On the previous pages I showed the Visual Volumes (VVs) of a person with normal sight and Alice (who has early-to-moderate AMD), and plotted the position of a newspaper article at three different levels of illumination. But it’s a little bit complicated to always think in 3D, so I’m going to take a moment here to simplify it, and go back to viewing it in just two dimensions.
Ah, but which two dimensions?
Bigger vs Brighter
Of the classic three dimensions (Bigger, Bolder & Brighter), the two conditions we can easily modify are the Bigger and the Brighter. The contrast of a document is not so easy to modify.
So, instead of taking a vertical slice of the VV to get the person’s Contrast Sensitivity Function at an arbitrary standard level of illumination, it would be more practically useful to take a horizontal slice of the VV to see the relationship between text size and illumination for text of this particular contrast (in this case, the local newspaper).
Here’s Alice’s 3D VV:
And if we take just the points that relate to one particular contrast level (newspaper), we get a flat plane:
The Brightness Sensitivity Function
Let’s take a closer look at just that layer.
It kind of looks like the CSF flipped upside down, but it shades off into nothingness down the bottom instead of up above, and that nothingness is black, instead of 50% grey (since it’s a luminance gradient, rather than a contrast gradient).
What shall we call this? There may be a proper name, but I’ve never seen this anywhere else. So on the off chance that I’m the first to discover it (if ‘discover’ is the word), I’ll mark its relationship to the Contrast Sensitivity Function by calling it the Brightness Sensitivity Function (BSF).
And (ta-DAA!) there you have it. Unlike the CSF, the BSF is the figure that will give you practical guidance on how best to use both magnification and illumination, our two main tools in low vision rehab, either separately or together. Remember though, this is contrast-specific. This is Alice’s BSF for reading the local newspaper. When Alice is reading a magazine with nice black print on white paper she will have a different BSF, and when looking at a pastel-shades bank statement she’ll have a different one again.
Using the BSF to Understand the Importance of Illumination
So, a patient comes in to you and says “I’m having trouble reading the newspaper.” We need more information. Where does the patient read the newspaper — in the living room? In their bedroom? What sort of lighting do they have in those places? Do they read during the day, or in the evening? Do they have trouble with the newspaper all the time, or do they find that it’s okay when they take it over near a sunny window?
When Alice is in the much dimmer room she can’t read the newspaper article at all, and it’s plain to see that giving her a magnifier will be futile in that situation. You could magnify the print for her until it’s as big as a house and she still wouldn’t be able to read it — it’s just too dim. She needs to turn the light on.
In the not-so-dim room, it would be possible to give Alice a magnifier (or give her a higher reading add) to let her read, but she’d need quite a lot of magnification — certainly a lot more than you’d expect from her (still pretty good) VA. (Remember, never trust a good VA in low vision).
Is that a sensible solution though? Not really. Magnifiers have their own problems , and even with a fair bit of magnification the text is still quite close to the seen/not-seen boundary, so Alice is still not going to be very comfortable. Even though a magnifier could work, the much simpler and more practical action is just to increase the lighting level.
Improving Reading Comfort
Let’s extend this a bit further. Even though Alice is reading in the well-lit living room, the newspaper is still going to be a struggle for her, as it’s almost on her seen/not-seen boundary. Could we make Alice happier? Could we give her a more enjoyable reading experience?
Alice could achieve a more comfortable reading experience by either using a low-powered magnifier or by arranging to read in even better lighting (either adding in some task lighting or moving to an even brighter room). Both of those options shift the Bigger/Brighter point to a position that is further away from the seen/not-seen boundary, deeper within the seen zone.
Using Magnification With Illumination
Here’s an illustration of Alice using magnification and illumination together (that is, an illuminated magnifier, or a magnifier lamp, or a higher add in a brighter room, etc):
When Alice uses both magnification and extra lighting, that brings the print to a point (the light blue one) that is even further away from the boundary (that is, it has more reserve), and so her reading should be even more comfortable.
I’ve also shown the situation where Alice adds in too much light, and the text position has shifted closer to the top boundary of the BSF. That represents a situation where the light has become so bright that it is causing Alice discomfort glare.