Fluent Essays

Look the Lecture Reading and documentation requirements. Complete the documentation. Lecture 2 for Weds Sept 8: https://www.youtube.com/watch?v=0fKEz2EeSlQ&list=PLZ8if_0QQBH-XzqdVyvsWaNQZf1vFa4s2&index=2 Knowing Nature, Fall 2021 Homework 1 - 45 points total Overview: We are using the topic of human color vision as a way to reflect on “direct” perception, subjectivity and objectivity, measurement, and representation. The readings contain a lot of detail on the physics of color perception, but this homework assignment is intended to focus your attention on the skills and themes that are most relevant to the class. Readings/listenings/viewings: 1. Radiolab episode on color - the first 10 minutes is excellent for giving background that will help with the readings, and the rest relates directly to themes from class. 2. Draft “Color” chapter by Kathryn Schaffer (part of a book on light) - on canvas 3. Very short youtube video on bees and crab spiders and UV light: https://www.youtube.com/watch?v=UjHvs5Xz_TI Please answer these questions on this sheet or another sheet of paper. Note that some questions do not have single “right answers.” Also be sure to use your own words and avoid paraphrasing any research sources for these answers. 1. (3 points) In class in week 2, we used spectroscopes to study different types of “white” light, including fluorescent light, flashlights, and light from the sun. Combining what we did in class and what you learned from the readings, is there a way to explain what “white” light is, in terms of the underlying wavelengths that are part of the light? Explain, referring to empirical evidence in your answer. 2. A person looks through a spectroscope at two light sources. When they look at light source A, they see a complete rainbow across the display, showing light at every visible wavelength, with equal brightness. When they look at light source B, they see one bright band at around 450 nm (“blue”), one bright band at around 500 nm (“green”) and one bright band at around 650 nm (“red”). The three bands are equally bright. (a) (4 points) Draw spectrum graphs to represent these different measurements, in the same way as we discussed in class, and in the same way as the graphs on page 190 of the readings. Label them A and B to show which is which! (b)(2 points) Speculate on how you think a human eye (without the spectroscope) would see these two types of light. Do you think they would be visibly different? Visibly similar? Why? 2. (3 points) In class in week 2, we looked at examples of “color illusions” and “color constancy.” Pick one of these topics, and explain why it shows that the color a human perceives is not an “objective” property of things. 3. One variant of color blindness in humans occurs when a person is missing L cones, and only has S and M cones. (a) (2 points) Draw a graph that represents the wavelength sensitivities of such a person’s cone cells (in other words, draw a graph that is similar to the one on page 193 of the readings, but appropriate for an eye with no L cones). (b) (3 points) Using the graph you have drawn, explain in words what happens when a person with this type of color vision senses “green” light (with a wavelength of around 500 nm) and what happens when the person senses “red” light (with a wavelength of around 650 nm). (c) (3 points) How is this different from how a non-color-blind human eye would eye would respond to light at those two wavelengths? Would you expect these differences to translate to differences in perception? 5. Many areas of scientific research, including climate science, use sensors to detect light and analyze it according to wavelength components. For example, instruments called radiometers, mounted on a satellite flying several hundred miles above the Earth's surface, receive incoming light that has been reflected by clouds, plants, atmospheric particulates, etc. The Multi-angle Spectroradiometer (MISR) instrument, as one current example, receives reflected light in 4 wavelengths (446nm, 558nm, 672nm, and 866nm). The instrument is very sensitive and records the intensity ("amount") of light received at each wavelength. It then sends these data back to Earth. A scientist then takes these data and, using the numerical values of intensity at each of the 4 wavelengths, measured at each location across a field of view, creates a composite image like the one on the next page. (a) (5 points) Compare and contrast this type of "observation" and the process by which a human looks at an object and judges it to be yellow. You should include examples of ways that it is similar and ways that it is not similar. (b) (2 points) Does this MISR instrument measure "color"? Please discuss. (c ) (5 points) In every form of measurement that we make, there is always a human at the end interpreting the information. A human viewing an object directly might interpret their sensations as “bright red.” A human viewing an image from a remote sensing system might interpret it as showing “more light in the infrared range of the spectrum than the visible.” Is the person making the same kind of interpretive judgments in either case, or are they different? Discuss, incorporating themes of objectivity and subjectivity as discussed in class and the readings. (d) (2 points) Is it possible, in your view, to remove subjectivity entirely from the process of measurement and interpretation? Use the example of a scientist viewing a remote-sensing image to explain why or why not, in your opinion. The image above is an example of a composite image made from MISR data. Data from each type of sensor has been given a different color and they have been merged together into one image. 