Abstract: Optical filters selectively transmit light in specific ranges of wavelengths while blocking the remainder. They are crucial in a plethora of applications from everyday photography to advanced scientific research. This paper reviews the fundamentals of optical filters, the main types, and their principal applications.
1. 1. Introduction
Optical filters manipulate the spectral properties of light. They can selectively transmit, reflect, or block light of various wavelengths. They have been employed for over a century in photography and are commonly used in microscopy, spectroscopy, chemical analysis, machine vision, astronomy, telecommunication, and medical imaging.
2. 2. Clarifying Key Terminology for Optical Filters: Even though optical filters have many of the same specifications with other optical components, it's essential to understand specific terminologies unique to this field. These terms can help you to determine the most suitable filter for your application.
· 2.1 Central Wavelength (CWL):
This term is associated with bandpass filters. The CWL refers to the midpoint in the spectrum where the filter transmits. When you encounter Traditional Coated Optical Filters, expect the highest transmission around the CWL. On the other hand, Hard Coated Optical Filters typically have a consistent transmission across the entire bandwidth.
· 2.2 Bandwidth:
Think of bandwidth as the 'window' of the spectrum that a filter allows light to pass through. It is usually defined as FWHM (Full Width-Half Maximum).
· 2.3 Full Width-Half Maximum
FWHM is a more specific measure of bandwidth. It describes the width of the spectrum over which a filter transmits. To pinpoint the FWHM, identify where the filter achieves half of its maximum transmission. For clarity, if a filter has its peak transmission at 90%, the FWHM is defined between the wavelengths where it reaches 45% transmission. A quick guide:
o FWHM ≤ 10nm: These are narrowband filters, suitable for tasks like laser purification and chemical sensing.
o FWHM between 25-50nm: Ideal for machine vision.
o FWHM > 50nm: These are broadband filters, common in fluorescence microscopy.
2.4 Blocking Range:
It's the spectral portion that a filter intentionally weakens or reduces transmitted light. The efficiency of this blocking is often measured using a term called "optical density".
Primarily related to edge filters (e.g., shortpass or longpass filters), the slope describes how swiftly a filter transitions from blocking light to allowing it through. It's defined concerning the cut-wavelength percentage. For instance, for a 500nm longpass filter with a slope of 1%, the transition from 10% to 80% transmission occurs over a 5nm bandwidth.
2.6 Optical Density (OD):
OD quantifies how effectively a filter can block or reduce light. A higher OD signifies lesser transmission, and vice-versa. To put it in context:
o OD ≥ 6: Suited for high blocking needs, like in Raman spectroscopy or fluorescence microscopy.
o OD between 3.0-4.0: Ideal for laser filtering, machine vision, and chemical detection.
o OD ≤ 2.0: Best for applications like color differentiation and spectral order separation.
Understanding these terms can immensely help in making informed decisions about choosing and using optical filters in various applications.
3. 3. Basics of Optical Filters
An optical filter’s function is based on interference, absorption, transmission, or any combination. Their performance metrics often include:
4. Types of Optical Filters
Optical filters, with their varied designs, are pivotal in a multitude of technical applications. As photonics and optical technologies continue to evolve, the importance and complexity of optical filters will only increase.