Substrate processing apparatus and method

A substrate processing apparatus comprising:

• a reaction chamber;
• a substrate rack for holding a plurality of substrates in the reaction chamber in a spaced apart relationship configured to rotate the substrate rack about an axis;
• and
• an illumination system constructed and arranged to irradiate ultraviolet radiation comprising a wavelength within a range from 100 to 500 nanometers onto a top surface of at least one of the substrates in the substrate rack from a side of the substrate rack;
• wherein a power controller is configured to at least one of switch off and limit power to a part of the illumination system to limit an amount of radiation hitting the substrate rack as it rotates around the axis.