Topic 7: Photographic Process

Transcription

1 V Topic 7: Photographic Process Aim: To cover the basics of the photographic process, and the properties of photographic material. Contents: asics of Photographic Process xposure and Transmittance urter and riffield Curve Characterisation of Photographic Process Two stage process Transmittance in coherent light. APPL PTC P PATMT of PYC Photographic Process -1- Autumn Term

2 V tructure of Photographic Material mulsion of ilver alide Crystals elatin Anti-halation layer ubstrate (acetate/glass) rain: mulsion of rains of ilver alide in elatin base. rain elatin 50! 500 nm (λ=10! λ=2) 3! 15 µm thick ach grain contains 10 11! ions, so large on atomic scale. ilver alide: Typically mixture of ilver romide, ilver odide and other ilver salts. ubstrate: sually acetate or polyester. lass use when flat stable material needed, (in holography and spectroscopy). Anti-halation layer: Light blocking layer to prevent internal reflections from substrate. ull details of photographic process is difficult chemistry, just supply an overview. APPL PTC P PATMT of PYC Photographic Process -2- Autumn Term

3 V asic Process xpose to Light 1. Quantum(s) of light free electron(s) in lattice. 2. lectron captured by silver ion (interstitial ion) 3. Migration of silver ions to neutralise local charge 4. eformation of lattice allows migrated silver ions to form form speck of silver in the grain. 5. pecks of silver known as Latent mage. ote: pecks much smaller than the grain, typically 50 to 100 silver atoms. Latent mage strength given by number of specks so proportional to incident number of photons (intensity) ormation of one latent image take about 10,4 s (limits film speed). n practice, need about 50 photons absorbed by a single grain to form Latent mage. This process results in the Latent mage recording the incident intensity pattern. APPL PTC P PATMT of PYC Photographic Process -3- Autumn Term

4 V evelopment Want to develop grains of silver halide exposed to light into solid (opaque) silver. Two basic processes, Chemical and Physical. Chemical evelopment: Treat film with weak reducing agent, 1. Add electrons to silver halide that liberates silver. 2. Liberated silver binds to latent image specks. 3. rains with strong latent image develop first. Process is highly temperature dependent, and slow (many minutes). (complex chemical process). Physical evelopment: asically same principle, but deposited silver mostly comes from silver salts in the developer rather than the silver salts in the film. (very little used due to cost of developer.) ixing emove remaining silver halide with stronger reducing agent. Typically also contains agents to harden the gelatin to prevent mechanical damage. APPL PTC P PATMT of PYC Photographic Process -4- Autumn Term

5 V xposure and Transmittance efine: xposure as nergy per nit Area incident. or incident intensity of g(x;y) for timeτ: (x;y)=g(x;y)τ We will typically imply (x;y) dependence, and write = gτ efine: ntensity Transmittance at a point (x;y) as T = Transmitted intensity at (x;y) ncident intensity at (x;y) T 0 T T = T 0 so 0 T 1 o in two dimensions we have T(x,y) f(x,y) g(x,y) g(x;y)= f(x;y)t(x;y) with g(x;y) f(x;y) APPL PTC P PATMT of PYC Photographic Process -5- Autumn Term

6 V xposure to Transmittance urter and riffield showed that 1 log 10 Mass of metallic silver T efine: ptical ensity as mass of metallic silver per unit area, so: = log 10 1 T =,log 10 (T) The film characteristics are then experimentally measured as plot of against. ote: ach film has a different characteristic plot. The shape of this plot can also be altered by different types of developer and by changing the development time. Most manufactured supply technical information on request. APPL PTC P PATMT of PYC Photographic Process -6- Autumn Term

7 V urter and riffield Curve s 3 houlder 2 f 1 Toe Linear egion 0-0 log () Low xposure: ptical ensity not dependent on. Constant og Level f. (Typically f 0:3) 2. Medium xposure: Linear region where log 10 () (seful region). 3. igh xposure: aturation (all grains developed into silver). Typically s 3:0 (0.01% transmittance) ynamic range of the file is = s, f 2:5 typical film APPL PTC P PATMT of PYC Photographic Process -7- Autumn Term