6. (4 points) According the Radiolab clip you listened to, when Isaac Newton used a glass prism to “divide”white light into a spectrum of wavelengths we see as a rainbow of colors, there was some question about the actual role of the scientific device - the prism - and what it was doing. What was the concern and how did Newton test the effect/role of the prism in generating a rainbow? 7. In the Radiolab section you listened to the interviewer Robert Krulwich asks a “visual ecologist” the following: “If a dog, and a human, and a crow were staring at rainbow, would they be seeing very different things?” The visual ecologist answers: “Yes.” Watch the following video, which is about a similar kind of situation: https://www.youtube.com/watch?v=UjHvs5Xz_TI As the video explains, flowers appear differently in reflected visible light and in reflected ultraviolet (UV) light. Bees apparently use these differences to navigate towards nectar sources. (a) (2 points) Looking at the illustration on p.198 of the “Color” chapter you read, what numerical range of light (in “nanometers / nm” is the bee likely seeing reflected off the crab spider that the an animal like a bird or human is not? (b) (2 points) The narrator in the video points of that the spider is white colored on a white flower and so “looks supremely camouflaged.” Camouflage is a term used to describe a situation where a creature is trying to hide and NOT draw attention to themselves. What animal might the spider be hiding from in that case, and why? (c ) (3 points) If humans can’t see ultraviolet light, how can we see “what a bee sees” in the image above? Explain briefly how a “ultraviolet camera” must work to make a image like the one above. extras/drafts 7. In the Radiolab section you listened to the interviewer Robert Krulwich asks a “visual ecologist” the following: “If a dog, and a human, and a crow were staring at rainbow, would they be seeing very different things?” The visual ecologist answers: “Yes.” Watch the following video: https://www.youtube.com/watch?v=UjHvs5Xz_TI The narrator in the video points of that the spider is white colored on a white flower and so “looks supremely camouflaged.” Camouflage is a term used to describe a situation where a creature is trying to hide and NOT draw attention to themselves. What animal might the spider be hiding from in that case, and why? If an animal sees reflected ultraviolet light, is flower appears differently: Looking at the illustration on p.198 of the “Color” chapter you read, what numerical range of light (in “nanometers / nm” is the bee likely seeing reflected off the crab spider that the an animal like a bird or human is not? If humans can’t see ultraviolet light, how can we see “what a bee sees” in the image above? Explain briefly how a “ultraviolet camera” must work to make a image like the one above. WK2 HW QUESTION on Remote Sensing: An instrument called a radiometer, mounted on a satellite flying several hundred miles above the Earth's surface, receives incoming light that has been reflected by clouds, plants, atmospheric particulates, etc. The Multi-angle Spectroradiometer (MISR) instrument, for example, receives reflected light in 4 wavelengths (446nm, 558nm, 672nm, and 866nm). The instrument is very sensitive and records the intensity ("amount") of light received at each wavelength and sends these data back to Earth. A scientist then takes these data and, using the numerical values of intensity at each of the 4 wavelengths, creates a "true-color image" like the one below. (a) How does this way of "observing" compare and contrast to the process by which a human looks at an object and judges it to be yellow? You should include examples of ways that it is similar and ways that it is not similar (b) Does this MISR instrument measure "color"? Please discuss. Comment by Kathryn Schaffer: This is a rich question... but since the chapter reading doesn't really deal with imaging, I am not sure what we are driving at in this particular context (c) Does the MISR instrument take a picture? Please discuss. (d) For every observation - a human interpretation is necessary to convert received information into usable data. For example, at the 866nm wavelength, the MISR satellite "sees" live vegetation as being very bright (it has a high brightness intensity