In fluorometry, which filter is used to select the excitation wavelength?

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In fluorometry, you must illuminate the sample with light of a specific excitation wavelength. To ensure only that excitation light reaches the sample, the excitation path uses a filter that shapes the light before it hits the sample—the primary filter. This filter selects a narrow band of wavelengths that matches the fluorophore’s excitation spectrum, so the molecule is efficiently excited without unwanted other wavelengths reaching the sample.

After excitation, the molecule emits light at longer wavelengths. An emission filter sits in the detector path to pass the emitted fluorescence while blocking remaining excitation light and other stray light, allowing accurate measurement of the emitted signal.

Secondary and tertiary filters aren’t typically used to define the excitation wavelength in standard fluorometry; they may appear in more complex setups for additional filtering, but they don’t perform the primary role of selecting the excitation wavelength.