标签: color vision

I previously wrote a column about a really amazing color vision experiment. I'm now a little closer to recreating this, but I could still use a little help... When I first saw this experiment performed a couple of decades ago on a television program it completely blew me away. But before we plunge headfirst into the fray with gusto and abandon, let's start with a very high-level intro as to the way in which we perceive objects as having different colors. Of course the way in which color vision works is amazingly complicated, so if you want to learn more, may I be so bold as to point you at my paper Color Vision: One of Nature's Wonders , which currently ranks #6 in a search for "Color Vision" on Google and is also found as an external link from the Color Vision page on the Wikipedia (sorry, I must admit to being quite proud of this paper). Setting the scene For our purposes here, let's begin by considering the first image below. The idea is that white light is made up of all the colors in the spectrum, but that we can approximate it using a mixture of red, green, and blue (RGB). In the case of an object like a tomato, which we perceive as being red, what is actually happening is that it is absorbing any green and blue light components and reflecting the red, so it's the red component that hits our eyes. Similarly, something we perceive as being green – like grass – absorbs the red and blue light components and reflects the green, so it's the green component that hits our eyes.   In the case of a pure white surface, this reflects all of the color components. By comparison, a pure black surface absorbs all of the color components (note that we're only talking about the visible spectrum here). Of course most real-world objects are more subtle than this in that they do not appear to us as primary colors, but instead comprise different amounts of red, green, and blue. Now, let's suppose we have two objects called Object A and Object B. Let's assume that it's a sunny day, we take our objects outside to look at them, and we see that Object A appears to be a brownish color while Object B has a pinkish hue. We return inside and place Object A and Object B on a table – surrounded by other objects of every color under the sun – and we illuminate them with three RGB light sources that, together, approximate a single source of white light. Under these lighting conditions, it comes as no surprise to discover that we still see Object A and Object B as being brownish and pinkish, respectively. Suppose we use a spectrometer (or something of that ilk) to measure the amounts of red, green, and blue light being reflected from Object A and Object B. It probably wouldn't shock us to discover that the RGB values being reflected from Object A are different to the RGB values from Object B. So, our knee-jerk reaction is that the RGB values being reflected by Object A uniquely define its brownish color. Similarly, the RGB values being reflected by Object B uniquely define its pinkish hue. This is where things start to get interesting... suppose that we fiddle with our three light sources, making one brighter and another dimmer and so forth until the spectrometer shows that the new RGB components being reflected from Object B are the same as the original RGB components we used to see being reflected from Object A. In this case, our knee jerk reaction is that Object B will now appear brownish (rather than pinkish) while Object A will seem to be some color other than its original brownish... What if I told you that in fact both objects appear to retain their original colors? Now read on... The experiment that inspired me Let's turn our attention to the experiment that originally inspired me. Consider the image below. The idea is that we have a panel formed from a large number of geometric shapes, each of which is a different color (actually, there's no reason why some areas should not share the same color). This panel is illuminated by three RGB light sources and we have a spectrometer that we can point at any of the areas so as to determine the RBG components that are being reflected by that area.   Originally, our three light sources are set up such that together they approximate a white light source. First we use our spectrometer to measure the RGB values being reflected from one area (let's say a brownish area to keep with our previous example).  

A few years ago I wrote a column about a rather amazing color vision experiment. I'm now a little closer to recreating this, but I could still use a little help... When I first saw this experiment performed a couple of decades ago on a television program it completely blew me away. But before we plunge headfirst into the fray with gusto and abandon, let's start with a very high-level intro as to the way in which we perceive objects as having different colors. Of course the way in which color vision works is amazingly complicated, so if you want to learn more, may I be so bold as to point you at my paper Color Vision: One of Nature's Wonders , which currently ranks #6 in a search for "Color Vision" on Google and is also found as an external link from the Color Vision page on the Wikipedia (sorry, I must admit to being quite proud of this paper). Setting the scene For our purposes here, let's begin by considering the first image below. The idea is that white light is made up of all the colors in the spectrum, but that we can approximate it using a mixture of red, green, and blue (RGB). In the case of an object like a tomato, which we perceive as being red, what is actually happening is that it is absorbing any green and blue light components and reflecting the red, so it's the red component that hits our eyes. Similarly, something we perceive as being green – like grass – absorbs the red and blue light components and reflects the green, so it's the green component that hits our eyes.   In the case of a pure white surface, this reflects all of the color components. By comparison, a pure black surface absorbs all of the color components (note that we're only talking about the visible spectrum here). Of course most real-world objects are more subtle than this in that they do not appear to us as primary colors, but instead comprise different amounts of red, green, and blue. Now, let's suppose we have two objects called Object A and Object B. Let's assume that it's a sunny day, we take our objects outside to look at them, and we see that Object A appears to be a brownish color while Object B has a pinkish hue. We return inside and place Object A and Object B on a table – surrounded by other objects of every color under the sun – and we illuminate them with three RGB light sources that, together, approximate a single source of white light. Under these lighting conditions, it comes as no surprise to discover that we still see Object A and Object B as being brownish and pinkish, respectively. Suppose we use a spectrometer (or something of that ilk) to measure the amounts of red, green, and blue light being reflected from Object A and Object B. It probably wouldn't shock us to discover that the RGB values being reflected from Object A are different to the RGB values from Object B. So, our knee-jerk reaction is that the RGB values being reflected by Object A uniquely define its brownish color. Similarly, the RGB values being reflected by Object B uniquely define its pinkish hue. This is where things start to get interesting... suppose that we fiddle with our three light sources, making one brighter and another dimmer and so forth until the spectrometer shows that the new RGB components being reflected from Object B are the same as the original RGB components we used to see being reflected from Object A. In this case, our knee jerk reaction is that Object B will now appear brownish (rather than pinkish) while Object A will seem to be some color other than its original brownish... What if I told you that in fact both objects appear to retain their original colors? Now read on... The experiment that inspired me Let's turn our attention to the experiment that originally inspired me. Consider the image below. The idea is that we have a panel formed from a large number of geometric shapes, each of which is a different color (actually, there's no reason why some areas should not share the same color). This panel is illuminated by three RGB light sources and we have a spectrometer that we can point at any of the areas so as to determine the RBG components that are being reflected by that area.   Originally, our three light sources are set up such that together they approximate a white light source. First we use our spectrometer to measure the RGB values being reflected from one area (let's say a brownish area to keep with our previous example).