at 866nm wavelength). A human might show these data as "bright red" on a graph. However, plants appear "green" to the human eye. Explain the difference and be sure to incorporate themes of objectivity and subjectivity from the reading, our discussions in class, and your understanding of how "observations" are made. Comment by Kathryn Schaffer: I am sort of worried that this question mixes two ideas... in my version I was going for a comparison between the act of subjectively judging color, and the act of comparing "objective" instrument measurements. But this question also invokes the representation choices we use to display non-visible colors, which I think is kind of another topic. Hope that makes sense? Color Suppose someone places an object in front of you, and asks you to list some of its properties. No matter what the object is, odds are one of the first things in your list will be a label associated with color. Color seems to inhere in objects as one of their most obvious physical attributes, so firmly that when philosophers discuss the abstract relationships between properties and things, color is often taken as a prototypical example of what we even mean by the concept of a property. Books on physics typically extend this color-as-property view to light, labeling the wavelengths across the visible portion of the electromagnetic spectrum with the names of the colors of the rainbow. I want to present a different view. Color is not a property of light. I would also argue that it is not a property of things or materials, counterintuitive as that may seem. Color is a perceptual experience, and its true locus is the human eye-brain system. The physics of light is certainly involved: when ambient light interacts with materials in our environment, those materials modify the light in ways that can trigger the visual perception of different colors. Yet, we cannot reduce the experience of color in a simple way to the physics of light and its interactions with matter. In keeping with this view, I have made a deliberate choice throughout this book to avoid any color imagery. We are trained from an early age to see "red" on a page, label it as red, and stop asking questions. But we should ask questions about colors on a page, just as we should ask questions about every other aspect of our visual representation systems, especially when we are using them to describe invisible physical phenomena like light. A lot is going on when we look at something like a page and label it with colors. Unraveling that process is one 182 COLOR of the aims of this chapter. The human brain shapes color vision a lot more than most people realize. Two types of phenomena that showcase this fact are color illusions and the experience of color constancy. The color illusions I want to highlight are cases in which two objects reflecting light in exactly the same way may be interpreted as different colors by the human eye-brain system because of differences in their immediate visual surroundings. The optical illusion below (known as the Munker-White effect) gives a simple greyscale version of this effect: all of the grey portions of the image are the "same" grey, in terms of what is printed on the page. The same pattern of light reaches your eye from each grey region, yet you see them to be different in the context of the striped pattern. Many similar illusions can be constructed with patterns of colored pigments on a page. A splotch of pigment may look yellow when surrounded by one set of colors and blue when surrounded by another set. We call this an illusion because there are measurable, objective facts about the sameness of the pigments in the splotches, despite our differing perceptions. But there is no measurable, objective fact about what color the splotch is. (Note that I am using the word "objective" here to mean something that can be measured in a way that does not depend on the direct perceptual judgments of any one individual.) Every perception of color is context-dependent. Color perception depends on lighting conditions, the perceptual processing in our eye-brain system, and our physiological state (e.g. color blindness, or the influence of diseases or drugs). COLOR IS A PERCEPTION 183 So while there may be an objective fact about two pigments being physically identical, that does not translate to any objective facts about what color to call them. Maybe a healthy adult human with normal color vision views a pigment in bright sunlight against a white background and calls it brownish-grey. Is that the "true color"? Only if we define these highly specific observing conditions as our normative standard for judging color. A non-human animal, a color-blind person, someone viewing the scene through sunglasses, or someone placing the pigment against a dark background might not make the same judgment. There isn’t anything less true about the color they see. Now, consider the visual phenomenon of color constancy, which refers to the way that our brains impose the