Technical points-Richard Flashcards
what are the two principal designs of a sensor?
CMOS - Complementary Metal Oxide Semiconductor
CCD - Charge Couple Devide
Sensor design
sensors work by converting light into an electrical signal
each sensor will include a number of ‘photo sites’ or pixels which collect light and turn it into an electrical signal
the more pixels, the better the resolution of the camera
what are 3 sizes of sensors
micro - 17.3 x 13mm
APS-C - 23.5 x 15.6mm
Full frame - 36 x 24mm
size of sensor
larger sensor means larger pixels which translates into more light gathering, better sensitivity, better performance and less noise, greater bit depth
sensor design
when a photo is taken, light hitting the sensor sills each pixel position which causes an electrical voltage to be produced.
charge is converted by an analogue to digital converter and is reconstructed to form an image
monochrome system:
-pixels where no light has hit appear black
-pixels where lots of light has hit appear pure white
-pixels somewhere in between appear a shade of grey
colour sensor design
most colour sensors include a special form of colour filter array known as a Bayer filter array
each pixel is covered with a filter which allows light of a specific colour to pass through
data has to undergo demosaicing
camera takes data from surrounding pixels and combines this to form a true colour image with each data point including colour data from the surrounding red, green and blue
what is bit depth
how many colours a camera can record
most high end camera are capable of 12 or 14 bit capture
image processing
once image has been produced it can be processed to form a jpeg file or remain in a raw unprocessed state forming a raw file
what does JPEG stand for
Joint Photography Experts Group
processed JEPG files
colour correction, sharpening, exposure correction applied by camera
compressed JPEG files
small file size, compression process can degrade and robs images of fine detail
converted to 8 bit colour JPEG files
only 256 colour per channel = 16.7 million colours
processed RAW files
requires importing into a RAW converter or software
post processing
most forensic providers or police have a Standard Operating Procedure (SOP) or process to follow
images are downloaded to a secure server where a security device known as a hash code is produced
if image needs to be published or processed, a copy of the original file can be taken and worked on
hash code is checked before every step to ensure that images have not been altered
image manipulation
sharpening - to improve clarity
brightness increase, contrast decreased - bring out more detail
slight colour correction
