Sun, Sky and HDRI
From LuxRender Wiki
Sun, Sky & HDRI
In this tutorial I will cover, as succinctly as possible, the subject of using both the SUN-LAMP and an HDRI environment in the same scene using LuxBlend25 (Though the methodology will be similar for all exporters).
HDRI and Sun lamps
The common problem people face with using an HDRI environment and a complementary light source is that there is no defined exposure range and associated light power scale used when creating an HDRI image. This means that we frequently need to adjust the gain of the HDRI environment to match out Scene's light level.
The solution is to calibrate the HDR image's gain to appropriately match a Sun-Lamp(even if the complimentary light source in your final scene is not). This is because the default Sun-Lamp when set to "SUN ONLY" has a fixed output level that matches our own Sun's output: insuring that the light levels emitted by the HDRI will correctly match all physically accurate lights with in any scene.
First you will need to add a Sun-Lamp and a HEMI-LAMP into the scene. Setting the Sun-Lamp to "SUN ONLY".
It is important to note at this point that the positions of the Sun-Lamp and the Hemi-Lamp are inconsequential. This is because the position is ignored due to the fact that both the Hemi-Lamp and the Sun-Lamp are placed at "THE INFINITE" and so only their rotation is used to control their mapping or apparent location.
Now within the Hemi-Lamp's "OBJECT DATA" Tab you'll need to add your HDRI image. Making sure that you've selected the correct "MAP TYPE" as you go.
Next you will want to provide both the Sun-Lamp and the Hemi-Lamp with their own "LIGHTGROUP". To do this: Head over to the "Render Layers" tab, locate the "LuxRender Light Groups" panel and add two Light Groups naming one "HDRI" and the other "Sun".
Then you will need to go to the "OBJECT DATA" tab of the both Lights and set the appropriate LightGroup.
The Test Render
Once you have everything set-up as above Start a Test Render. Then in LuxRender under the "TONE MAPPING" widget make sure the "KERNEL" is set to Linear and adjust the Tone-mapping settings until the scene is lit to a reasonable level
Finding the Correct Gain
When the image is clear enough for you you to see most of the details (around 500S/p is usually enough) head on over to the "LIGHT GROUPS" sub-tab and adjust the HDRI group's Gain until it looks about right when compared to the Sun. Make a note of this value as you'll need it in the next step.
You might find that you need to increase the Gain passed the end of the slider - if that is the case follow the next step and then do another Test Render and repeat this step.
Setting the Calibration
Head back into Blender and hop over to the Hemi-Lamps "OBJECT DATA" tab again. This time Adjusting the Gain value to what you found worked best in the previous step.
That's all there is to it! Now that you've calibrated the HDRI for your scene: go a head and make light work of it - Excuse the pun.
And don't forget to share your work on the Forums and feel free to ask there about anything your unsure of.