6.2 The lux heatmap

What it shows

The light actually landing on a surface: each machine’s lumens spread over the image it makes there, discounted for a beam that arrives at a slant, and added up over every projector that reaches the point. Whatever is in shadow gets none of it.

Three consequences worth internalising:

Bigger image, dimmer picture. The same lumens are spread over more surface, and it is the area that grows — so brightness falls away faster than the extra width suggests.
Angle costs light. The more obliquely a beam lands, the further it is smeared across the surface, and the dimmer it reads.
Overlaps add. A blend region gets both machines’ light, which is exactly why it shows up hotter.

The lux heatmap across a two-projector blend — Figure 6.2 The demo rig’s main screen. The two images read the same green; the vertical band where they overlap is visibly hotter — that is the blend region, carrying both machines’ light.

Set Max lux to the top of the range you care about. The default of 1000 lx puts a typical rig’s overlap bands in the middle of the scale.