Introduction to Optical Filters
Optical filters are used in many optical systems because they have the ability to selectively transmit specific parts of the spectrum and block or slowly attenuate unwanted wavelengths. These filters play a vital role in applications such as microscopy, spectroscopy, chemical analysis, and machine vision. Optical filters come in different types and precision levels, each tailored to meet the needs of a specific application. Understanding the technology behind optical filters, their key specifications, and their different classifications is essential to choosing the best filter for a specific application.
Terms Related to Optical Filters
In this article, we will briefly introduce several parameters related to optical filters that play a decisive role in the product.
Center Wavelength (CWL)
The center wavelength (CWL) defines the midpoint of the spectral bandwidth of the light transmitted by the optical filter. Bandpass optical filters use CWL as the main specification, and traditional coated optical filters usually achieve maximum transmission at this wavelength. In contrast, hard-coated optical filters maintain a more uniform transmission profile within a defined spectral bandwidth, making them more stable and reliable in precision applications.
Bandwidth and Full Width at Half Maximum (FWHM)
The bandwidth of an optical filter refers to the specific wavelength range that incident light is allowed to pass through. This specification is often expressed as the full width at half maximum (FWHM), which quantifies the spectral region over which the transmission reaches 50% of the peak value. FWHM provides insight into the filtering characteristics, distinguishing between narrowband, standard, and broadband filters:
Narrowband filters (FWHM ≤ 10nm): Ideal for applications such as laser cleaning and chemical detection that require precise wavelength selection.
Standard bandpass filters (FWHM between 25nm and 50nm): Commonly used in machine vision systems and industrial imaging.
Broadband filters (FWHM > 50nm): Suitable for fluorescence microscopy and general optical illumination.
Blocking Range and Optical Density (OD)
The blocking range of an optical filter defines the spectral interval over which incident light is significantly attenuated. This blocking capability is quantitatively expressed in terms of optical density (OD), which measures the level of attenuation or suppression within the blocking spectrum. The relationship between OD and transmission is as follows:
Different OD values correspond to different levels of blocking efficiency:
OD ≥ 6: Used in applications requiring extreme blocking, such as Raman spectroscopy and fluorescence microscopy.
OD 3.0 – 4.0: Suitable for laser separation, machine vision, and chemical detection.
OD ≤ 2.0: Used in color sorting and spectral order separation.
Dichroic Filters
Dichroic filters are a special type of filter that selectively transmits certain wavelengths while reflecting or absorbing others. These filters work on the principle of interference, making them useful for efficient optical applications. Dichroic filters are commonly used for long-pass and short-pass filtering, allowing precise wavelength separation with minimal loss of transmitted light. Their ability to reflect unwanted wavelengths makes them ideal for fluorescence microscopy, beam splitting applications, and multi-wavelength imaging systems.
Conclusion
Our optical filters at MOK Optics are indispensable in scientific, industrial, and medical applications, providing precise control over wavelength selection and light transmission. Understanding fundamental parameters, including center wavelength, bandwidth, blocking range, optical density, and transition slope, is essential to selecting the most effective optical filter to meet specific needs. By leveraging the capabilities of advanced filter design, researchers and engineers can optimize the performance of their optical systems to achieve superior results. If you are interested, please contact us via the following email:
Email： info@mokoptics.com