This image processing system comprises: a device (PRJ, SI1) for projecting a first light beam (FL1) to form a first image (I1) on a screen (ECR) on which a second light beam (FL2) coming from an observation area (ZO) forms a second image (I2) and a device (CAM, SI1) for acquiring a third image (I3) formed on the screen (ECR) and corresponding to the superimposition of the second image (I2) and at least a portion (I1′) of the first image (I1). This system further comprises control means (CTR) for: obtaining a first signal (S1) representing the portion (I1′) of the first image (I1), obtaining a second signal (S2) representing the third image (I3), and calculating a third signal (S3) by subtracting at least part of the first signal (S1) from the second signal (S2) to form an image (I2′) representing the observation area (ZO).