Apparatus suitable for transforming a radiation beam so that it can exit with a predetermined alteration. The apparatus includes a first variably diffractive radiation element for transforming a characteristic of the radiation beam from an original state; a second radiation element juxtaposed to the first radiation element for transforming the same characteristic in a complementary way; and, means for physically deforming the first variably diffractive radiation element so that its diffracting pattern can change in a known way.