expectation that objects should appear to have a constant and consistent color despite lighting conditions. As humans, we have a strong bias to consider color to be a static property of ordinary physical objects, much like how we know size to be a constant property of most objects despite visual effects like foreshortening. Such a bias helps to simplify our complex sensory experience, encoding an assumption that many objects and materials do not change in any important ways from one moment to the next. At the same time, the perception of color constancy requires that we interpret dif- fering sensory inputs to be identical. It is a sophisticated trick of the neuroscience of vision. For example, suppose that you are reading a book on a porch in the late after- noon, and continue reading until the sun sets. 184 COLOR In such a scenario, you will see the pages as white the whole time. Sure, things look a bit different in evening light, but not different enough to say that the actual color of the book pages changes. If you take photographs of the book in afternoon and evening (with the same camera settings), the color in the photographs will change. One photograph will show the pages to be blue, and the other orange. Both photographs are likely to be annoying, because neither will look "right" as representations of what you see. That’s a clue that something funny is going on here. Your eyes and the camera both receive the same light: sunlight reflected off of the book pages. The sunlight changes as time passes due to the effect of the atmosphere. Yet you and the SPECTRA 185 camera disagree about how the changing lighting conditions affect the color of the book. The difference is that you have a brain shaping your perception, and the camera does not. The camera, by itself, just records light according to its technological design. Your brain, on the other hand, actively constructs what it sees. The camera does not have an expectation that the pages of the book should stay the same from one moment to another, but the brain does. The brain makes the paper white. As organisms operating in an object-filled environment, we actively impose the expectation that most of them have static properties. To do this, we change the way we interpret colors on the basis of context. Since people usually design cameras with the intent to mimic human vision, they often build in ways to "correct" camera images, even though the camera is not making any mistakes when it creates a blue image of the book in one case and an orange image in another. "White balance" is a technique in photography that modifies colors to ensure, for example, that white pages always look white. It is not easy, though, to make a device mimic the complex perceptual biases that shape human visual experience. For this reason, achieving photographic color that appears realistic to a human is something of an art. "Realistic" in this case is not a matter of recording and presenting information as it is is "truly" present in the environment. It is about replicating aspects of a constructed and context- dependent experience outside of the original context. Really, that is what we are doing when we create a photographic representation, and it is a subtle process. Color constancy and color illusions illustrate what I mean when I say color is not a property of objects or materials, nor of the physical properties of the light that we see. Color constancy phenomena show that differing visual stimuli can give rise to the perceptual experience of identical colors. Color illusions show that the same objects or pigments can give rise to the experience of different colors in different contexts. Color is a perception. It involves light, but depends greatly on neuroscience, conscious and sub-conscious expectations, and the peculiarities of the instrumentation in our eye-brain system. The word spectrum (plural spectra) is applied in two distinct ways when discussing 186 COLOR light, and we will need both of them to explore color phenomena in more depth. The first way we use the word is to refer to an abstract collection of light with varying wavelengths, possibly within some restricted range of interest. This is the sense in which I have used the word previously in this book, to refer to the abstract collection either of all possible light waves (the electromagnetic spectrum), or of visible light in particular (the visible spectrum). There are minimum and maximum wavelengths (and corresponding maximum and minimum frequencies) of light that the human eye can register. Light waves with too short a wavelength (too high a frequency) are not visible, and light waves with too long a wavelength (too low a frequency) are also not visible. Any light wave with a wavelength between these extremes, provided the amplitude is great enough, will be visible to the human eye. The visible spectrum – that is, the visible range of the electromagnetic spectrum – is roughly between 390 nm (nanometers, or billionths of a meter) and 700 nm. The