Subject open for debate – my reply is for Bryce 4:
Ranged Falloff causes the light to change abruptly from full intensity to zero (no light) after a defined distance (range) based on Bryce units.
(Units are arbitrary - you can consider them feet, inches, centimeters, whatever you wish)
This range can be confusing because '0' appears to be full intensity light while 100 seems to be no light!
What this means is that when a ranged light is set at 0 (zero) it will behave as if its 0 <zero> feet, inches, etc, from an object.
Maximum light range = 0 falloff.
The light source is zero feet from the object if you will.
When range is set at 100, it is infinitely far away.(in Bryce 4)
Absolute distance away = no light can be seen.
Let's say you create a default terrain, which has a high point ('Y' Axis) of 20.48 units.
If you drop in a default radial light, which is also 20.48 units in size by the way, it will be placed at 30.72 'high' directly above the terrain.
The range difference between the two objects is 10.24 units. Right?
When you change to radial light setting to a ranged falloff of 90 (the 10 unit difference between the two objects) you will notice that only the top of the terrain is highlighted.
At 80 it illuminates approximately 1/2 to 2/3 of the terrain.
At about 70, the complete distance between the light and terrain bottom (30.72 units) is illuminated.
The brightness does not change.
As you decrease the falloff, the distance between the light and object is decreased.
So, as you slide down from 90, 80, 70, 60, 50. . .more light is hitting the object (terrain in this case).
Ummm, as for faster rendering time, I don't know about that.
Ranged falloff lights can be as slow as volumetric lights because of how they interact with materials and scene in general.
In fact they can be slower depending upon how they are used.
If you use several ranged lights the render time can increase exponentially.
'Fast' lights are linear and squared falloff with shadows disabled.
I used to get falloff reversed, until I did a waterfall scene (with no water - falloff lighting can do some amazing things) using about 80 ranged falloff lights.
It was done as an exercise to see how the light reacted to terrain mats.
Only then did I notice the constants are reversed logically.
0 = all light and 100 = no light.
Very confusing at first!
I have 2 tutorials at 3dcommune using falloff for sky effects and show how to make the waterless waterfall. Hahahahaa...sounds stupid but it works.
The hardest, and most aggravating, thing you will probably run into is placement of the lights.
Make sure you are familiar with Anchor based alignment if you want to avoid headaches.
My head still hurts!
Keep the light(s) grouped with the object especially if it's far into the scene.
One little wrong move and > boom, boom out go da lights.
In Bryce 4 you can find the light settings by selecting then clicking [E] next to the light.
There is a small drop-down arrow at the bottom right corner of the Edit Lights Preview where you can select types and properties (shadows, disable shadows, surface, volume or infinite, visibility) of light as well as apply Pict and Texture (preset mat) gels.
Here's B4 light edit:
