Lab 4: Optics this week. HW 9: Read through before lab. Lab will help you better understand creating images and how lenses work. Topics: This week –Finish cameras, Digital Cameras –Optical communication We want to have a special expert guest lecturer on Friday, April 23 rd, 3pm. How eye works, visualization, science behind optical illusions, etc… This class would be substitute for class on Thursday, April 22 nd. Class points as usual for attending Friday lecture. If you can’t make it, we will provide alternative means (equal time) for earning those points: a. I can make it on Friday’s at 3 pm. b. I cannot make it on Friday’s at 3 pm.

A and B’s C’s D’s

The Basic Idea Object Distance Image Distance Object Lens With focal length f Shutter/Iris Film Dark Box

Put film, Retina here! Lens bends the light. Focal length (f) Send in parallel rays from distance light spot: Distance to focus is focal length Focal length (f) More curved lens is… More bending power Shorter focal length Focal length: the shorter the focal length, the more bending power

Lens equation: Distance to object Focal length  Distance to image o f 11 1  i f f Object Distance (o) Image Distance (i) f f compound lenses- same basic physics, but 1) hard to make simple piece of glass to perfectly match Lens eq. (specially for different colors). Easier to stick two pieces together. 2) Multiple lenses allow more adjustments- bigger Mag., same object and image distance (zoom lens), really short f. l., big M (microscope)

(o) (i) f f What will happen to image if we increase focal length of lens? a.Image is same size, same place b.Image is same size and further from lens c.Image is bigger and further from lens d.Image is smaller and closer to lens Lens equation: (o) (i) f f Answer is c: Like Zoom Lens. Extra-special lens w/ variable f, increase f to zoom in, decrease to zoom out Increase f  Less bending power  Further distance to image  Bigger image.

(o) (i) f f What will happens to brightness of image? a.Less bright b.More bright c.Same brightness Lens equation: (o) (i) f f Answer is a: Same amount of light captured by lens, but now spread over larger area… less light per area… dimmer. How can we compensate for less light per area of film?

(o) (i) f f Lens equation: (o) (i) f f How can we compensate for less light? 1. More light sensitive film 2. Larger lens / Open Iris all the way 3. Longer exposure time 4. Use Flash

Two objects different distances from lens f f Screen at Focus of What will happen at screen if close down iris in front of lens? a.Images brighter and object 1 more in focus b.Images dimmer and object 1 more in focus c.Images brighter and object 1 more out of focus d.Images dimmer and object 1 more out of focus

Depth of Focus (a demo) Two objects different distances from lens f f Light Rays from “1” diverging behind focus point. Rays through outer part of lens diverging strongly. Closing down iris blocks rays that are diverging most, Improves depth of focus. ( squinting – improves vision. Nighttime – iris open, poor vision) Screen at Focus of 2. Answer is b.

Digital vs Film Cameras Mechanical Electrical Shutter Mechanical Iris Object Distance Image Distance Object Lens With focal length f Film Electronic Light Sensor (semiconductor device) Dark Box Battery Required! You have already seen same physics principles in other applications: Lens/Optics, Color/Light, Resolution/Pixels, Semiconductors, P-N junctions, capacitors

Film vs Electronic Sensor Object Distance Image Distance Object 24 mm 36 mm FILM Electronic Light Detector (MUCH SMALLER) 6.6 mm 4.4 mm With 35 mm camera, stand 15 ft away from tree, whole tree just fills film. How should the lens system be set up in the digital camera so that when you are standing 15 ft away from tree, the whole tree just fills the digital camera’s detector: a.the lens and lens-detector distance must be identical to that of the 35mm b.the lens curved more and the lens- detector distance shorter c.the lens curved less and the lens-detector distance shorter d.the lens should curved more and the lens- detector distance longer

Film vs Electronic Sensor How should the lens system be set up in the digital camera so that the view captured is the same as the 35 mm camera when I am about the same distance away from the subject: Answer is b. The lens curved more and the lens-detector distance shorter. Need more bending power (curvature). To focus entire image in smaller area. (o) (i) f f (o) (i) f f Digital Camera: Lens has more curvature  More bending power  Rays converge to focus faster  smaller image Film Camera: Lens has less curvature  Less bending power (refraction)  Rays converge to focus slower  larger image

Semiconductor Device (converts light to electrical charge) Most digital cameras use: charge-coupled device (CCD Array) The Electronic Detector 4.4 mm (1200 pixels) 6.6 mm (1600 pixels) Lots of tiny light detectors (each 1 pixel) The more light photons that hit, the more electrical charge builds up on the pixel Cannot discriminate between colors… all colors treated equally

To capture information about color: put colored filters in front of each pixel Transmit 100% of Orangeish-yellow Transmit 25% of Reddish-orange

To capture information about color: put colored filters in front of each pixel Question: If the camera is taking a picture of an orange, light will be detected by: a. blue pixels b. red pixels c. green pixels d. both b and c e. both b and a Answer is d. Both green pixels and red pixels. Red filters will pass larger fraction of orange light than green filters will. Ratio of light detected/recorded is indication that color was orange.

Interpreting color Computer determines true color at center pixel is determined by looking at amount of light collected at surrounding pixels… that way you have info on red, green, and blue.

Resolution 24 mm ~”2000” pixels 36 mm (~3000 “pixels”) FILM 6.6 mm (1600 pixels) 4.4 mm (1200 pixels) Traditional film (~200 iso) has effective resolution of ~3000 x 2000 pixels ….Captures more detail than most CCD arrays. Question: To improve the amount of detail captured by the digital camera, you need to: a.Increase number of pixels on the CCD array b.Increase the physical size of the CCD array, keeping the total number of pixels the same c.Both a and b. 2.1 million pixels 2.1 Megapixels Answer is a. actually chunks of chemical

Optical Zoom vs Digital Zoom Optical Zoom: Real Image is larger on array Digital Zoom: Real Image is same size on array, uses subset of pixels and then guess at detail…. NO EXTRA INFORMATION.

A pixel of the CCD (Charged-coupled Device) Array Energy  Thin Insulator Thin Conductor P-Type semiconductor N-Type Excess negative charges Excess positive charges Depletion Region Question: What will happen if apply a positive voltage at A? a.Nothing, no electrons can move b.Electrons in N-type will flow to the right and pile up at insulator until charges balance c.Electrons will flow from P-type to N- type and pile up at insulator until charges balance d.Electrons will flow continuously from P to N to A. A