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The diameter affects the “softness” of the edges (penumbra) of shadows cast by objects under the spotlight.  When the diameter is set to 0”, the light is emitted from a single point and the penumbra is a sharp line. As you increase the diameter, light is emitted from an area (globe?) of the specified size and the penumbra softens.

If there are no objects to cast shadows from the spotlight, you will not notice a difference between settings.

EDIT:

In the attached example, two spotlights shine straight down on a bar positioned above a surface.

The left spotlight has a 0” diameter and the right spotlight has a 2” diameter.

The edges of the light from the spotlight are the same.

The bar shadow cast by the left-hand spotlight is sharp, provides strong contrast/definition, but may not be ideal for those looking for photorealism.

The bar shadow cast by the right-hand spotlight is softer/less defined, but may be considered more realistic.

When using this setting, consider/use the light source you are attempting to imitate. For example, a standard incandescent bulb is approximately 2.5” in diameter.

The spotlight should be used where only a generic, directional lighting is required. IES light sources will provide better results. 

Cheers,

Andrew

Thanks Andrew—this is very helpful.  Is this sort of info anywhere in the Shaderlight documentation?

E.g. https://artvps.com/media/onlinehelpSU/ search “diameter” comes up with no results!

I’m glad it helped.

I took a look at the documentation and it appears to predate the diameter setting. So…“no” you will not find it in current documentation.

Andrew

Thanks, Andrew. Are you on the Shaderlight team?

No just a user/moderator.

If it helps you further, I wrote a guide for interior lighting.  The goo.gl link that is posted elsewhere in the site is about to expire, but this is the direct link.

https://drive.google.com/file/d/0B32DwESKy7fbVTk1LUd2cnlQRTA/view

Bravo!  I just downloaded it and will read it. I’m sure it would have saved me immense frustration.  Here is my first effort, and my final effort on a project I am now working on.

final

Beautiful!

Wonderful Render - Congratulations !  :-)
Thanks to both, Sjschloeder and Andrew - a great contribution here again since this entire artificial lighting thing certainly is not a strength of mine !!!  ;-)

Sjschloeder, although Andrew is not in the shaderlight team he should at least be appointed to a honorary member there long ago ;-)  ;-) deservedly!

One request to you Sjschloeder - could you please also show impressions of your SkUp model and describe in more detail how you have generated the lighting of your in fact wonderful work?

Thanks & Best