So, how do we represent changes in illumination on the CSF? Having different illuminations means dealing with different CSF plots, so to help me understand what’s going on I like to consider them arranged together in sequence, darkest-to-brightest, in a 3D space.
If we connect up the bottom of each plot (the maximal contrast part of each plot), then we get this, as forming two walls of a space:
I know representations of 3D shapes can be ambiguous — so, just to be clear, the two top walls are the back walls of the space, with the bottom wall being like the floor. Looking towards the right wall is kind of like looking towards the sunrise on a misty day — very bright at the horizon, with the brightness fading into haze above, while behind you shades into night.
Plotting CSFs in 3D
Remember this image? CSF plots for normal healthy eyes over a range of illumination levels?
Let’s plot (some of) them in the 3D space:
Notice that I’ve modified the left side of each curve (the part that represents high spatial frequency, aka ‘big things’) so that they don’t dip down. I’m doing that because I’m planning to use this mostly for discussion of making text readable, and since the print/background junction is abrupt rather than gradual there isn’t the reduction in visibility.
CSF in Very Bright Conditions
There’s also one plot missing, the very back one. That should represent the really bright conditions we normally encounter — let’s say outside on a bright sunny day. At that point things are too bright, we’re suffering discomfort glare, and so we don’t see as well (until we put on our sunnies). That doesn’t appear in the graph from the study we were looking at, but we can predict what it will look like — something somewhat worse than the optimal, anyway. For low vision work it’s important to consider those high illumination levels, so let’s add that in now:
Can you see how these all start to form a framework for a 3D surface? Considered all together, they create what I call a Visual Volume (VV).