We encounter this all the time in low vision work, and most especially with patients who have AMD. Patients often perform a whole lot worse at home than they do in our consulting rooms, because they are reading text that isn’t high contrast and because they are reading in ambient illumination that is way below our consulting rooms.
A classic example is when people read in bed — perhaps a book before bedtime, or sitting in bed reading the newspaper in the morning. The level of light in a typical bedroom (in the evening especially) is often very low, the only light source being a bedside lamp with some sort of lampshade. You might prescribe them reading glasses that let them read perfectly in the consulting room, but they struggle when reading at home, so they’ll come back complaining that their new reading glasses don’t work.
So if a patient with any sort of vision impairment is having trouble with reading, it’s so important to ask about what they’re trying to read (to get an idea of print size and contrast) and also in what lighting conditions they’re trying to read it.
Don’t just ask “Are you reading with good light?” Everyone thinks they’ve got good light. “Yes, the strongest you can get… I have a 100W bulb on the ceiling.” Remember your inverse-square rule, and also think about the angle at which the book might be relative to the light source (the light’s not much help if the page is in shadow).
You’d think that people with a luminance deficit would be very aware of the need for extra light, but they often aren’t. I’ve puzzled about this, and I think the key is that patients will often have a luminance defect only in the macula, but they judge how well an area is lit with their whole retina. Remember, the macula is a quite tiny part of our overall vision, so it’s quite possible for patients to feel they are in a perfectly well-lit room even while their macula is struggling.