The invention relates to a method for determining a distance between charged particle beamlets in a multi-beamlet exposure apparatus. The apparatus is provided with a sensor comprising a converter element for converting charged particle energy into light and a light sensitive detector provided with a two-dimensional pattern of beamlet blocking and non-blocking regions. The method comprises scanning a first beamlet over the pattern, receiving light generated by the converter element, and converting the received light into a first signal. Then the two-dimensional pattern and the first beamlet are moved relatively with respect to each other over a predetermined distance. Subsequently, the method comprises scanning a second beamlet over the pattern, receiving light generated by the converter element, and converting the received light into a second signal. Finally, the distance between the first beamlet and second beamlet is determined based on the first signal, the second signal and the predetermined distance.