This is due to Purkinje shift. At scotopic conditions, we are more sensitive to blue than to red.

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1 1. We know that the color of a light/object we see depends on the selective transmission or reflections of some wavelengths more than others. Based on this fact, explain why the sky on earth looks blue, but the same sky on moon looks black. Why? The wavelengths of light transmitted or reflected by the moon s atmosphere is different than those of the earth s atmosphere. Moon s atmosphere probably transmits more of the longer wavelength and hence the red appearance. 2. Your carpet is stained and you are trying to clean it. Often you will find yourself concentrating on the edges of the stain and trying to remove it rather then the whole strain. And it works. A casual viewer finds it difficult to locate the stain when the edge is removed. Why? Visual system is more sensitive to edges. We have edge feature detectors. So, when the edge is removed it is difficult to see it. Also, our contrast sensitivity function is less sensitive to both very low and high frequencies. 3. The preferred color of the runway lights (used usually for nighttime landing and taking off) is usually blue, and not red. Why? This is due to Purkinje shift. At scotopic conditions, we are more sensitive to blue than to red. 4. Cataracts are tissues that develop clouding the eye s lenses. Cataracts mostly happen in the old age. But there are also juvenile cataracts that occur in babies and often are existent from birth. In 1960s, when the art of cataract removal perfected, they operated on old people and cured them. They also operated on a bunch of young men, who had juvenile cataracts. But this did not cure the cataracts. Why? The cataract in older people was probably due to age. So, these people had in their earlier life spend some time without cataract and learnt how to see with them. But the juvenile patients essentially were never exposed, and as we know from selective adaptation experiments with kittens that they became selectively adapted to seeing a skewed view of the world with the cataracts. 5. The response, R, of human eye to intensity of light, I, can be described as R = I 1/2. The following experiment is used to correct the gamma of any display. Create a image with rows of gratings of the form Sin 2 (x) with increasing values of x along the vertical axis. Display this image on your monitor. You will be seeing some interference patterns if your gamma is not correct. These are called Moire patterns. Adjust your gamma until the moiré patterns disappear. This is the correct gamma setting for your display. Explain this phenomenon. (Hint: Think about linear systems and how they behave).

2 A linear system passes sine waves without changing its phase or frequency. The square of a sine wave is related to the sin(2x) linearly. You want the combination of your display and your eye to be linear. Till you get that correct combination, the sin waves will be distorted in frequency creating interference patterns. When you find the correct gamma suited for you, the linear system will pass them correctly and the Moire patterns will disappear. This is actually a test used to correct the gamma of the display in the image production industry. 6. The picture on the left appears like ridges in the sand. Now, invert page and look at the same image, it will look like steps. Which perceptual phenomenon can explain it? We are used to having light from above. 7. Let the (x,y,y) representation of color C 1 be C 1 = (x 1,y 1,Y 1 ). Find the XYZ coordinate of a color C 2 that has chromaticity coordinates as half of the chromaticity coordinates of C 1 and luminance 1.5 times that of C 1. C 2 = (x 1 /2, y 1 /2, 3/2 Y 1 ) From this you can find I 2 = 3Y 1 /y 1 Hence, x 1 /2 = X 2 /I 2 From this, X 2 = 3x 1 Y 1 /2y 1 Z 2 = I 2 -X 2-3/2Y 1 = 3/2Y 1 (1/y 1 -x 1 /y 1-1) 8. C 1 and C 2 are colors with XYZ coordinates (50,75,10) and (25,50,100) respectively. Find the XYZ coordinate of a color C 3 with same luminance as C 1 and chromaticity coordinate half way between C 1 and C 2. How would this color look? C 1 in (I,x,y) notation is (135, 50/135, 70/135). C 2 in the same notation is (175, 25/175, 50/175) Y 3 = 75. x 3 = ½(50/135+25/135) = 0.25 y 3 = ½(70/135+50/175) = 0.4 I 3 = Y 3 /y 3 = X 3 = I 3 *x 3 = Z 3 = I 3 -X 3 -Y 3 =

