The glow of a lens flare is centered around the source object of the flare. The parameters on the Glow panel let you control each aspect of the glow.
Interface

- Size
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Specifies the diameter of the glow of the lens flare as a percentage of the overall size of the frame. This value is separate from the overall size value set in the Flare Properties. This parameter can be animated.
- Hue
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Specifies the gradation of color for the glow. Clicking the green arrow button lets you animate this control. This parameter can be animated.
- Hide Behind Geometry
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Places the glow behind the geometric forms.
- Gradients
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Use radial, circular, transparency, and size gradients. Glow gradients are subtler than flare gradients, because they are glowing an area larger than a pixel.