corresponding frequency of the long-wavelength end of the spectrum is 430 THz (terahertz, or trillions of cycles per second), and the corresponding frequency for the short-wavelength end of the spectrum is 770 THz. As we discussed, we can pick either the wavelength or the frequency as our basis for differentiating and sorting light, as long as we keep track of the fact that they vary inversely. The most important light source in the human environment is the sun, and its light includes every wavelength across the visible spectrum, combined together into a complex and chaotic waveform that we perceive as "white." If we use a prism or a spectrograph (a more sophisticated prism-like tool) to spatially separate the components of sunlight by wavelength, we will see a smooth and continuous pattern of different colors that we recognize as a rainbow. How we delineate the rainbow into a set of 6, 7 or more distinct colors is largely a matter of cultural convention. A typical human eye can recognize millions of different distinct colors in the rainbow, but in English we only normally label red, orange, yellow, green, blue, indigo, and violet. If we follow this convention, the rainbow spans the wavelengths and frequencies of the visible spectrum as shown below. SPECTRA 187 In this table, the long-wavelength end of the spectrum (at the top of the rainbow) is the portion we usually call "red" and the short-wavelength end (at the bottom) is "violet." Most of us do not deal with units like nanometers and Terahertz every day, so it is worth taking a moment to note that visible light involves waves that have almost incomprehensibly high frequencies and tiny wavelengths. The final entry in the table (which corresponds to the violet end of the rainbow) represents a traveling wave that goes through 714,000,000,000,000 cycles per second! Looking at that light, your eye would be bombarded with 714,000,000,000,000 repetitions of the wave pattern in a single tick of a clock. Meanwhile, given the high speed of light (2,998,792,458 m/s through empty space) this wave only travels 0.00000042 meters (420 nanometers) in the course of one cycle, with more than 2000 complete cycles (wavelengths) fitting in the span of a millimeter. The scale on which visible light waves oscillate is so ridiculously tiny it is almost impossible for a human to even conceive. A second point I want to make about the visible light spectrum is that there are infinitely many possible wavelengths or frequencies between any two entries in this table. There are infinitely many numbers between 428 and 484, so there are infinitely many possible frequencies between 428 THz and 484 THz. This does not mean that there are infinitely many colors (in the perceptual sense) between red and orange in the rainbow, though. Human eyes do not have the ability to discern subtle differences between frequencies. A 428 THz wave and a 428.5 THz wave might be the same to our view. Somewhere between 428 and 484 THz we would start to perceive orange instead of red (under typical daylight conditions), 188 COLOR but the boundary is not sharp, and it is dependent in part on how we have been culturally trained to distinguish one color from the next. A third and final point on the visible spectrum is that the rainbow – the set of possible wavelengths between 390 and 700 nm – does not account for all colors that a human can see. Pink, purple, brown, white, grey-green... the majority of colors in our actual environment are not rainbow colors. Light of a single wavelength is referred to as monochromatic, but color phenomena usually involve polychromatic light containing many wavelengths. A different sense of the word spectrum comes into play when we want to analyze polychromatic light. As we discussed in the previous chapter, most of the time light waves are complex and "contain" light of many wavelengths at once. If we want to talk about the wavelength or wavelengths of this light, it is rarely practi- cal to list every component. Instead, we typically use a graphical representation that shows the amplitude of each component as a function of wavelength. Such a graph, or the associated concept of wave components organized in this way, is called a spectrum. A spectrum in this sense refers to something concrete and measurable about real light, rather than referring to an abstract organizational scheme. If we take real light and use a prism or spectrograph to separate out its components, we can individually measure the brightness of each, and construct a spectrum graph. The horizontal axis of the graph usually shows wavelength, often covering only a restricted range of possibilities, like the range of wavelengths that are visible to humans. For the graphs in this chapter, the vertical axis will be labeled "intensity," which is something we measure by seeing how bright the light is when it is re- ceived in a sensor. I will show (and discuss) numerical values for the wavelength but leave off numerical values for the intensity, since