3 9. What is the dominant wavelength of a color with chromaticity coordinate (0.4, 0.5)? What is the chromaticity coordinates of this wavelength? What is the chromaticity coordinates of the complement of this color? Solve this from the chromaticity diagram. The dominant wavelength is roughly between nm with chromaticity coordinates of (0.41,0.59). The complement is around ( ). 10. Look at the image on the right for 30 seconds or more and then shift your gaze to a white paper. What do you see? What is this phenomenon called and why does it happen? This is the standard afterimage phenomenon due to adaptation of the opponent color cells. 11. The image on the right shows six different objects A, B, C, D, E and F, which have different reflectance. It also shows the gray scale values at the edges. Assuming that the illumination is continuous throughout the image, find the reflectance of B,C,D,E and F with respect to A. How would you verify that the illumination is indeed continuous? Retinex theory says that we combine ratios across edges to calculate the ratios of intensities of regions which are spatially separated. So, just find a path and calculate ratios across the edges. For example, to find F with respect to A, you can use a path of A- B-C-F and that will give you: A:B = 1:3, B:C = 3:2. Hence, A:C = 1:2. Then C:F is 1:3. Hence, A:F is 1:6. Note that it will be consistent if you take a different path. For example, if you take the path of A-B-D-F, you will get 1/3 * 2/3* ¾ which is also 1/6.

4 12. The figure on the right shows an image segmented into three parts, A, B and C based on the detection of edges between them. The XYZ coordinates of the colors on the both sides of the edge between A and B are given in blue. The same for the edge between B and C is given in red. Which of these two edges do you think is due to reflectance and why? The proportions of X:Y:Z across the B-A edge is same, 1:2:2. So, most likely this is an illumination edge since the color of the object is not changing. On the other hand, for the opposite reason C-B edge is probably more likely to be a reflectance edge. 13. A person was presented an annular ring of luminance of 100cd/m 2 with a circular inset of 50cd/m 2. In an adjacent region, he was presented with a circular inset of 150cd/m 2 and asked to adjust the luminance of the annular ring surrounding it so that it creates similar perception as that of the adjacent pair. What luminance do you expect him to adjust to? In the first outside:inside = 2:1. We match the ratios. So, for an inset of 150, we will match the outside to A two month old was being very jittery and crying in a room lit by a whitish tube light. In order to pacify him, the mother suddenly puts on a very bright yellowish light. The kid suddenly finds renewed interest in everything around him and stops crying. Can you explain this? Till the kid adapts chromatically, the environment looks entirely new to him and creates interest. 15. You are going on a train. The track is lined by electric poles placed regularly along the track so that it is just a few feet away when you look out of the window. When the train just starts you can make out these poles clearly as they keep moving away from you. As the train increases speed, slowly the poles become blurred as they move away from you. Can you explain this phenomenon with CSF? Our temporal CSF has low sensitivity for high frequency. Initially, when the speed of the train is low, temporal frequency of the poles are low and hence we decipher them. As the temporal frequency increase with speed, we can no longer decipher them.

5 16. In an movie auditorium, the projection system is used with mean luminance of foot Lamberts for refresh rates of 48 Hz. However, it is said that a refresh rate has to be increased to 60 Hz for higher luminance projection system. Can you explain this with CSF? As per our temporal CSF, our sensitivity to higher frequency increases with higher luminance. So, for us not to be able to perceive a bunch of frames as a motion and not separate frames, we have to increase the temporal frequency as the light increases, so that now this raised frequency is well beyond our sensitivity. 17. Let the maximum spatial frequency detected by a human at luminance L be 30 cycles/degree of the angle subtended on the human eye. Nyquist sampling condition says that to generate a spatial pattern of a certain frequency f on a display, at least 2f number of pixels are required. From this find out the minimum display resolution needed for a person at a distance d, so that he can feel the experience of seeing a natural scene on the display. Plot the r with respect to d. We know f <= d*r*pi/180*s From here r>= 180*2*30/d*pi So, r is inversely proportional to d. It is a hyperbolic plot. 18. A bright lighthouse is not visible in the day, but is easily visible at night even though the power of the light remains unchanged. Why? This is due to Weber threshold. Due to high intensity of sunlight, the lighthouse is not sufficiently above sunlight for us to detect it. But at night, due to low surrounding light, it crosses the threshold and we detect it. 19. The image of your TV looks washed out. The technician says that the intensity response curve of the TV is linear and hence the problem. To correct the problem, he has to make it non-linear. Why? What kind of non-linear response do you think he will put in? The linearity of the TV will make the combined response of TV and eye as I A where A < 1.0. This is a compressive response and you would feel darker colors are washed out. But if you make the TV as I 1/A, then the combined will be linear, reproducing the correct grays. 20. We know that the color of a light/object we see depends on the selective transmission or reflections of some wavelengths more than others. Based on this fact, explain why sunsets and sunrises in polluted areas are found to be much more spectacular than other area. Hint: Sunlight passes through the environment before reaching the eye. Same argument as Q1.

