@Martin

It's a little joke,because it looks like indirect light,but it's not true. My settings were PATH-0...but ENHANCED EXTERNAL BRIGHT...and in combination with sS-SILK and shining walls it looks like this...

Nice trick to simulate indirect light with a rendering time less long! [:)]

You discovered that the Enhance external brightness option brings light to the interior of a room with large openings. [;)]

image size: 600 x 450 pixels
advanced settings: aAmax 1, gI default, dB 0, sS glossy

without Enhance external brightness option (rendering time: 16min 57.9s)


with Enhance external brightness option (rendering time: 35min 14.1s)