Fig. 3. Two setups for varying a laser beam diameter.

a Constant-irradiance setup: 4f optical configuration enabling the setting of an illumination diameter D₁ at the sample location S by adjusting the diaphragm diameter D₂, according to the relation $D_{\mathrm{1}}{=D}_{\mathrm{2}}\cdot f_1{/f}_2$. Note that for the 4f configuration to be appropriately used, the optical power density impinging on the diaphragm D has to be uniform, not Gaussian. b Constant-power setup: optical configuration, similar to a when removing lens L₂, enabling the setting of an illumination diameter D₁ at the sample location S, by adjusting the displacement δx of L₁, according to the relation $D_1=\delta x\cdot D_2{/f}_1$. Note that L₁ can be the objective lens of a microscope