V-ray plane light diameter

[Streetwise]

After upgrading to 3.2, I just noticed that there's a controller on the plane light to make it appear circular. Does that produce the same volume of light at any point on the circle, or does the light fall off beyond the traditional rectangle?

[spyzor]

I think this is just to show the direction of the light. By default the light is emitted at 180°.

If you lower the direction then the same global amount of light will be directed to the central direction.
Therefore you will have more light in one direction.

(at least if it hasn't changed since 2.5)

[Zdravko.Keremidchiev]

The Disk light type works in the same fashion as the rectangular one.
Although it produces the same volume of light at any point on the circle there is a directional option which controls the disperse of its light in different directions creating fall off effect. You can decrease this effect by increasing the directional parameter value, just like in the rectangular light type.
Furthermore you can preview the dispersion range in the viewport by enabling the preview option.