The BloomEffect simulates the camera viewing a very bright light. It causes brighter colors to glow, similar to applying the neon Material to everything, including the Sky. Multiple BloomEffect objects can be applied at once and they will compose their effects together.
Like other post-processing effects, BloomEffect will only work while Enabled and when parented to Lighting or Workspace.CurrentCamera. Also, it may render differently depending on your Studio settings (see the Quality Level settings in Rendering → Performance).
For more details on this effect and others, see Post-Processing Effects.
Intensity determines how intensely the colors that bloom (determined by the Threshold) will additively blend with themselves. Higher values will produce brighter colors.
Size determines the radius of the bloom effect in pixels in a similar manner to BlurEffect.Size. Larger values create a wider bloom effect, and a value of 0 will disable the bleed (but not the color adjustment).
Threshold determines how bright a color can be before it blooms. If set to 1, only pure white colors will bloom. If set to 0, all colors will bloom.