A radiation beam modulator (1; 500; 600) that can be employed for modulating a radiation beam (60) in a radiation system. The modulator (1; 500; 600) comprises a bath (590; 690) containing a radiation attenuating fluid (520; 620) and multiple low radiation attenuating bars (570; 670) immersed in the fluid (520; 620) to form an array of multiple columns (525; 625). A height adjuster (530; 630) is arranged for adjusting a respective level of immersion of the bars (570; 670) in the fluid (520; 620) thereby to adjust the heights of the fluid columns (525; 625) such that the columns (525; 625) collectively form a radiation beam modulating profile.