These effects are seperate. The additional light coming out of the opening is created by the blue part of the spectrum of the LED hitting it again and being converted to more yellow-green light (it changes direction after the conversion, it’s scattered). This increases the intensity and also dramatically changes the tint (more yellow-green).
If you used a mirror (collar) that doesn’t reflect the blue part of the spectrum there wouldn’t be a large increase in Luminance. You would just cut off 75% of the LEDs total emitted light.