When you render an image, the light does not display a self-luminous surface (glow).
Symptom:
Issue: In the lighting fixture family, you specified an Emit from Shape of Circle or Rectangle. (See Define the Geometry of a Light Source.) In the lighting fixture parameters, you selected Emit Shape Visible in Rendering. (See Parameters for Lighting Fixtures and Light Sources.) Therefore, you expect the light to display a self-luminous surface in the rendered image. However, the luminous surface does not display because the Soft Shadows option is turned off.
Solution: Select the Soft Shadows option on the Render Quality Settings dialog. (See Define a Custom Render Quality and Advanced Render Quality Settings Reference.) Then render the image again.