Week 12: Optical Instrumentation

Question 1

Human eye is comparable to the design of a photo camera name the comparability of these below: iris, cornea & lens, retina, brain

Answer 1

Iris = aperture stop
Cornea & lens = objective lens
Retina = photo sensor
Brain = image processing computer with algorithms

Question 2

In contrast to a photo camera….

Answer 2

• The eye is not centred at a common axis
• Performance of the ‘light sensors’ are different
  -The resolution of the camera’s detector is higher in the central retina than the peripheral retina

Question 3

Describe Entrance & Exit Pupils

Answer 3

• Cornea forms an image of the pupil (aperture stop)
• Exit pupil of the eye is the image of the same aperture stop formed by the lens;
• E is the centre of the entrance pupil and E’ is the centre of the exit pupil
• Marginal ray passes through the edge of the lens
• Chief ray passes through the centre of the aperture stop (pupil)

Question 4

Describe Visual axis

Answer 4

The line between the fixated point 01 and fovea by nodal points N and N’

Question 5

Describe Nodal ray

Answer 5

The angle to the ray’s incident on the optical axis is equal to the angle to the refracted part

Question 6

Describe Object & image distances

Answer 6

Are measured from primary & secondary principal planes

Question 7

Describe Cross section of the primary & secondary plane

Answer 7

At the optical axis are called Primary (P) and secondary (P’) principal points

Question 8

Describe Line of sight

Answer 8

Line between the fixated point 01 & centre of entrance pupil E

Question 9

Describe Pupillary axis

Answer 9

Line passing thorough entrance pupil & perpendicular to the corneal surface

Question 10

Describe Optical axis

Answer 10

Line connecting centre of refracting/reflecting surfaces, cornea, pupil, lens, retina

Question 11

Describe Kappa k

Answer 11

Angle between the optical and visual axis, usually 5°

Question 12

Describe Fixation axis

Answer 12

The reference axis for eye movements, determined by object point 01 & centre of eye rotation (C)

Question 13

Describe how speed cameras operate briefly

Answer 13

• Detect the speed of vehicles by using piezo electronic detectors embedded into the road surface or RADAR & LASER technology
• The piezo detectors deflect slightly when a vehicle is driven over the detectors, which then triggers an electronic device that accurately measures the speed of the vehicle

Question 14

Describe Fixed Speed Cameras

Answer 14

• Fixed speed cameras can monitor 1 to 4 lanes in the same direction
• The original recorded images are stored electronically on a WORM (Write Once Read Many) disk, which cannot be overwritten or altered

Question 15

What is Gatsometer Digital Rader Camera System – Parabolic (DRCS-P)?

Answer 15

• RADAR technology is used to measure how fast a vehicle is travelling & to trigger the camera into act
• If a vehicle is driving above the road speed limit then two photos are taken in quick succession
• It is a legal requirement to have a secondary measurement for speed

Question 16

Describe Photocells / Photoelectric Cells briefly

Answer 16

• They are also known as CdS cells or light-dependent resistors (LDR) or photoresistors
• Photocells are sensors that detect light
• They are small, inexpensive, low-power, easy to use and do not wear out
• Photocells are resistors that change their resistive value depending on how much light is shinning onto the squiggly face

Question 17

Photocells are useful for most light-sensitive appliances to detect…

Answer 17

• Is it light or dark?
• Is there something in front of the sensor (that would block light)?
• Is there something interrupting a LASER beam (break beam sensors)?

Question 18

List the Photo-Cells Types

Answer 18

1. Photoconductive
2. Photovoltaic
3. Photoemissive

Question 19

Briefly describe Photoconductive

Answer 19

• The front surface has a lens on top & the back has light-sensitive materials made of calcium sulfide with a snake-pattern of electrical connection running across it
• In darkness or normal light, the LDR have high resistance but when light is shone the resistance decrease dramatically by converting incoming light into electrical energy

Question 20

Briefly describe Photovoltaic

Answer 20

• These are like diodes, made from two layers of semiconductors placed on top of another
• The top layer is electron rich & bottom layer is electron poor
• When the light is shone on top, electrons leap up from the bottom layer to top, making a voltage that can drive through external circuit providing the solar power

Question 21

Describe Photoemissive

Answer 21

• Are oldest way of turning light into electricity
• They are sealed glass vacuum tubes inside which there is a large metal plate that serves as negative terminal with a smaller, positively charged, rod-like terminal running inside it

Question 22

Explain Fibre-Optics

Answer 22

• Used to transmit data, voice & images by the passage of light through thin, transparent fibers
• Also a basis of the fiberscopes used in examining internal parts of the body or inspecting the interiors of manufactured structural products
• The basic medium of fiber optics is a hair-thin fiber made of plastic but often glass
• First data from a device is converted from electrical information to series of light pulses using LASER. Then, the LASER is fired to the fiber-optic cable
• Photoelectrical cells turn the pulses of light back into electrical information
• Light travels via a fiber-optic cable by bouncing repeatedly off the walls

Question 23

Describe flat-screen/LCDs

Answer 23

• Liquid–Crystal Displays (LCDs) use same technology as calculators, cell phones & digital watches
• In 1888, Fridrich Reinitzer discovered liquid crystals
• Picture is made from millions of tiny blocks called pixels
• Each one of these is effectively a separate red, blue or green light that can be switched on or off very rapidly to make moving colour picture
• Pixels are switched on or off electronically using liquid crystals to rotate polarised light

Question 24

How about LED displays?

Answer 24

• LEDS uses Light-Emitting Diodes for backlights instead of fluorescent
• LEDs usually have better picture quality