Transcript Pinhole Cameras

PINHOLE CAMERA
What do we need to know?
•
Light travels in straight lines
•
A pinhole is a tiny hole
CAMERA OBSCURA
•
Camera Obscura (latin) > Dark Room
•
Projects a view of the outside onto the inside walls
•
The image is upside down
The history…
5th
Century
BC:
Collecting
Place /
Locked
Treasure
Room
Mo-Ti
322 BC:
965 AC:
Aristotle
Alhazen
1500s:
Leonardo
Da Vinci
Pinhole:
Small Hole
>> Sharper
Image
Bright
Object
Small
Hole
Light
reflects
off bright
surface
Pass
through
small
holes
Bigger Hole
>>Brighter
Image
Light rays
cross
Upside
down
image on
wall
The history…
Leonardo Da Vinci:
"When images of illuminated objects ... penetrate
through a small hole into a very dark room ... you
will see [on the opposite wall] these objects in
their proper form and color, reduced in size ... in a
reversed position, owing to the intersection of the
rays". Leonardo da Vinci
• Used for observing eclipses
1558 > Lenses and mirrors
1558: Lens Based
Camera Obscura,
1568
Correct for
reversed image
1700: Hooke described ‘picture
box’ in a paper to the Royal
Society
1558
1568
1700
1728: Reversed the
specimen to overcome
upside down image
1728
The history…
Translucent screen +
45° mirror: Override
two inversions of
image
1700 –
1900
1898
Silicon Image Detector, 1970
1970
The history…
• Kodak handheld
camera
•
1888
Camera sent back to Kodak
for processing
• Digital Cameras/ SLR
• Reflect IR beam off subject +
focus lens
J-Phone:
• Cost: $ 500
• 110,000 pixels (0.1 MP)
1959
2000
Ray diagrams:
Making a pinhole camera …
•
It can be very simple!
•
Dark box or tube + a tiny
hole in the front
•
We’ll use a screen for
viewing on the back
instead of film
STEP 1
•
Roll a tube of black card
OR
•
make a box with two open
ends
STEP 2
•
Use the tinfoil to cover one end
(Blocks light from entering)
STEP 3
•
Make a tiny pinhole in the
front
STEP 4 4
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Add a screen to the back
•
The image will appear here
STEP 5
•
The image will be dark so
you’ll need to cover the
screen
•
Make a cover that wraps
around the screen.
TEST IT!
Problems with pinholes
image plane
r  ( x, y , z )
y
optical
axis
effective focal length, f’
z
pinhole
x
r '  ( x' , y ' , f ' )
r' r

f' z
x' x

f' z
y' y

f' z
Problems with pinholes
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Pinhole size (aperture) must be “very small” to obtain a clear image.
•
However, as pinhole size is made smaller, less light is received by image plane.
•
If pinhole is comparable to wavelength of incoming light, DIFFRACTION
effects blur the image!
• Sharpest image is obtained when:
pinhole diameter
Example: If f’ = 50mm,
λ = 600nm (red),
d = 0.36mm
d 2
f '
Image Formation using (Thin) Lenses
• Lenses are used to avoid problems with pinholes.
• Ideal Lens: Same projection as pinhole but gathers more light!
o
i
P
P’
f
Gaussian Lens Formula:
1 1 1
 
i o f
• f is the focal length of the lens – determines the lens’s ability to bend (refract) light
• f different from the effective focal length f’ discussed before!
Two Lens System
d
final
image
object
f2
i2
o2
i1
f1
o1
image
plane
lens 2
intermediate
virtual image
lens 1
• Rule : Image formed by first lens is the object for the second lens.
• Ray passing through focus emerges parallel to optical axis.
• Ray through optical center passes un-deviated.
• Magnification:
i2 i1
m
o2 o1
The Eye
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The human eye is a camera!
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Iris - colored annulus with radial muscles
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Pupil - the hole (aperture) whose size is controlled by
the iris
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Photoreceptor cells (rods and cones) in the
retina
PINHOLE CAMERA
How to Build It