In the preceding pages I discussed how the Brightness Sensitivity Function (BSF) can help us understand how to best help a vision impaired patient with lighting and/or magnification.
Now I want to more carefully examine just how we represent the effects of low vision aids on the BSF.
Magnifiers Makes The Print Move Left
Pure magnifiers make the print appear bigger, so the point on the BSF moves to the left from its origin. The greater the magnification, the more the point is moved.
In this image, what’s important is the direction and length of the vector for each magnifier. At this stage, just ignore each arrow’s starting position on this plot. The thing to take away is, whatever combination of print size, contrast and illumination you start with, using that magnifier will mean applying that vector to move the print that far to the left of its origin point.
Increasing Illumination Jumps the Print Upwards
Adding illumination results in a vector that moves upwards (or, if you’re making something dimmer, it shifts downwards). It’s a little harder to represent the magnitude than it was with magnification. After all, if you use a task lamp or illuminated magnifier, it doesn’t really make much difference whether you start in a normally-lit room or a pitch black room — the result is going to be pretty much the same level of illumination on the page. So I’ve represented that with a curved arrow, by which I’m meaning that no matter where you start, the text illumination ‘jumps’ to that certain level of brightness.
Filters Shift the Print Downwards
Filters (sunglasses, etc) act to dim down the illumination (not of the document, but the brightness of the image entering the eye), in a proportional manner. If you start in dimmer light, you get a dimmer result than if you’d started in bright light. So, just like magnification, filters have a direct vector rather than ‘jumping’.
Vectors are Additive
Low vision aids that use both magnification and illumination combine these vectors. We can use this combination to effectively ‘burrow’ deep into the ‘seen’ zone of the BSF.
Here are some other examples:
- A telescope provides magnification, but reduces illumination. This can be represented by a vector to the left (magnification) plus a vector downwards (reduced illumination).
- Similarly, high adds typically result in just a magnification vector, but if the document working distance comes in so close that the page starts to be shaded, we need to introduce a negative illumination vector as well.