Formaldehyde is an important combustion intermediate, which is often used for the analysis of turbulent flames, internal combustion engines or droplet/spray ignition processes. Due to its high concentration in hydrocarbon fuel combustion, laser induced fluorescence (LIF) of formaldehyde can easily be excited (e.g. 339, 343, 353 and 355 nm) and measured (360–550 nm). However, it is often superimposed by background emission from other combustion intermediates as, e.g. polycyclic aromatic hydrocarbons (PAHs).We present formaldehyde LIF excitation spectra obtained with a XeF excimer laser in combination with a spectrometer and an ICCD camera in dependence of pressure (50 mbar to 20 bar). For formaldehyde LIF and background excitation, we selected the wavelengths 353.373 and 353.386 nm, respectively. LIF measurements at the background wavelength contain a rising formaldehyde contribution with increasing pressure. We present a novel method for the calculation of the real background. This method is applied to the ignition of single gas-to-liquid (GTL) diesel droplets. However, the method can also be applied to other fuels. In the case of a GTL diesel ignition experiment (628 K, 3 bar) the real background amounted to only 30% of the emission which was measured at the background excitation wavelength (353.386 nm).