So let's go to the way that we actually see the physical world, and I'm going to make the point that it's very strange. And the reason for telling you this is not only that that's the way it really is but to get you hooked on the idea that vision is really pretty mysterious in terms of visual perception. There's a huge discrepancy between what we're aware of seeing and the physical measurements that are out there in the world that we make with simple instruments. And that discrepancy is going to be the theme of the course that we're going to come back to again and again, and try to explain what that discrepancy is all about. Why is it there? Let's first take a look at the perception of luminance. So the definition of luminance is a physical measurement. It's something we measure with a photometer. Again, as I said a minute ago, you all have one of these in your cellphones, when you take a picture with your cellphone. Luminance, the intensity of light that's falling on the detector in the phone, is being measured. That's the physical measurement, and that corresponds as all measurements of physical stuff and qualities that we see in response to that. There's a psychophysical subjective definition of the response to luminance, and that's what we call lightness or darkness. When we see an intensity of light, we judge it to be light or dark, lighter or darker. And those subjective impressions of what we see are the psychophysical correspondences of the physical luminance. So always there's a physical parameter, and there is a psychophysical parameter. And in this case we're talking about the physical parameter of luminance and the psychophysical parameter of lightness. So let's consider these two circular patches presented to you on the same background. And if you took your photometer to them, you would find that this one measures exactly the same number of units of light intensity, those are physical units. They are typically expressed in candelas per meter squared. We don't have to go into that, but just to let you know that there is a physical measurement. Those two little patches are identical, photometrically, physically. Now, when we look at them and they're in the same background like this, they look the same. So no big deal, they're identical, and they look identical. But now let's [COUGH] surround these patches with a different surround. So now, one patch is surrounded by a dark surround, and the other patch again, we haven't changed the patches. These are the same, but they're now in different surrounds. And you can see now that they look different. This one looks a little bit brighter or lighter than this one. So again, the brightness, darkness, the subjective impression of the same luminance is different. And that's really quite puzzling, why should that be? Why is that? Well, first of all you might say, the difference between these two perceptions, not very great. Maybe that's just a little side effect of the imperfections of our visual system. And of course, all biology that's evolved, or anything that's evolved, is presumably not perfect. But I think I can quickly [COUGH] disabuse you the idea that this kind of difference between physical reality and psychophysical subjective experience is different in a pretty big time way. So let's look at this example. And here I want you to direct your attention to the perception, the lightness or darkness of this area versus this area, or this area versus this area. And I think you see them as I see them, as everybody sees them, as this region of the scene being very much lighter than this region of the scene. Again, this region of the scene being very much lighter than this region of the scene. Well if I take the context away, the surround away, again like before, if I take the surround away, you can see that these patches are exactly the same physically. If we measure them photometrically, they also have exactly the same reading in candelas per meter square of the light falling on those surfaces. But as soon as I restore the surround, you once again see them as very different in lightness or darkness. And this happens as quickly as I can take on or take off the mask. So this is not at all like something you experience when you're going into and out of a movie theater and you have to dark adapt. It takes many minutes for you to see everything in the theater once you go in or to readjust when you come out in the sunlight. It's not that at all. This is instantaneous, and that raises the question of what's going on. So another sort of way out of this, you might say, well okay, maybe that's something that's pertinent to lightness and darkness. But what about other qualities of vision? We see color of course. We see geometry, we see motion. We see many other qualities of vision, and what about those? Are they subject to the same peculiarities, the same strangeness? Well the answer is yes, they are. And there are many, many examples I could show you of this, but let me just show you this one. Here I want you to focus on the appearance of this patch and this patch on the surface of this multi-colored cube. And this patch I think you see as I see for those of you who have normal color vision. Abnormal color vision is not that rare. And there will certainly be some people viewing this who don't have normal color vision, who are so-called color deficient or color blind in quotation marks. But for those of you with normal color vision, the vast majority, this patch looks orange, and this patch looks kind of a rich brown. And again, when I take away the context, you see that these perceived colors are coming from patches that are physically identical. So this patch really is the same as this patch in terms of its spectrophotometry. So we'll talk later about what color is and how one differentiates color vision from black and white vision. But suffice to say, for the moment that this measured with a spectrophotometer, and that it has to do with the distribution of power across the range of wavelengths that we human beings are capable of seeing. We'll talk about that in a bit. So the same strangeness that we talked about in black and white is apparent in color. And again, that happens as quickly as I can take on or take away the full set of information in the scene.
