That's something that I've just realized - lol

Transparent Background but still a source of light for the scene... 

That was something I was exploiting a while back, something that is beneficial if you are rendering in back-, mid-, and foreground layers.  I was thinking about revisiting this feature.  Could you post a screencap of the settings? 

hborre posted at 10:53 PM Fri, 17 September 2021 - #4427650

That was something I was exploiting a while back, something that is beneficial if you are rendering in back-, mid-, and foreground layers.  I was thinking about revisiting this feature.  Could you post a screencap of the settings? 

The scale must be -1, 1, 1
You may control the rotation: the vertical axis is set using the second value, it is in radians: 0 is on the right, 1.57 towards the top of the screen, 3.14 towards your left, 4.7 towards the bottom of the screen, negative values accepted. Using HDRIHeaven's bitmap, I often get interesting effects using 2.2...
You may even control the vertical position: enter small positive values in the second position to lower the bitmap

Then, turn off the background object. It's obvious but from time to time, I forgot this... lol, or Afrodite Ohki's EnvSphere...

And check the "Transparent Background" on the Render Settings Dialog  box:

Thanks, I'll give it a try.

You can also do this with any object if you go to the object's properties and tick off Visible in Camera while keeping Visible in Raytracing on. Good for light effects :)

This panel should be used more often, too - I also like playing with the "Casts shadows" for things like placing a wall behind my subject while still casting all the light from HDRI without having the light be blocked by the wall.

You can also use it for reflections. This image has an invisible sphere for the reflection of the window and door but was rendered over a gray background.  I could have hidden the background to use it in Photoshop. And this works with both render engines.

Conversely, you can leave visible in camera checked but uncheck ray tracing to keep items out of reflections, like a vampire in a mirror. However, this might effect some shaders that need ray tracing.