Holographic interferometry and finite element analysis have been applied to determine the strain in a large optical window installed in a high‐pressure chamber. Two different shapes of windows were chosen, and the outside and inside surface strains of the windows were followed as a function of internal pressure by taking double exposed holograms at 69‐bar intervals up to 483 bars. The results were compared with the values calculated by a standard finite element code with an assumption of no displacement at the load‐bearing edge. The agreement between two results was found to be good for both cases.