6 21. What will be the difference of our perception of Hermann s grid if we had lateral excitation instead of lateral inhibition (i.e a center surround receptive field that has inhibition in the center and excitation in the surround)? Ish sent this to the class when we covered this part. 22. Why do we see better in the dark if we avert/skew our gaze a little? By not looking directly, we use the rods more than the cones. Rods are more sensitive in the dark. 23. Do the following experiment. Look at the above picture. Then cover your left eye with hand for 10 minutes. So, now you are looking at the image with just your right eye. After 10 minutes, uncover your left eye and cover your right eye. Compare what you see with what you were seeing with your right eye before. You will see that the blue flower looks brighter from your left eye than from your right eye. Why does this happen? Ignore this question. The experiment was designed to show the effect of Purkinje shift. But I did not explain it properly. What you are fixating at is important which was not clear from the question.

7 24. A bunch of kittens were reared from birth by seeing only vertical stripes. They were in an enclosed area with just stripes all around them. They wore neck ruffs so that they cannot turn their heads to see the vertical stripes in a different orientation. After this, they were compared with normal kittens. What difference would you expect in the sensitivity of these kittens from the normal kittens? Why? Which area of the brain is responsible for this difference and how? I think you will see selective adaptation to vertical stripes. Also, they will not learn to see other orientation of stripes. So, they will suffer in detection all kinds of orientations. This will probably take place in the visual cortex. 25. The left picture is a simple line drawing of a face while the right image is a low resolution picture. The right hand side image has a lot more information, yet we perceive the face of the man with the line drawing much better. What does it say about our visual system? What are the different areas and cells in the visual system that the left image would stimulate? We detect features, especially edges. These are probably triggering the feature detectors in the higher visual areas in the brain.

8 26. The color gamut of a printer is given by triangle ABC on the chromaticity chart where A = (0.15, 0.65), B = (0.6, 0.3) and C = (0.15, 0.125). The color gamut of a monitor is given by triangle DEF on the chromaticity chart where D = (0.2, 0.05), E = (0.55, 0.4) and F = (0.05, 0.55). How many sides does the polygon that represents all the colors that can be produced both by the printer and the monitor have? Find the coordinates of all the vertices of this polygon. Draw on the chromaticity chart. You will have the common region to be a 5 edged polygon. Finding the coordinates is essentially finding the intersection of edges between two gamuts. 27. You have difficulty in reading very small text on the screen. However, this difficulty reduces as you increase the size of the fonts. Explain this using CSF. When fonts are small, they subtend higher frequency in the eye. We are less sensitive to higher frequency as we know from CSF. As font size increase, the frequency subtended reduces increasing our sensitivity and hence legibility of the text. 28. You are working in your office which receives good sunlight in the day time. Around dusk when it is getting dark, you find that it is difficult to read the text on the book. You light your table lamp and the text is again legible again. Explain this phenomenon with CSF. At low illumination, our eye acts like a low pass filter, rather than a band pass filter, have a much lower frequency cut off. Hence, in dusk our legibility of text decresses.

9 29. In the image on the right, one X looks yellow and the other looks gray though they are the same color as you can see from the place where they are connected. What is this phenomenon and why does this happen? Simultaneous color contrast.