Circular polarization illumination device and plant growth regulation method

A plant growth regulation method comprising irradiating a plant with a source of light irradiation,

• wherein the source of light irradiation is a circular polarization illumination device,
• the circular polarization device comprising:
• a light-emitting light source;
• a reflective polarizing plate that transmits one of a plurality of polarization components of incident light from the light-emitting light source and reflects a different one of the plurality of polarization components, wherein said one of the plurality of polarization components is p-polarized light, and said different one of the plurality of polarization components is s-polarized light;
• a reirradiation unit for changing a polarization state of light reflected from the reflective polarizing plate to reirradiate the reflective polarizing plate with the light;
• and
• a circular polarization conversion unit for converting light having been transmitted through the reflective polarizing plate into circularly polarized light,
• wherein:
• the reflective polarizing plate is any one of a total reflection-type polarizer, a polarizer as a laminate of thin films having different types of birefringence, a wire grid-type polarizer, a thin film-type polarizer, and a birefrigent polarizer;
• the circular polarization conversion unit is a λ/4 wave plate, and
• plant growth is promoted by 20% or more if the plant is irradiated with right-circularly polarized light and plant growth is promoted by less than 20% if the plant is irradiated with left-circularly polarized light.