So, here we have a normal Visual Volume (VV):
Just as a document with print of a certain size with a certain contrast can be plotted on a standard Contrast Sensitivity Function plot, we can extend that to consider a document with print of a certain size with a certain contrast at a certain illumination in this 3D space. If that point falls within the VV, it can be seen. If it falls outside of the VV, it can’t be seen.
Plotting a Print Task
So, let’s consider a common visual task… reading a newspaper article in a normally-lit living room. That’s around N10, moderate-but-not-great contrast, and illumination of pretty much the middle of the CSF plots. Those three dimensions together intersect at one single point. I’ll represent it as a green dot.
The point in is well within the normal VV. Of course, that’s unsurprising — it would be a poorly designed newspaper if a person with normal sight couldn’t easily read it in normal home lighting conditions.
Reducing the Illumination Makes the Task Marginal
But now let’s consider — what if the our reader is so engrossed in their newspaper that they haven’t realised that evening is falling, and so the ambient light levels have dropped. The text is still the same size, and it’s still the same contrast, but its position has now shifted along the illumination axis:
I’ve marked this new point in yellow. It’s still within the VV, but it’s very close to the boundary. That means it’s seen, but not easily. (Take note there — proximity to the boundary starts bringing in the concept of Reserve, which is going to be a big part of the conversation soon). This is the point where the person would probably be thinking “Hey, this is getting hard to read, maybe I should switch on the light.”
Further Reducing Illumination Makes the Task Impossible
What if the illumination drops further? The sun has gone down now, and our reader still hasn’t turned on the room light. Again, the text contrast and size are unchanged, but the illumination is even lower, so it moves even further along that axis.
This time I’ve marked the point in red, because it’s now outside of the VV, which means our newspaper reader can no longer make out the article text — it’s just too dark.
The next page will consider the same situation for people with low vision.