter effectively attenuates energy. Its blocking effect is usually quantified by optical density. A higher optical density value indicates lower transmittance and stronger blocking effect; conversely, a lower value indicates higher transmittance.
The conversion relationship between optical density and transmittance is as follows
Transmittance (%) = 10^(-OD) × 100%
OD = -log(Transmittance/100%)
Typical applications of different optical density values:
OD 6.0 or higher: Extreme blocking requirements, such as Raman spectroscopy, fluorescence microscopy
OD 3.0-4.0: Laser separation and purification, machine vision, chemical detection
OD 2.0 or lower: Color sorting, spectral order separation
Slope is an important specification of edge filters (such as short-pass or long-pass filters), describing the bandwidth required for the filter to transition from a high-cutoff state to a high-transmittance state. MOK Optics defines slope as the wavelength distance between the 10% transmittance point and the 80% transmittance point, usually expressed as a percentage of the cutoff wavelength. For example, a 500nm long-pass filter with a nominal slope of 1% will complete the transition from 10% to 80% transmittance within a 5nm bandwidth (1% of 500nm). The steeper the slope, the sharper the edge characteristics of the filter and the higher the wavelength selection accuracy.
Start Wavelength and Cutoff Wavelength
Start Wavelength (λcut-on): Used for long-pass filters, indicating the wavelength position where transmittance rises to 50%.
Cutoff Wavelength (λcut-off): Used for short-pass filters, indicating the wavelength position where transmittance drops to 50%.
These two parameters are key indicators for quickly evaluating the operating range of edge filters.
How Dichroic Filters Work
A dichroic filter is a type of filter whose transmission or reflection characteristics are determined by wavelength. Light within a specific wavelength range passes through the filter, while other wavelengths are reflected or absorbed. This characteristic makes dichroic filters ideal for both long-pass and short-pass applications, especially in optical systems where beams need to be separated by wavelength.
The performance of dichroic filters is highly dependent on the incident angle. When used at an angle deviating from the design angle, the transmittance and wavelength specifications of the filter will change: increasing the incident angle will shift the transmitted wavelength towards shorter wavelengths (blue direction); decreasing the angle will shift it towards longer wavelengths (red direction). Therefore, the installation angle of the filter must be strictly considered during system design.
Comparison of Absorption and Dichroic Filters
Optical filters can be divided into two main categories based on their working principle: absorption and dichroic. The core difference between the two lies not in which wavelengths are filtered, but in how the filtering is achieved.
Absorption Filters
Absorption filters achieve light blocking based on the absorption properties of the glass substrate. The blocked light is not reflected back into the system but is absorbed by the filter material itself. This characteristic is particularly valuable when there is excess light in the system causing noise, because absorption filters effectively eliminate unwanted light energy instead of reflecting it to other parts of the system and causing interference.
A major advantage of absorptive filters is their angle insensitivity—light can be incident from various angles, and the filter maintains stable transmission and absorption characteristics without requiring precise control of the incident angle.
Dicochromatic Filters
Dicochromatic filters achieve their filtering function by reflecting excess wavelengths and allowing the desired spectrum to pass through. This characteristic is extremely useful in certain applications because it allows light beams to be separated into two independent light sources based on wavelength.
The manufacture of dichroic filters involves alternately depositing multiple layers of materials with different refractive indices on a glass substrate, utilizing the interference effect of light to achieve wavelength selection. When light moves from a low-refractive-index material to a high-refractive-index material, reflection occurs; only light at specific angles and wavelengths can produce constructive interference and pass through the filter, while other light is reflected due to destructive interference.
Unlike absorptive filters, dichroic filters are extremely sensitive to the incident angle, and the angle must be strictly controlled during use to ensure compliance with specifications.
Optical Filter Manufacturing Technology
Conventional Coating vs. Hard Sputtering Coating
MOK Optics manufactures dichroic bandpass filters using two main technologies: conventional coating and hard sputtering (also known as ion-assisted deposition). Both technologies achieve unique transmittance and reflection characteristics by alternately depositing multiple layers of materials with high and low refractive indices on a glass substrate. Depending on the application, a single filter may have more than 100 layers deposited on each side.
Conventional Coated Filters: Layers of materials with different refractive indices are deposited on multiple substrates, which are then stacked together. This technique can result in thicker filters, and the overall transmittance may decrease due to cumulative absorption and reflection losses as incident light passes through multiple coating layers.
Hard Sputtering Coated Filters: Materials with different refractive indices are deposited on only a single substrate, forming a multilayer film using a precise ion-assisted deposition technique. This technology produces thinner filters with higher transmittance and better environmental stability and lifespan.
Common Optical Filter Types
MOK Optics offers a variety of optical filter types to meet different application needs:
Bandpass Filters: Allow a specific wavelength range to pass through, blocking wavelengths on either side.
Long-Pass Filters: Allow light above the cutoff wavelength to pass through.
Short-Pass Filters: Allow light below the cutoff wavelength to pass through.
Narrowband Filters: Extremely narrow bandwidth, suitable for laser applications.
Neutral Density Filters: Uniformly attenuate all wavelengths.
Diclonal Filters: Separate the beam according to wavelength.
Fluorescence Filters: Designed specifically for fluorescence microscopy.
Raman Filters: Ultra-steep edge filters for Raman spectroscopy.
Thermoreflective Filters: Reflect infrared light while allowing visible light to pass through.
Color Filters: Used for color separation and correction.
Conclusion
Choosing the right optical filter requires comprehensive consideration of multiple parameters such as center wavelength, bandwidth, optical density, and slope, and a trade-off between absorptive and dichroic types, and between traditional and hard coatings, depending on the application requirements. MOK Optics offers a comprehensive product line of optical filters and professional technical support to help customers find the most suitable filtering solutions. Whether for precision spectral analysis, industrial machine vision, or biomedical microscopy, MOK Optics can provide high-quality optical filters to meet your needs.