oh yes, balls are the way to do it

or the use of styrofoam balls to evaluate light sources.

After thinking about ways to evaluate softboxes, brollies and the like, I decided to try polystyrene balls as a target to better see what is going on and it works really well.

 Here is a set of balls I got from hobbycraft - middle sized and big ones alternating - hanging in front a bare naked flash (Yongnuo 565), set to 24mm with the diffuser in place.

The balls are all lit with a hard edge around the circumference of each ball, there is a little reflected light showing from the white ceiling and other balls, but not enought to matter really. Below is a zoom in on the centre 3 balls.

Next here are the same balls with a Lastolite stripbox added to the flash. The flash is in exactly the same position w.r.t. the balls as before.

You can see that while the light now wraps round the balls much more (and mostly in the vertical plane 'cos the stripbox is vertical) there is a big drop off in illumination as the balls move further above or below the centreline. This is evident even in the balls immediately above and below the centreline. The balls next to the top and bottom of the box (the top one and the 1st in from the bottom) are over a stop down.

Finally I have added my own mods to the stripbox to even out the illumination. Not only are the outer balls closer in illumination level to the centre ones, there is less variation between the 'equator' and 'poles' of the balls, there is a more evenly distributed wrap round than in the standard version.