the important points I want to make relate only to comparisons of intensity, not absolute measurements. To give an example of how we might use such a graph, the sketch below repre- sents light from a monochromatic light source producing 580 nm waves. SPECTRA 189 Monochromatic light is the idealized version of a light that is purely "one color," really just one wavelength. In this case, the light (with a wavelength of 580 nm) would most likely be perceived as "yellow," in typical viewing conditions. On the opposite extreme, "white" light is often described as the idealized concept of light that is all visible wavelengths mixed together in equal amounts. That concept might be represented with the spectrum below. While this graph works to represent the concept of white light, no natural or artificial light source ever has such a spectrum. In fact, light sources that we perceive to be white can vary tremendously in their spectra, and the details actually provide some clues about the way that human visual perception works. In the next section we will compare spectra for real-world white light sources and use this to begin to unravel the physics of human color vision. 190 COLOR Let’s take a look at some real-world "white light." Consider the spectrum of sunlight, shown below in a graph that compares the spectrum measured above the atmosphere and the spectrum measured at sea level. Dotted lines delineate the visible range. This spectrum graph, which is based on measurements of light from the sun, is a lot more complicated than the idealized spectra shown in the previous section. The wavelength axis of the graph extends to the ultraviolet and infrared, because quite a lot of the light that the sun produces is not in the visible range. However, the peak brightness of the sun is in the visible range (between the dashed lines), and not coincidentally: we evolved to interpret a world lit by this spectrum! The sensitivity of our eyes is thus tuned to match the range of wavelengths where sunlight, interacting with the objects around us, will provide the most information. I have overlapped two versions of the graph, one based on measurements taken from space (above the atmosphere) and one based on measurements taken on the earth, at sea level. We see from the graph that the atmosphere is not perfectly transparent. That is, some light is always reflected or absorbed, so that the in- tensity of the light at the surface of the earth is less than it is in space. Also, the transparency of the atmosphere depends on wavelength. This is especially true WHAT IS WHITE LIGHT? 191 outside the visible range of the spectrum, where some wavelengths of light in the infrared are almost wholly absorbed before the light reaches the earth. Across the visible range, there is some bumpiness to the solar spectrum, meaning that it does not quite resemble the idea of "perfect white." Moreover, as you might imagine, exact atmospheric conditions in a given context can change things substantially. The graph shown here is an average, for somewhat idealized daytime viewing conditions when the sun is overhead. Depending on factors like weather, pollution, or time of day, there could be significant variations in the real spectrum of sunlight we might measure. For example, during morning and evening, our viewing perspective means that sunlight must pass through more of the earth’s atmosphere to reach us than it does at midday. The graphs below zoom in just on the visible-light component of the overall solar spectrum, and compare a noon-time spectrum to an evening-time spectrum. In the middle of the day, there is comparatively more light in the short- wavelength part of the spectrum. When the sun is on the horizon and sunlight passes through a lot of the atmosphere before it reaches us, these wavelengths of light tend to be scattered or filtered out. The light we receive is comparatively skewed towards the longer-wavelength end of the spectrum. These differences in lighting conditions account for the different comparative colors that a camera would record when viewing a "white" piece of paper in mid-day compared to evening. Thus, the exact properties of "white" sunlight vary quite substantially depending on conditions. But there are even greater variations in the types of "white" light that we encounter in the built environment. Below are spectra typical of three common types of interior lighting: white LED lights, fluorescent overhead lights, 192 COLOR and incandescent bulbs. These spectra look nothing like one another! They also look nothing like the solar spectrum, and nothing like the idealized concept of "white" light presented in the previous section. Thus, the concept "white," if we try to apply it as a descriptive term for light, is ambiguous at best. We cannot even fall back on the notion of white light "containing" all wavelengths of visible light, since there are clearly cases (like the light from white fluorescent lamps) in which broad bands of visible wavelengths are wholly