Don t twinkle, little star!

Transcription

1 Lecture 16 Ch. 6. Optical instruments (cont d) Single lens instruments Eyeglasses Magnifying glass Two lens instruments Microscope Telescope & binoculars The projector Projection lens Field lens Ch. 7, Visual processing We are here 14 October, Don t twinkle, little star! Adaptive optics corrects for atmospheric turbulence, requires computer control. Adaptive optics is an additional mirror that can be deformed to correct the wave fronts that are blurred by atmospheric turbulence. Recall that you can see heat rising from a hot surface due to changes in the index of refraction with temperature. 1

2 Binoculars Simply folded telescopes for each eye. Internal reflections invert the image. 3 What does 7 x 50 binoculars mean? 7x is the magnification 50 is the diameter of the front lens (the objective lens) in millimeters 6 x 30 binoculars are easy to carry 7 x 50 binoculars are heavy, but these give a brighter image 15 x 80 binoculars need a tripod 4 2

3 35 mm slide projector Field lens is used to put the most light on the slide. Field lens or condenser Color slide Projection lens Mirror The condenser focuses the light from the filament through the projection lens to increase brightness. 5 Viewgraph projector Mirror Projection lens Fresnel lens (condenser) and viewgraph location Curved mirror returns light otherwise lost 6 3

4 Ch. 7 Visual Perception Parts of the visual processing system Retinal processing: Lateral inhibition Hermann grid Receptive field Motion illusion Craik O Brien illusion & simultaneous lightness contrast Lightness and brightness Other optical illusions We are here Next time 7 Components of the visual processing sytem Photoreceptors (rods and cones) Horizontal cells (connect between photoreceptors Ganglion cells Bipolar cells & Amacrine cells (connect between ganglion cells) Axons Optic nerve Optic chiasma Visual cortex of the brain 8 4

5 Brain anatomy Visual cortex is where you see Brain damage at this location hurts vision. Optic chiasma Left field of view goes to right brain Right field of view goes to left brain from both eyes 9 Optic chiasma Brain damage on the left side hurts vision of things on the right side. 10 See text fig

6 Processing without thinking The retina is wired to quickly recognize lines and shapes. Motion is quickly recognized. The brain is wired to quickly recognize faces. 11 Channels A channel is a subsystem in the brain that responds to specific features Motion channel, keeps firing after watching a moving object, causing motion aftereffect. Size channel, size is judged relative to other objects. Large people make you look small. This is called simultaneous size contrast. Orientation channel, the surroundings (a floor) are used to help you determine orientation. A stream can look like its flowing uphill if the surroundings slope down more steeply. 12 6

7 Connections Photoreceptors: rods and cones connected to the bipolar cells connected to the ganglion cells, funnel data through axons into the optic nerve sideways connectors (these help with analysis) horizontal cells, next to the photoreceptors amacrine cells 13 Layers of the retina Light 14 7

8 Layers of the retina 15 See text fig. 7.2 Layers of the retina From the following article: Neurobiology: Bright blue times Russell G. Foster Nature 433, (17 February 2005) doi: /433698a a, The rods (R) and cones (C) convey visual information to the ganglion cells (G) through the bipolar cells (B). Horizontal cells (H) allow lateral connections between rods and cones. Amacrine cells (A) allow lateral connections between bipolar and ganglion cells. The optic nerve is formed from the axons of all the ganglion cells. A subset of ganglion cells (MG cells) also detects light directly; for this, they require the photopigment melanopsin, as now confirmed 1, 2, 3. b, Light, via melanopsin, causes changes in Ca 2+ levels in MG cells 9 (a fluorescent Ca 2+ indicator was used here). Counterintuitively, light passes through the transparent ganglion layer to reach the rods and cones. 16 8

9 Layers of the retina 17 Retinal processing Amacrine and horizontal cells turn down the signals from areas adjacent to bright areas. See text fig

10 Receptive field center and surround are lit up separately Yellow is light and blue is dark See text fig Receptive field (again) The yellow is the region receiving light. See fig

11 Edge detection is enhanced Half illumination gives bigger signal 21 Simultaneous lightness contrast Craik O Brien Illusion Contrast at the edge affects your perception of the center. Are the small gray patches below identical? See fig

12 Craik O Brien Illusion Simultaneous lightness contrast These are the patches without the surround. 23 Here is a related illusion involving lateral inhibition Each vertical band has equal light intensity across its width. However the left side of each bar appears darker than the right side due to lateral inhibition at the edges One side is next to a lighter bar while the other side is next to a darker bar. less inhibition (looks lighter) more inhibition (looks darker) 24 12

13 Simultaneous lightness contrast (again) Checker shadow illusion Which square is lighter in shade, square A or square B? 25 Simultaneous lightness contrast Checker shadow illusion Slide them together and compare. A is surrounded by light squares and B is surrounded by dark squares in the previous slide

14 Hermann grid illusion: dark areas are from lateral inhibition The red areas show the receptive field. Lateral inhibition is greater at 1 than at 2. The fovea has a smaller receptive field. So the lateral inhibition is the same everywhere in the white area

15 White space is larger than receptive field, hence no illlusion. 29 It is blacker away from a corner where there is more inhibition

16 31 The music A. Kitaoka 32 16

17 Does the center stripe have constant lightness? Or is the center stripe darker in the middle and at the ends? 33 The center stripe has constant lightness

18 Victor Vasarely, Zebras. The black/white boundaries outline the necks. There are no lines in this drawing! The artist has made use of the tendency of the eye to find lines. The regions of color don t have edges, but appear to. Picasso 36 18

19 French artist George Seurat used edge enhancement by lateral inhibition to make figures stand out sharply Lighter just before edge Darker just before edge 37 El Greco 38 19

20 The white crosses are an illusion. Victor Vasarely, artist. The edges of the squares seem lighter because of the dark surrounds. 39 Which creature is larger? Previous experience in tunnels tells us that the creature in back is further away, and hence must be larger

21 Previous experience interprets these flat images as being from 3-dimensional boxes. The shadows tell us what is a floor and what is a wall. Victor Vasarely, artist 41 A B Previous experience effect: Here, the eye is fooled into thinking the light is from above. The panel A has lots of light, so it must be really dark. But B must be lighter because it is in the shade

22 A B Previous experience effect: Here, the eye is fooled into thinking the light is from above. The panel A has lots of light, so it must be really dark. But B must be lighter because it is in the shade. 43 Contrast constancy effect The image at left has too much contrast, so the eye turns down contrast which makes the central gray squares more alike. On the right, the central gray squares look more different (higher contrast)

23 Lightness and brightness Brightness: amount of light Lightness: property of a surface newspaper = 0.65 (reflectance) printer paper = 0.84 photo quality paper = Lightness and brightness Lightness constancy: brain and eye correct for amount of light so that white, gray, and black look the same independent of brightness. Weber s law: we think lightness is equally spaced when the ratios are equally spaced Example: lightness 0.5, 0.25, look equally spaced. These numbers are ½, ¼, 1/8 etc. The spacing that looks equal is not 0.9, 0.8, 0.7, etc. Demo: gray sheets of paper 46 23