Rendering a semi transparent object on a transparent background I have a problem rendering a sphere with a semi-transparent material on a transparent background.
When rendered in preview mode, the result is correct, the invisible parts of the sphere don't show up :



But when I calculate the final render, I only get a totally transparent image. 
When the transparency box in the Render Properties is unchecked, it works fine : the problem only occurs when the background transparency is enabled.
For the material, I used a noise texture linked to an emission shader which I added to a transparent one :



Would you have any idea where it comes from ?
Your problem is that you are adding the shaders instead of mixing them. Therefore, the material is interpreted by Cycles as being 100% transparent, regardless of what else it is.

You can fix this by swapping out your Add Shader for a Mix Shader and plugging in your texture to the Fac value instead of the Strength of the Emission node.