missing from the spectrum. The only thing that all these forms of white light have in common, in the end, is that a human perceives them to be similar. And that boils down to the specialized properties of the light-sensing apparatus that is built into a bio-typical human CONE CELLS IN THE EYE 193 viewer. Below is a sketch of the human eye, which we first saw in Chapter 1. The zoomed in portion on the right highlights the structures in the retina, where light is absorbed and visual information is processed to send to the brain. The pointy things on the far right of the sketch illustrate light sensing cells called cone cells, which normally come in three types for detecting light of different wavelengths. The three types are annotated S, M, and L, for "short", "medium", and "long." The cone cells themselves do not differ much in size; the names relate to the wavelengths that stimulate each type of cone cell the most. S cones respond most to short-wavelength visible light, M cones to medium-wavelength visible light, and L cones to long-wavelength visible light. In between the cone cells are the rods, which are also light-sensing cells. The rods are all of a single type. They are sensitive across the full visible range, and give us the ability to discern differences in brightness even in low-light conditions. Individuals vary substantially in their retinal anatomy. For example, someone who is colorblind may lack one type of cone, or have a cone type that behaves atypically. Some people have far more of one type of cone than the other two. Some people even apparently have an extra type of cone cell, which may give 194 COLOR them enhanced color vision. Given the intrinsic impossibility of replicating subjective experience, it can be difficult to tell to what degree these variations change perceptual experience. What we can do in a rigorous, controlled way is ask people to sort and discriminate objects based on comparisons of their color. On the basis of these kinds of tests, we know that individuals who lack one or more of the cone types cannot differentiate as many colors as those with three cone types. But among individuals with all three (and in some cases four) cones, there is a great deal of consistency in how people sort colors despite large variations in retinal anatomy. This is likely due in part to the culturally-specific training we give young children in recognizing colors and assigning their names. However, the relatively "objective" consistency in how people typically sort colors says nothing about whether their subjective experience of individual colors is similar at all. Whatever the distribution of cell types in a given eye, a normal retina is densely packed with millions of rods and cones. Thus light from even a pencil-point-sized dot in the nearby visual field will fall on an area of the retina that can process it with multiple sensors at once, including all four sensor types. (If an object is so small in our visual field that light from that object only falls on one sensor cells, we will typically still see it but be unable to determine its color.) It is the combined pattern of response from a set of nearby retinal sensors that we use to construct the experience of color. When I use the word "response" here, what I mean is that when light hits the cone cells, they will absorb some wavelengths more efficiently than others. This takes place through the same kind of resonance process that we discussed in the previous chapter. Inside each cone type, there are pigment molecules. Again, pigment molecules are molecules that tend to vibrate resonantly in response to certain wavelengths of light, largely ignoring other wavelengths. When light energy is absorbed in this way, it is transferred into motion energy on a molecular scale. This can trigger chemical changes and tiny electrical currents, which are the kinds of things that constitute neural signals. Thus, selective absorption of some wavelengths of light in a cone cell can generate a signal that the brain receives and processes as part of color vision. Just as we can use a spectrum graph to show how the intensity of light varies with wavelength in a given scenario, we can draw similar graphs to show the degree of response or sensitivity each cone cell type has to light of different wave- CONE CELLS IN THE EYE 195 lengths. Again, I will leave numbers off of the vertical axis, since the important information for our purposes is a comparison of relative sensitivity. What we want to represent in such graphs is the wavelengths that make the pigments in each cone cell respond a lot, versus wavelengths that do very little. While it is an oversimplification, I like to think about the cone cells jiggling when light hits them, analogous to the vibrating paperclip pendulum example we discussed in the previous chapter. I think about this graph describing how vigorously each type of cone cell will jiggle when it is struck by light at each visible wavelength. S cones, then, jiggle a lot in