Welcome back to this second module in the course visual perception and the brain. And I've titled this module Seeing Lightness, Darkness and Color because we're going to dig deeper in to those qualities of vision. Remember I mentioned there were several basic qualities of vision including seeing lightness and darkness, seeing color, seeing forms, seeing motions, seeing motion and distance and depth those are all basic qualities of vision. And we're going to dig into two of those today seeing lightness and darkness number one and seeing color number two. So let me begin by discussing lightness and darkness. This is absolutely the fundamental quality that we need to have vision at all. Some animals, for example, don't have color vision, but all animals see lightness and darkness, if you don't see lightness and darkness you of course are effectively blind. So this is the most basic property, quality of vision that we are aware of in our [COUGH] perception of the world. And let me remind you of some definitions in this first lesson because they're going to be important for you to remember, not only for today's discussion but throughout the course. First of all, luminance is an objective measurement of light intensity per unit area. That is, it measures the number of photons following on some detecting surface, the detecting surface that we are interested in is of course the retina and luminance has a variety of physical units. The most common ones is candelas per square meter, that's not very important for what were talking about, the important thing is to realize that it is a physical measurement. It's adjusted for the sensitivity of human vision when we're talk about human psychophysics because of course it wouldn't make much sense to look at aspects of radiation, photons that we weren't sensitive to. So, luminance has an asterisk beside it in the sense that it is a physical measurement but it's a physical measurement adjusted for the particular sensitivities of human beings. Lightness is quite different. That's a subjective measurement in quotation marks, I'll talk about that in a second. It's the subjective impression of the intensity of light reflected typically from the surface. We can also use it to talk about lightness instead of lightness the intensity of light coming from a light bulb or some other light source. The flame of a candle, etc. But my point in this, first lesson is to make sure that you understand the difference between physical measurements of the objective world taken with some kind of a measuring device. Photometers in the case of luminance, you have those in your smartphones. And that's very distinct from lightness which can be measured, in a sense, but it's very different. It's a subjective impression. And the only way you can get out subjective impressions is to ask the person who's seeing whatever it is, well, what's your impression of that? How do you get people to report that? Well, there are a whole variety of ways. You can think, for example, of doing it in a very precise way, by asking people what the threshold is for seeing light at all? So you could put a person in the dark room and ask them to detect or report when they can detect seeing a flash of light as you gradually raise the intensity of a light source from zero to some value that indicates the threshold of human sensitivity for seeing light at all. You can have people report psychophysically what they are seeing. What they're perceiving by other ways ranging from questionnaires that are used very commonly in psychology. To comparative measurements, put on a stimulus that's a reference stimulus, and ask a person to judge a second stimulus, when it looks the same as the first stimulus. That's a way, by comparison of measuring, getting a person to report what they see. So there are a whole variety of ways of doing psychophysics. And as I say, we'll need to keep this distinction in mind throughout the course, the difference between physical measurements and subjective measurements.
