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Explanation of Terms Related to Filter Parameters (Part 1)

Optical filters are optical devices used to select specific wavelength bands of radiation. They can selectively transmit a portion of the spectrum while blocking the transmission of the rest. Optical filters are commonly used in microscopes, spectroscopy, chemical analysis, and machine vision.

Transmittance (T): Suppose the initial light intensity is 100%. After passing through a filter, some light is lost. If the measured transmittance after filtering is only 80% of the initial value, then we say that the optical transmittance of this filter is only 80%.

Center Wavelength (CWL): The center wavelength used to define a bandpass filter is the midpoint of the spectral bandwidth over which the filter transmits. Specifically, it refers to the wavelength actually used in the filter’s practical application—for example, if the main peak of a light source is an 800 nm LED, then the required center wavelength would be 800 nm.

Full Width at Half Maximum (FWHM): The full name is “full width at half maxima,” often abbreviated as FWHM. It is used to characterize the bandwidth of the spectral range transmitted by a bandpass filter. Specifically, the upper and lower limits of this bandwidth are defined by the wavelengths at which the filter’s transmission reaches 50% of its maximum value. For example, if a filter has a maximum transmission of 90%, the wavelengths at which the filter’s transmission drops to 45% will define the upper and lower limits of the FWHM. An FWHM of 10 nm or less is considered narrowband and is commonly used in laser purification and chemical sensing applications. An FWHM between 25 and 50 nm is typically employed in machine vision applications; an FWHM exceeding 50 nm is regarded as broadband and is frequently used in fluorescence microscopy applications.

Explanation of Related Terms for Filter Parameters (Part 2)

Fluorescence filter – Fluorescence imaging filter

Explanation of Related Terms for Filter Parameters (Part 2)

Fluorescence filter – Fluorescence imaging filter

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