Understanding Vision
Aspects of Vision 3: Brightness
Eyes are better than any camera
The human eye is able to see over a remarkably wide range of illumination, from a bright sunny day all the way down to a moonless night. Even the very best cameras can’t match the eye’s ‘dynamic range.’ That’s not to say that our eyes see equally well in all light conditions though, there is definitely an optimal range.
All light is detected by special cells in the retina called photoreceptors. One of the reasons we have such a good range is because we have two different systems of photoreceptors.
Rods are just for low-light conditions
The first, and simplest system is made entirely of photoreceptors called rods. They work best in low light conditions, and are responsible for our night vision. They are spread fairly evenly across our retina, but there aren’t any at all in our fovea (which is why, if you’re looking at a faint star in the night sky, you’ll see it better if you look a tiny bit to the side of it). Rods are not densely packed, so they don’t provide much in the way of detail vision, but they’re good for getting around in low light conditions.
Rods don’t work at all in bright conditions, or even in what most people would consider normal lighting, so people who have conditions where only their rods are working need to wear dark glasses to see.
Rods can’t tell see colour. Our night vision is just shades of grey. They can’t detect red light at all, so red objects appear black.
Cones give us normal vision — in normal to bright light conditions, with colour and detail
Our vision in normal light conditions is handled by a more complex system of three different types of photoreceptors known as cones.
Each type of cone is sensitive to a different colour of light. The fact that there have different sensitivities allows the retinal nerve fibres process the information from all three types of cone together to determine the colour of an object, so this system gives us our colour vision.
Cones are spread fairly thinly across most of the retina, but have a much higher density in the macula, which is how the macula is able to give us fine detail vision. The very densest area of cones is in the fovea, where they are packed absolutely as tightly as possible, in order to give us that one spot of ultra-high detail vision.
Cones have evolved to work best in daylight conditions. They don’t work so well in lower light, and don’t work at all in very low light conditions (such as night-time), which is when we rely on the rods. Vision from rods is reasonably poor though, so whenever we can we add extra light to our environment so we can see with our cones instead.
- We turn the light on, or light a candle.
- We put in street lighting.
- We turn on the car headlights.
- We use a torch/flashlight.
Colour vision deficiencies
Some people have a problem with one or more types of photoreceptor, which gives them a colour vision deficiency. It’s generally genetic, so there’s nothing you can do about it (although perhaps one day we’ll be able to use gene therapy to fix it?).
Most people call it colour blindness, but it’s more accurate to say colour vision deficiency, because most are quite mild. Almost all people with a colour vision problem can see colours — it’s just that what they see is a relatively restricted range, and they can’t tell the difference between some shades of colour that most people would see as quite different.
The most common colour vision deficiency is a reduced sensitivity to shades of green, with the next most common being reduced sensitivity to the colour red. The colours we see in the real world are always a lot of different coloured photons (light particles) mixed together, just the same way as a TV screen uses just three different colours of pixels mixed together to show a whole range of colours. So having a reduced sensitivity to one colour affects our perception of a whole variety of colours, not just that one colour.
Glare — too much of a good thing
We use the word glare to describe any situation where there is too much light, but it’s worth considering the differences between various types of glare,
Ambient glare is when everything is simply too bright. At a certain level vision becomes less comfortable, and at an even higher level the cone photoreceptors just get overworked and can’t keep up (we say that they are ‘bleached’). Simply using sunglasses works well to bring things back to a light level that is comfortable, or we move somewhere else, into the shade or inside.
Glare sources are when there is something uncomfortably bright in amongst more normal lighting conditions. Good examples include driving towards the sun when it’s low in the sky, seeing the sun reflected off the windscreen of the car ahead of you, or talking to someone who is silhouetted in by a bright window. You can wear sunnies, but really the best thing to do is to move (yourself or something else) to get the bright light source out of your field of view — in the car you’d put down the visor, or move your position relative to the person you’re talking to so the window is no longer directly behind them.
Veiling glare is when bright light illuminates something that is on a clear surface in between you and what you’re looking at. An example would be when we have a lot of dust on the windscreen of our car. In most situations it’s fine, but when direct sunlight strikes the dust the dust particles reflect light, and it’s hard to see past them to whatever we are supposed to be looking at — especially if that thing is dim. We could wear sunnies, but it doesn’t help all that much. The most effective thing is to clean the windscreen, restoring the surface to clear.
Another example is having dust or scratches on our spectacle lenses. We can clean dust off, but not scratches. If we put our hand up to block the sun landing on our glasses, that removes much of the veiling glare.