up or down is it an illusion? Tate Modern 12 4 2018

This wasn't intended to be an optical illusion, but largely due to the way the handrail and the stairway's wall diverge, one gets the impression that the stairway is going down as it turns right. The steps themselves say otherwise.
(For the 'Time for Experiments' group) Whether this is an experiment is a moot point. It is the first time that I have produced something with an element of optical illusion to it - but it was by accident.