TL;DR version:
Go into LIGHTS -> Sunlight -> Caustic Photons and change the EMIT RADIUS there. The units are in your model units. And the center starts from the sun's location.

Full explanation:
Vray projects a beam with an 'Emit radius' when rendering caustics. Where that edge ends, you'll see a cut-off line.
In 3ds max you can set this emit radius' direction and size visually. Buuuht, not here.

In Sketchup the sun is the emit source, so depending on time of day(!) you'll have to fiddle with the emit radius.

The attached image has three swimming pools: at 34' from the origin of the blue axis (0,0,0), then again at 60' and again at 90'.

At 1:00pm the sun catches all the pools nicely, but the 2d and 3d pools aren't getting enough caustic photons to show caustics properly (top half of image), because the emit radius is set to 700, which doesn't extend far enough.

NOTE: The units of this model is inches, so that's 700 INCHES or 58.3'. The yellow circle has a 59' radius, and you can see the 'dark line' cutting through the pool.

The second render shows the emit radius set to 1080" (90'). This time the third pool is captured and begins to render caustics.

Tadah!

vray 3.6 sketchup causticss affected by emit radius setting under Sun->caustics menu

@ely862me said:

What software do you have ? Photoshop can help you with both. There is a Diffuse Glow Filter which you can play with and also a Lens flare effect under the Render filter.

And perhaps you want it done inside Vray, then, I am sorry for my misplaced answer.

This is the first image retouched with those filters in photoshop. If you have a well formed eye you will make it way better.

Thanks for the reply. I've never used the diffuse glow filter in PS, but I'll give it a shot. And yes, I was specifically referring to understanding how to adjust the Vray glare/bloom effects inside VfS.

And imabzeous, thanks for the link. I'm not really sure what to take from those images, and all the accompanying text is in a language I don't know... I know how to turn the effects on, and how to alter the weight, size, etc. That much is apparent from the sliders. What I'm trying to figure out is how to control the source of the effects, to limit them to the brightest areas of the image, and to perhaps set what that threshold is myself, if possible.

At the moment, I'm just working around this by saving the effects-heavy image and then raising color clarity in Adobe Camera Raw to cut down on the 'vaseline on the lens' appearance...

thanks, Andybot. I had been putting it in the wrong spot... in fact, you made me realize I've been tweaking the parameters on my fresnel reflections wrong, until now! No wonder it felt like such a guess & check process to get the results I wanted...

oy! Thanks, andybot. Much appreciated

I switched hardware acceleration off briefly and didn't notice the effect... but anyway, I started noticing a couple other weird graphical issues outside of SU, so I figured it was a graphics driver issue. I had already tried updating the driver, but this morning I did a full wipe and reinstall of it, and it seems to have worked. Thanks for your help, Dave.

you got an old version update to lastest and then leave the opacity 100% and tick "use color texture for transparency" in the diffuse slot, that's it. for proper shadow apply the png on both positive and negative faces.