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When observing with the slitless Low Resolution Spectroscopic (LRS) mode of the MIRI instrument on board the James Webb Space Telescope (JWST), photons at wavelengths shorter than the nominal wavelength range (5 μm - 12 μm) can reach the detector. In this paper, we study the impact of such photons in terms of opportunity to get spectral information below the nominal cut on, and in terms of possible contamination of the nominal spectrum by those photons. First, we present new laboratory measurements done to characterize the transmission and dispersion of the LRS disperser element, a Double Prisms Assembly (DPA), in the 3-5 μm wavelength range. We confirm a transmission peak at the level of about 30% around 3.7 μm and validate the dispersion optical model. We use the information to simulate how the short wavelengths will impact the spectroscopic images obtained during slitless LRS observations of bright stars; we see a bright spot at the pixel corresponding to the 3.7 μm. Such a spot has indeed been seen in the spectral images obtained during the commissioning of MIRI in May-June 2022; the level of the observed spot is lower than predicted. Such a feature could be useful when observing exoplanets, as it can bring addition information on the exoplanet atmosphere. The effect of spectral contamination is difficult to assess precisely; but, according to the modelisation we did, it is low: at most 7 % in the 5 - 6 μm range and negligible at longer wavelengths.
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