Wavelength conversion element, phosphor wheel, light source device, and projection display apparatus

A wavelength conversion element comprising:

• a first phosphor region including first phosphor particles;
• and
• a second phosphor region including second phosphor particles, wherein
• the first phosphor region is on the second phosphor region,
• the first phosphor particles have a peak emission of fluorescent light,
• the second phosphor particles have a peak emission of fluorescent light,
• the peak emission of the first phosphor particles is same in color as the peak emission of the second phosphor particles, and
• an average particle size of the first phosphor particles is larger than an average particle size of the second phosphor particles,
• the wavelength conversion element further comprising a third phosphor region including third phosphor particles, wherein
• the second phosphor region is between the first and the third phosphor regions,
• the third phosphor particles have a peak emission of fluorescent light,
• the peak emission of the third phosphor particles is same in color as the peak emissions of the first and the second phosphor particles, and
• an average particle size of the third phosphor particles is larger than the average particle size of the second phosphor particles.