[music] Now we are going to discuss brightness and contrasrt. What detertmines the brightness and contrast of the objects we perceive? Let us start with the obvious. Of course, the lightness and contrast of the perceived objects depends on their physical properties. Every object can absorb and reflect a certain amount of light. The intensity of light that hits a surface is called illuminance. The amount of reflected light is called reflective power. In the same lighting conditions, objects may reflect different amounts of light. For example, a white surface reflects 90% of light, while a black surface only reflects 3%. If we calculate the ratio of luminosity to reflective power for a white and a black surface, the result will be around 30 to 1. The brightness of reflected light depends on a lot of different parameters: on the intensity of light hitting the surface, on the reflective power (or albedo) of the surface, on the surface's orientation relative to the viewer, and various other factors. However, not only the objects' physical properties determine how they are peceived. There are effects that indicate the complexity and ambiguity of lightness perception. So, now we are going to discuss contrast and assimilation effects. I would like to show you two illusions. Please look at this slide. Here you can see that a figure against a light background seems darker than the same figure against a dark background. Now, please look at a seemingly very similar, but actually different effect. The figure surrounded by light areas seems to be lighter than the figure surrounded by dark areas. The first image is used to demonstrate the so called contrast effect. The second picture demonstrares the assimilation effect. Let us think about what these two effects have in common and what are their differences? The common property is that in both cases we perceive stimuli that have the same brightness as being different. We could hypothesize that the lightness or the darkness of these similar lines are due to their thickness or spacial frequency. However, it has been shown that the assimilation illusion is preserved in both low and high frequences. Maybe there are some physiological mechanisms that control lightness perception? In fact, there are theories that try to explain these effects from a physiological perspective. We will discuss them in this lecture and in the upcoming lectures. However, these assimilation and contrast effects are not the only contradiction that we are going to encounter. There are a lot of contradictions in lightness, brightness and contrast perception. We will have to face them in our quest for the mechanisms of viual perception.