I think the thread title is pretty self-explanatory. Is the raytrace preview window available in PP2014 accurate for using BB's Light Meter, or do I need a full blown render?

But bagginsbill will tell in his time you are out there BB ...right?

Looks like I answered my own question. Turned off my lights and only left a fairly far off wall with ambient on, and the light meter still shows almost all red. Meaning it's not really adjusting as I would expect in raytrace preview.

The light meter does react to changes in lighting, but you're mistakenly expecting preview renders to look the same even though the rendering engine is different.

If you were to switch on "Progressive Mode" in render settings, with Pixel Samples = 3, you'd find your main render and preview render are almost identical.

Progressive mode is using a completely different set of rules for lighting.

Here is a scene lit ONLY by my Env Sphere, i.e. entirely by IDL.

The main render (left) is set to progressive mode and matches the preview render. (right)

I think it's still usable though. I put in two light meters, and I adjusted one of them for a readout in raytrace preview to produce about the same result as the standard one in render. 

For me, the preview was producing less reaction, so I increased the shader sensitivity in the center of the light meter. I changed the Diffuse_Value from .8 to .88 and this made the meter show in preview what the unmodified meter showed in main render.

Thanks, BB. Odd, though ... my light meter in Raytrace Preview is fully red, and the scene is in fact pretty dark.

Do you find the progressive render options use a fair amount more resources?

I don't use progressive render at all, so I don't know what resources it uses.

If Progressive Render uses a different lightning model, maybe the light meter does not need to be changed. If you want to do the final render with Progressive, your should use the unmodified light meter, IMHO. If you use Progressive as a preview for the normal render style, you would probably need a changed light meter.

I found Progressive too blurry for my taste, also the results were different from what I've expected. I use it as a preview, but even then I do seperate preview renders before doing a final.

After investigating it more, it's pretty clear that progressive mode leaves you hardly any control over a bunch of settings ... not likely I'll be using it.

Quote - Do you find the progressive render options use a fair amount more resources?

It will use less memory, but in most cases more time/CPU. It's doing everything by brute force ray tracing, so there is no irradiance or occlusion cache, no dicing into micropolygon grids, no shadow maps, etc. Hence there are no options for those features, as they're not needed. IDL is calculated with naive path tracing, so the quality of IDL depends directly on the number of primary rays (= pixel samples * pixel samples).

Thanks for the explanation, Stewer.