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One way to adjust the brightness of Global Illumination is to work with the Exponent setting here in the Caustic and Global Illumination attributes of the light. The Exponent works similar to the Decay Rate. We'll use the Decay Rate to determine how quickly the intensity of the light falls off when the light is set to cast direct light. Then we're creating indirect light by casting photons, the Exponent works in the similar way. However, in order to increase the brightness, you actually decrease the Exponent.
So the default settings is at 2; if I decrease this to 1, you'll notice, here's what it looks like with the default setting of 2. If I store this image and set Exponent to 1 and then create a test render, we'll see that it is significantly brighter. In fact, again, it looks like a nuclear blast. So, this is a very sensitive setting. You can just lower this down to 1.8, and you'll start to see how this affects the overall brightness of the Global Illumination effect.
So here is where the setting of 2, and here's a setting of 1.8.
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