define as the capture of a luminous image with metallic silver

produced through the reduction of silver halide to metallic silver

1839 Louis Daguerre coated glass with emulsion==> positive images produced

1885 George Eastman emulsion on cellulose nitrate

Any energy source can be used to form a silver-based image

light==>photons, usually less than 10 eV, need several to sensitize a silver grain
SEM==> photons from CRT
TEM==> electrons, usually greater than 20 KeV, need only 1 and can sensitize several grains

paper................................................film
------coating to protect surface------

               emulsion

AgBr grains (30-2500 nm) in gelatin
gelatin maintains dispersed silver

paper -----base----------------- plastic

exposure of film or paper to photons/electrons produces a latent image

over time, the activated silver grain will turn black on ints own. Developer increases the speed of this reaction significantly

reaction time varies with

1) the pH=more alkaline, faster development
2) temperature=higher temperature, faster development

Fixer== removes unexposed silver grains

fixes(coagulates) protein/gelatin into place

Wash== to remove everything else

residual sulfur compound would form silver sulfide in presence of light and humidity==leaves brown silver sulfide and can convert silver to colorless form

developer gel is smeared between print and negative in Positive/Negative format

the negative film has 10x the resolution of the positive

on negative, sensitized silver is reduced to metallic silver; on positive, have inverse reaction take place

film or paper is covered with a layer of silver grains with different sizes
 
 

resolution ==smallest measurable distance between two distinct dots

speed ==sensitivity of film/paper to be exposed
 
 

for 10 electrons/photons

the larger the grains, fewer total electronsphotons hitting the film or paper are needed to produce a grain

this would make a faster film/paper

smaller grains, more total electrons/photons hitting the paper or film are needed to produce enough silver grains to form an image

larger grains make for a grainy image, so resolution is reduced
 
 

Spectral sensitivity==wavelengths that material is sensitive to

safelights are relative--won't expose the material in a reasonable length of time

silver bromide alone is sensitive to blue, violet and UV

to increase this range, additional dyes are linked to the silver grains

orthochromatic film== green sensitized==red insensitive, very high contrast

pancromatic film== all wavelengths sensitized==numerous shades of gray

can produce an image curve to show rates of exposure

graph density vs. log of exposure

A->B underexposed

B->C useful region

C->D over exposed

contrast is function of slope

greater the slope, greater the contrast

Fiber vs. Plastic (Resin coated)

Graded vs. Polycontrast

paper finishes: F glossy, M matte

instead of film, use electronics and computers

use frame-grabber to capture a single frame of video

use computer to average several frames to improve signal to noise

noise is the random spots created by the electronics of the system, not the image

use frame grabber to grab higher resolution slow scan

video is 30x scans per sec

slow scan has longer dwell time per spot==better resolution

both of these rely on a YAG (Ytrium Aluminum Garnet) scintillator in detector to convert electron signal to light image and then capture that image

3. TEM==use single crystal YAG (Ytrium Aluminum Garnet) to convert electron signal to light image and then capture that image with a high resolution camera

use a CCD charge coupled device==set of polysilicon electrodes in silicon substrate and separated by an oxide insulation layer

photons from scintillator hit surface of CCD and raise the electrical potential at each pixel==this is counted by computer and used to form an image

1. dynamic range==ratio of the range of values from the lowest to the highest

for film, it is only 200:1, for CCDs it is 100,000:1

2. linearity==fully linear for CCD, variable for film

3. handling==computer stored, no darkroom 'instant image'

resolution==film is about 10,000 pixels by 10,000 whereas CCDs are 4Kx4K (but getting bigger and cheaper)

cost==for highest resolution, $90,000 for 2K square chip plus computer , storage, etc. Then need some way to get the image printed

Inkjet printer with a cartridge that shots microdots of colored dye to make spots of image

Laser printer: similar to a xerox machine

laser creates line of charged particles on rotating drum. As the drum passes toner, it attracts colored toner particles onto charged drum. This toner is heat transferred and bonded onto paper, creating an image on the paper (which is why paper from printer or xerox machine feels warm)