Colour constancy: how do we see in colour?
You might have heard of photoreceptors: the cone cells, and the rod cells. Basically, when light hits your eye it stimulates these cells, which send a message to your brain. Your rod cells are great at seeing in dim light but can only see in shades of black and white, whereas your cone cells only work well in brighter light and allow you to see colour. But it turns out there’s more to it than that, as thanks to physics, when we see a colourful object, what we’re really seeing is the colour of light reflected back to us from light hitting that object. Katie Haylor spoke to colour fanatic Anya Hurlbert from the University of Newcastle...
Katie - Can we start off with a very brief rundown of the basics? I'm currently looking at a computer screen with a very bright blue background. How am I perceiving that as blue?
Anya - Your photoreceptors in your eye are picking up that light signal and analysing it in terms of its different wavelength components in a sort of bluish band, a greenish band and a reddish band. The cells further on in the visual pathway are comparing how much bluish, greenish, reddish light there is from your computer screen relative to the background. So when you're seeing colour, you're always seeing relative light amounts from the thing you're looking at, relative to the background.
Katie - So that means that the quality of the colour is dependent upon the light source, right?
Anya - Absolutely. And since you were talking about your computer screen, I was sort of thinking of your computer screen as the light source. But of course, most things we look at are not light sources, they're surfaces. They absorb light incident on them from a light source and then they reflect the remaining light to the eye. So what we're seeing when we see colours of objects, is the intrinsic material properties of the object that enabled them to reflect certain wavelengths more than others. But the light that hits the eye is a combination of the light that's shining on it from the light source and the inherent reflectance properties of the object.
Katie - Does that mean that if we're looking at something under natural light compared to artificial light, there's quite a difference in what we're actually perceiving in terms of colour?
Anya - Well, there's a huge difference in terms of the light that's hitting the eye. If I take an apple and I look at it in bluish sunlight, it will be reflecting much more bluish light to my than it does when I take it inside and look at it under say tungsten light, or candlelight, which has much more orange light in it. So that red apple would reflect more of that orange light to my eye. But what my whole visual system is doing, from eye to higher levels of the brain, is sort of filtering out that difference in the light shining on the apple. So that I can continue to see it as red.
Katie - Thing is, if we step outside and light sources aren't constant, you might have clouds in the way or other variations. So how does something that I perceive as red keep staying red?
Anya - Well, that whole phenomenon of seeing that apple as red under, as you say, hugely varying lighting conditions is due to this phenomenon that we call colour constancy. And it's deeply embedded in the way we see colours and it's deeply embedded in our whole visual system, in human vision. We're constantly accommodating and compensating for these changes in the light shining on objects.
Katie - So what's the brain doing in order to achieve that?
Anya - It's thought to be doing many different things on multiple levels, from simply adjusting the sensitivity of the light receptors in the eye initially, so that if all the light in the room suddenly goes very reddish, the so-called red cones in the eye will adjust their sensitivity down to compensate for that change in the illumination, to keep the light coming off objects relatively stable. Or to keep the light signals in response to the light coming off objects relatively stable. If there's more red light coming off them, but it's due to the illumination, you want to turn down your sensitivity to that and that's what the receptors in the eye do.
But it's not just in the eye, the brain is continually assessing and adjusting to these changes in the illumination. That can include levels of, for example, memory. So the fact that I have a memory colour for a banana as being yellow might help me to adjust to changes in the colour of the illumination because I can compare the light reflected from the banana at this moment in time with what I know it's colour should be, based on my memory colour. And then calibrate for those changes in the illumination accordingly.
Katie - That's incredible. I imagine that's not conscious, right? That's going on below the surface!
Anya - Yes, it's completely unconscious. We're totally unaware of that. What our brains are doing is really, really phenomenal in day to day, millisecond to millisecond vision. And colour constancy is one of those fascinating phenomena that is really very, very complicated, and we know it's a really massive achievement of the human brain because it's so difficult to enable cameras to achieve the same thing.