A hemispherical silicon solid immersion lens (SIL) was used to improve the spatial resolution of front-side thermal IR imaging in lock-in mode. The bottom of the SIL was coneshaped to reduce the footprint of the SIL to the size of the imaged region. Caused by the lock-in operation mode, the detection limit improves by 2-3 orders of magnitude, and scattered light does not limit the image contrast. By using this SIL in combination with an IR camera working in the 3-5 μm wavelength range, a spatial resolution of 1.4 μm was obtained for thermal IR imaging. An automatic SIL positioning facility was constructed to place the SIL exactly in the center of the imaged region and to easily remove it after the detailed investigation.