IB Physics Topic 14 Digital Technology
Define magnification.
The ratio of the length of the image on the CCD to the length of the object. M=Hi/Ho
List the advantages of uses of a CCD compared with the use of film.
•each photo does not require film (cheaper) •greater quantum efficiency (faint images can be photographed) •may be enhanced by electronic processing techniques •may be viewed virtually (no processing time) •easy storage of high capacity
Outline how the image on a CCD is digitised.
An electrode measures the potential difference developed across each pixel and this is then converted into a digital signal. The pixel position is also stored.
What is the path difference between light from pit and land.
As destructive interference occurs, path difference is ½λ
Solve problems involving the conversion between binary numbers and decimal numbers.
Binary numbers are represented by counting in powers of 2 so each bit represents 1, 2, 4, 8, 16 etc.
Describe a range of practical uses of a CCD.
CCDs are used for image capturing in a large range of the electromagnetic spectrum. DIGITAL CAMERAS: •Greater shareability, convenient, store more pictures •Some resolution lost during colourising process, image quality can be lower than traditional film unless high quality camera used (which is expensive). VIDEO CAMERAS: •Better quality, easier to store and transport, convenient, record longer videos, searches easier tand faster to perform •Pixelation with cheaper video cameras as they have low resolution TELESCOPES: •More sensitivity allowing detailed analysis over a range of frequencies, exposure times reduced, allow for remote operation e.g. Hubble telescope MEDICAL X-RAY IMAGING: •Have better contrast, able to be processed, images easily compared using imaging techniques, quickly and easily shared
What is a byte.
Eight separate bits of information.
Describe different means of storage of information in both analogue and digital forms.
LP's: Stored as variations in a track on the LP, analogue CASSETTE TAPES: Stored as variations in magnetic field orientations in the tape, analogue FLOPPY DISCS: Stored as series of magnetic variations in the disk, only two magnetic variations used, digital HARD DISCS: Stored as series of magnetic variations in the disk, only two possible variations used, digital DVD'S: Variations of light and sound are stored as series of optical bumps or "pits on the DVD track, digital
Discuss the implications for society of ever-increasing capability of data storage (moral, ethical, social, economical and environmental).
MORAL: •Information that is potentially problematic for society can be easily recorded and shared. •Censorship (should some countries censor accessibility of certain websites?) •Viruses, Spyware and malware which put integrity of digital data at risk ETHICAL: •Makes it easier to infringe copyright •Ease of plagiarism •Identity records and identity fraud SOCIAL: •Can be used to record and highlight abuses of human rights •Access and storage of information can control societies opinions and views. •Individuals without access to internet will be disadvantaged •Data must be backed up on regular basis •As records not on paper, something needs to be done to ensure records available for future generations. ECONOMICAL: •Cost saving •Distribution of data implies more perfect information and thus more competitive markets ENVIRONMENTAL: •Saving in resources •Paper still wasted from unnecessary duplicate printing •shift in use of natural resources towards need for digital data storage
Define capacitance.
Measure of the amount of electric charged stored per unit electric potential difference. C = q / V C in farad F 1F=1CV⁻¹
How far apart must points be on a CCD to be just resolved.
Need to be two pixels apart.
Discuss the advantage of the storage of information in digital rather than analogue form.
QUALITY: better reliability as each copy replica of original (analogue: very liable to damage or corruption) REPRODUCIBILITY: each retrieval virtually identical (analogue: process of retrieval affects quality of future retrievals) RETRIEVAL SPEED: simple data at great speeds, more complex data takes longer (analogue: significant time) PORTABILITY OF STORED DATA: large quantities of data can be stored in a very small device (analogue: occupy significantly larger volume compared to digital alternative) MANIPULATION OF DATA: easy manipulation without significance corruption (analogue: all manipulation increase possibility of corruption)
Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed image.
QUANTUM EFFICIENCY: The greater the QE, the greater sensitivity of the device MAGNIFICATION: Greater magnifications means more pixels are used to produce the picture, giving it more detail RESOLUTION: Greater resolution, greater amount of detail recorded but will occupy more memory.
Define quantum efficiency of a pixel.
Ratio of the number of photoelectrons emitted to the number of photons incident on the pixel. QE=Ne/Np
What is the most significant bit?
The largest power (first digit in a binary number)
Calculate an appropriate depth for a pit from the wavelength of the laser light.
The laser light experiences destructive interference as the light travels an extra distance equal to ½λ. For this to occur the depth of each pit must be ¼λ.
What is the least significant bit?
The smallest power (the last digit in a binary number.
Describe the structure of a charge-coupled device (CCD).
•A CCD is a silicon chip divided into small areas called pixels. •Each pixel can be considered to behave as a capacitor. •Used to electronically record an image focused onto its surface.
Describe analogue signals.
•Analogue techniques involve codes and signals that can take a large number of different values between given limits. •Analogue signal continuously varies with time.
How colours produced.
•Different colours are produced by an array (or grid) of tiny coloured lenses covering the photodiodes. •Primary colours used: red, green, blue
Describe digital signals.
•Digital techniques involve codes and signals involve a large number of Binary digITs, or BITS. •Each BIT can only take one of two possible values, (1 or 0, high or low, ON or OFF)
Outline how the image stored in a CCD is retrieved.
•Each capacitor transfers its charge to a neighbouring capacitor. •The voltages on the pixels along one edge of the array are then transformed into digital form and stored. •The next line of pixels are then transfers their charge to the outside edge and the process is repeated.
Explain how interference of light is used to recover information stored on a CD.
•Information is read by sensing amplitude of the reflection of a laser beam reflecting off the bumps and pits •Speed of rotation is controlled so constant length of track is scanned in a given time •When beam reflects from a land or a pit, a strong signal is received •Beam reflects from edge between a land and a pit, destructive interference takes place and a weak signal is received. •A strong signal represents 0, weak signal represents 1
Problems involving the data storage capacity.
•Resolution is a problem •RAYLEIGH CRITERION defines when two images or objects as "just resolved" which depends on the wavelength of light and the angular separation of the objects. •1 bit of data is produced by a pit bump pair
Describe the conversion between analogue and digital forms.
•Sampling of input information at regular intervals •Converting each sampled signal into one value •Converting each sampled quantum level into digital form ACCURACY OF INFORMATION CAN BE IMPROVED BY: •increasing the sampling frequency •increasing the number of available quantum levels THE OUTPUT PROCESS: •involves converting series of bits back into analogue signal
Explain how incident light causes charge to build up within a pixel.
•light photons arriving at CCD's photodiodes releases electrons due to the photoelectric effect, accumulating an electric charge •the build of charge is proportional to the intensity of the incidented light •bright light produces a higher charge on each pixel •each pixel is assigned a digital number depending on the charge it posses
