The actual temperature rise and time-rise constant of the mask membranes during x-ray irradiation was measured using metal resistors fabricated directly on the mask. The experimental results are compared with the calculated. A model of thermoelastic distortions of the printed image during x-ray beam scanning was developed. In addition to the shift of the features the model predicts blur of the edges. Experimental results for the thermal wave effects are presented, and indicate that the effects are noticeable only during operation in a vacuum environment. The effects are negligible during normal operation in a helium atmosphere. © 1990.