In the field of optics, optical filters play a vital role. They can selectively transmit or block light in a specific wavelength range and are widely used in many fields such as photography, scientific research, and optical communications. In order to better understand and select optical filters, it is necessary to master their related parameters and terminology.
- Central Wavelength
The central wavelength refers to the wavelength of light corresponding to the highest transmittance of the filter, usually represented by λ₀. This is a key parameter that determines the spectral region where the filter mainly acts. For example, for a filter used in fluorescence microscopy, if its central wavelength is 550nm, it means that it has the best transmittance effect on excitation light or emission light near this wavelength, and can accurately separate the target fluorescence signal and eliminate the interference of other stray light, so that users can clearly observe the structure of biological samples with specific fluorescent labels.
- FHWM
Bandwidth refers to the wavelength range corresponding to a filter at a certain transmittance (such as half-width, the wavelength range where the transmittance is half of the maximum value), recorded as Δλ. Narrower bandwidth can achieve more precise spectral screening and is suitable for scenes with extremely high requirements for light wavelength accuracy, such as laser spectrum shaping, which requires precise limitation of a very narrow band of laser light to meet specific experimental or industrial processing requirements; while wider bandwidth allows more light of similar wavelengths to pass through, and is used in some lighting systems that are not so demanding on light purity and pay more attention to light flux, such as ordinary stage lighting color filters, which can create a wide color effect.
- Cut-off Wavelength
The cut-off wavelength is divided into long-wave cut-off wavelength (λ₁) and short-wave cut-off wavelength (λ₂). The long-wave cut-off wavelength refers to the wavelength above which the transmittance of light drops rapidly to a very low level, while the short-wave cut-off wavelength refers to the wavelength below which the transmittance of light drops sharply. Taking the filter in the UV-visible spectrophotometer as an example, in order to prevent stray light in the UV region from interfering with the measurement in the visible region, a filter with a short-wave cut-off wavelength of about 400nm is used to effectively block UV light with a wavelength less than 400nm, ensuring that the detector mainly receives visible light signals and improving measurement accuracy.
- Transmittance
Transmittance is an important indicator to measure the light transmission ability of the filter, which is represented by T. Its value is equal to the ratio of the transmitted light intensity to the incident light intensity (T = Iₜ/I₀× 100%). High transmittance means that more light can pass through the filter, which is particularly critical in optical systems such as astronomical observation telescopes. It minimizes light loss as much as possible, allowing weak celestial light to fully reach the detector and capture clearer celestial images; on the contrary, in protective glasses that need to block strong light, protect optical components or human eyes, the transmittance of specific harmful bands should be as low as possible. For example, the transmittance of welding goggles filters for ultraviolet rays and strong visible light must be controlled within a very small range to ensure the safety of operators.
- Peak Transmittance
Peak transmittance is the maximum transmittance value of the filter at the central wavelength, which reflects the optimal degree of light transmission performance of the filter at the ideal working wavelength. In the optical path of high-precision optical instruments such as spectrometers, the higher the peak transmittance of the configured filter, the stronger the instrument’s ability to identify and analyze weak spectral signals, and can accurately detect extremely subtle changes in the spectral characteristics of substances, helping researchers to accurately analyze the components of complex samples.
- Cut-off Depth
The cut-off depth indicates the degree to which light outside the cut-off wavelength is suppressed. It is generally measured in decibels (dB). The larger the value, the better the filter’s blocking effect on the light in the cut-off band. In the field of optical communications, filters in wavelength division multiplexing systems need to have a high cut-off depth to prevent optical signals of different channel wavelengths from crosstalking with each other and ensure high-speed and accurate data transmission. For example, for dense wavelength division multiplexing channels with a spacing of only a few nanometers, the cut-off depth of the filter is often required to reach 30dB or even higher to ensure that each channel transmits information independently and cleanly.
- Filter material and coating
The material basis of the filter determines some of its basic optical properties, and common ones include glass, quartz, plastic, etc. Glass material has low cost and easy processing, and is widely used in ordinary optical instruments; quartz has good UV transmittance and thermal stability, and is suitable for optical systems in UV and high temperature environments, such as filters in UV curing equipment. Coating is a key process to improve the performance of filters. By depositing multiple layers of thin film materials with different refractive indices on the substrate, the interference principle of light is used to accurately control the spectral characteristics of the filter, and accurate screening of specific wavelengths is achieved to meet the needs of complex and diverse optical applications.
Understanding the parameters and terminology of these optical filters, whether it is an optical engineer designing a precision optical system, a scientific researcher conducting cutting-edge experiments, or a photography enthusiast pursuing a perfect shooting effect, can select the most suitable optical filter according to actual needs, so that light can play the maximum efficiency in various fields.
