Projection system and light tunnel thereof

A projection system, comprising:

• a light source;
• a color wheel assembly disposed on one side of the light source;
• and
• a light tunnel disposed between the light source and the color wheel assembly or disposed on one side of the color wheel assembly opposite to the light source, the light tunnel having a first optical element, a second optical element a third optical element and a fourth optical element, wherein the first optical element has a first recess;
• the second optical element is disposed opposite to the first optical element and has a second recess;
• the third optical element connects the first optical element with the second optical element and has one end disposed in the first recess or the second recess;
• the fourth optical element is disposed opposite to the third optical element, connects the first optical element with the second optical eiement, and has one end disposed in the first recess or the second recess;
• and inner surfaces of the first optical element, the second optical element, the third optical element and the fourth optical element form a light passage the first recess has a concave portion disposed adjacent to the end of the third optical element or the fourth optical element.