response to light at around 450 nm, but are almost wholly unaffected by light around 500 nm. M and L cones have very similar behavior, but if the incoming light is 550 nm in wavelength, the M cones will jiggle more than the L cones. If the light is longer wavelength than 550 nm, then the reverse is true. This graph is based on measurements of the absorption of light by the pigments within real cone cells. From the graph, we can read off the pattern of cone cell responses to any given stimulus. Take, for example, monochromatic 580 nm light, like the kind graphed earlier in the chapter. Receiving such light, both the M and L cone types would respond quite strongly, but the S cones would not respond at all. Incoming 530 nm light would make the M cones respond more than the L cones, perhaps causing the S cones to vibrate just a tiny bit. By comparing these distinct patterns, the eye-brain system can tell the difference between the two 196 COLOR experiences perceptually. We would call the 580 nm light "yellow" and the 530 nm light "green." Looking back at the spectra for sunlight and other so-called white light sources, the thing they all have in common is that they stimulate the set of three cone types in a roughly similar way. The experience of perceiving white is an experi- ence of "all of my cone cells are responding at the same time." This experience can be created by light that is bright across the whole visible spectrum, but it can also be created by various combinations of monochromatic light sources tuned to trigger a similar response in the three cone types. There are substantial physical differences between the light from a fluorescent lamp and the light from the sun, but the perceptual differences are subtle because they both create comparable response patterns in the eye. Imagine that the average human eye came equipped only with rods. We would still have the ability to see light across almost the whole visible spectrum. We would not be blind, but we would be colorblind. We would be able to discriminate lightness and darkness in a general sense, but we would have no way to tell the difference between a 580 nm stimulus (normally called yellow) and a 480 nm stimulus (normally called blue). The two experiences would be perceptually ambiguous. Humans, and most animals, evolved to be able to visually identify distinct ob- jects in our environments. To do so we need to be able to draw contrasts in our visual field where one object ends and another object, shadow, or background surface begins. Objects vary in how much light they reflect overall, but you get more information if you can distinguish different ways that those objects affect light across the whole spectrum; that is, if you can sense color. Single-sensor vision seems to have too much ambiguity for navigating our complex visual en- vironment successfully, thus many organisms that are active during the daytime evolved to see with sets of cones in addition to rods. Many animals, and some humans, see color using two cone types. This is called dichromatic vision. With two distinct cone types (perhaps analogous to the , a 580 nm stimulus and a 480 nm stimulus might now be perceptually distinct, AMBIGUOUS COLORS 197 and might be labeled yellow and blue. However, the colors that most humans label as red and green might remain ambiguous, depending on how the cone cell response curves overlap. For humans, individuals with dichromatic vision are considered colorblind. Humans, some primates, and a few other animals evolved to see colors predom- inantly through trichromatic vision, with three distinct cone types to distinguish colors. With trichromatic human vision, red,yellow, orange, green, blue, and violet all become visually distinct from one another. The human eye becomes capable of discriminating and visually organizing millions of different colors in our environment, which is perhaps a hundred times more color variety than a dichromat can distinguish. Trichromacy makes the world a much more colorful place, but it does not mean that a person or animal sees every color that is available to see in the world. Many animals (and possibly some people, as I mentioned) have four or even more cone types. Some animals also supplement cone-cell-based vision with additional ways of sensing light that have other functions. Insects, sea creatures, and birds often have vision that extends into the ultraviolet or infrared. Any of these animals can see more colors than a human, and some of them can see features of the world that are wholly invisible to us. 198 COLOR Comparing the spectral sensitivities of cone cells for a few different animals shown in the sketch, we can see that a bee will have a much more difficult time differentiating shades of red and orange than a human, because they only have a single type of cone cell covering the longer-wavelength portion of the visible spectrum. But, they will see colors the human eye cannot see at all, extending into the ultraviolet. To people, the dog is …