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The CSF tells us nothing about how well a patient sees at different levels of illumination.
I’ve heard people claim that, just as magnification moves the text leftwards on the CSF (that is, it gets larger), illumination moves it downwards.
At first glance it seems reasonable. But it’s not true. Sure, things look more ‘contrasty’, more black & white when you get a good light on them, so it’s easy to think the contrast has improved. But that’s not how contrast works.
Contrast Doesn’t Change With Illumination
When we’re talking about a text document, the contrast indicates the brightness of the print compared to the brightness of the background paper. For example, in this picture, the paper has a reflectance of 50% (not great), while the print has a reflectance of only 10%. If we increase the illumination one-hundredfold, it makes the paper brighter, but it makes the print brighter too. They still have the same proportionate relationship — which is the Contrast.
Where does this leave our CSF?
Our two main tools with low vision aids are magnification and illumination. So, how do we represent the effect of illumination on the CSF?
Different Illumination Means a Different CSF
The answer is — we don’t. A CSF is measured at one constant illumination level (for a standard CSF it’s measured at a close to optimal level of illumination). If you change the illumination level, you get a whole new CSF line. And that’s where things get really interesting, because in the real world we’re all dealing with brighter and dimmer situations all the time.
Here’s an illustration of how the CSF of healthy eyes changes over a range of illumination levels:
What I find interesting to see in this diagram is that the top three or four curves are all pretty similar — it demonstrates that healthy eyes see pretty well over a wide range of illumination. I can’t remember where I got the figure (let me know if it’s from your study!), but I’m pretty sure the top curve represents daylight outside, just below that is inside in a brightly lit room (like an office), then a not-quite-so-well-lit room (like a living room), and then down from there. There’s not a huge difference between living room and daylight outside, we see close to optimally in both.
That’s absolutely not the case in most eyes with pathology (especially retinal and optic nerve pathology such as AMD, diabetic retinopathy and glaucoma). Most people with low vision see worse at normal household levels of illumination, which means they will have quite different CSF plots for normal room lighting and strong lighting. The challenge is maximising the light levels without causing glare. More on that later (but, spoiler alert, task lighting is a big part of the answer).