Corona Light Meter

Mental ray used to have a light metering tool that could display the light levels hitting a surface.

It would be reallt useful for those of us in archviz and design to have a similar method of checking light levels. It could be as simple as a helper or it could be some sort of rendertime effect like a heatmap or even a grid texture that can display the lux/lumens at user specified grid spacing.

Not only is this useful for when youre recreating real world scenes and have light measurements, its also useful for interior lighting designers when checking the suitability of luminaires.