Unknown icon with iray preview: what is it?

javier_4c3512252b

When I select Iray preview, a new icon appears at the left of the draw settings: it has a + and a - sign, and a small sun. 

 

Richard Haseltine

it's an exposure meter - click on it, then click a reference point in the viewport to set the exposure level to give sensible lighting for that point (though it doesn't seem to work well for me).