8 V ilm peed ilm peed: Measure of for given xposure. olographic materials measured in µjcm,2, to give a good holographic exposure. Photographic material subjective speed measure when developed in standard developer. Two typical measures: AA Linear with Linear with (log of exposure) Typical ilm peeds: AA Type 5 Lithographic film 25 Very slow portrait film 100 ormal b/w or colour film 400 ast b/w or colour 1000 astest normal film > 1000 pecial processing sed as a guide only: (eg: 400 AA film requires 1/4 xposure of 100 AA). ifficult to get quantative measure. (ee tutorial) Always get trade-off low speed mall rains igh esolution igh speed Large rains Low esolution APPL PTC P PATMT of PYC Photographic Process -8- Autumn Term

9 V n linear region we have Linear egion = γlog 10 (), 0 where γ is the gradient (Called ilm amma). We have that =,log 10 (T) & so that the intensity transmittance = gτ T = 10 0,γ = 10 0(gτ),γ elation between xposure and intensity transmittance T is - LA. Key esult Low Contrast: γ 1 igh speed lack/white file, (P-5, or Tri-X). mall changes in ptical ensity with Large change in exposure. ilms normally ast. igh Contrast: γ 2! 3 Lithogrphic copy film, very Large changes in ptical ensity with mall changes in exposure. utput often binary (black or white). ilms noramlly low. APPL PTC P PATMT of PYC Photographic Process -9- Autumn Term

10 V Variation of γ or filmable to modify γ by changing exposure and development conditions. ncrease amma: Low exposure plus long/hot development. Weak latent image, only brightest point developed to full darkness, high contrast. ecrease amma: igh exposure plus short/cold development. trong latent image. Latent image only partially developed, low contrast. or a typical /W white film (P-5), we get γ evelopment Time (minutes) so will careful development we can select the γ required. APPL PTC P PATMT of PYC Photographic Process -10- Autumn Term

11 V Two tage Process Consider two stage photographic process. 1) orm egative ncident intensity of f(x;y) for time τ with γ. egative with T = K ( f τ ),γ K = constant 2) orm Positive lluminate egative, project onto second film. T Photographic Material Constant g(x,y) = T n second film, intensity g(x;y)=t (x;y) econd film has γ P and exposure τ P, transmission T P = K P (gτ P ),γ P = K P (K τ P ),γ P( f τ ) γ γ P = K( f τ ) γ γ P o if we choose γ γ P = 1 then we have the original incident intensity. T P (x;y) f(x;y) ntensity Linearity possible APPL PTC P PATMT of PYC Photographic Process -11- Autumn Term

12 V egatives in Coherent Light orm a negative with intensity f(x;y), intensity transmittance T = K( f τ),γ where for coherent light f(x;y)=ju(x;y)j 2. lluminate negative with coherent light, want Amplitude Transmittance. lluminate with constant beam, transmitted amplitude is v(x;y)= p T(x;y) assuming that there is no optical path differences in the negative, (no phase effects). efine: Amplitude Transmittance, which we will usually write as: T a = p T = 10 0=2 ( f τ),γ=2 T a = p K( f τ),γ=2 K = constant Key esult This will be used in holography in the next lecture. APPL PTC P PATMT of PYC Photographic Process -12- Autumn Term

13 V Practical Problem elatin surface not flat after development elatin urface olid ilver ubstrate hrinks to cover solid silver, (also cracks). ips of up to 2µm common, so sever phase problem. olution: ptical ate: ptically lat lass ndex Matching fluid elatin film se oil/solvent with same refractive index as gelatin, (n = 1:53) Light silicon oil or xylene (both rather unpleasant). ptical gate required when critical optical measures are being made. APPL PTC P PATMT of PYC Photographic Process -13- Autumn Term