Rectangle Light - Arrow Size

[andy_smith]

We all know that the direction of the arrow in a VRay rectangle light in Sketch-up shows the direction. That the main light rays will take, with the directionality value further influencing how much the light is scattered away from that direction.

I also know that the rectangle can be scaled in Sketchup to make it physically larger (wider and/or longer).

There are also U Size and V Size values which confuse me: changing them does not affect the size of the rectangle in the Sketchup interface so how do these settings and the scaled size of the rectangle affect things?

Finally, the scale tool in Sketchup is often used in video tutorials to “stretch” the arrow of a rectangle light object to make it longer. What affect does changing the arrow length have in the render?

For example, in many tutorials I see someone lighting a whole interior scene with a single large rectangle light positioned outside a window, pointing inwards. The rectangle may be 10 feet high and 12 feet wide but often the length of the arrow is left default (around 2 feet long I’d guess). And then they add an accent LED strip light under a cabinet in a kitchen in the same scene and they intentionally scale the arrow to make it very long. Why would they do this?

[konstantin_chaos]

We all know that the direction of the arrow in a VRay rectangle light in Sketch-up shows the direction. That the main light rays will take, with the directionality value further influencing how much the light is scattered away from that direction.

The "arrow" shows the light normal and the direction, this is correct.
The "directionality" parameter allows you to focus the light rays more towards the light normal direction.
The amount of light emitted remains the same even when the directionality is increased.
At the same time the light appears brighter since the same emission is focused over a smaller area.

I also know that the rectangle can be scaled in Sketchup to make it physically larger (wider and/or longer).

This is correct.
The SU scale will be treated by V-Ray as a transformation which allows for each light instance to be different in size.

There are also U Size and V Size values which confuse me: changing them does not affect the size of the rectangle in the Sketchup interface so how do these settings and the scaled size of the rectangle affect things?

The U and V sizes are shared between all light instances (between the light Component instances).
They allow you to change the light source size for all instances at the same time.
The viewport light widget will also change when you change those values.
The "internal" size is multiplied by the scale transformation and both give you a more flexible workflow and more control.

Finally, the scale tool in Sketchup is often used in video tutorials to “stretch” the arrow of a rectangle light object to make it longer. What affect does changing the arrow length have in the render?

Scaling a light along the normal vector will not affect the light emission or intensity.
The U and V sizes as well as the scale in those directions will make a difference...

For example, in many tutorials I see someone lighting a whole interior scene with a single large rectangle light positioned outside a window, pointing inwards. The rectangle may be 10 feet high and 12 feet wide but often the length of the arrow is left default (around 2 feet long I’d guess). And then they add an accent LED strip light under a cabinet in a kitchen in the same scene and they intentionally scale the arrow to make it very long. Why would they do this?

Personally I don't think this is done intentionally and also it should make no difference.

Regards,
Konstantin