FIgure 1 – Schematic showing the creation of a latent image upon exposure to light in a silver bromide emulsion. (c) 2013 DE Wolf

You will recall that photography was invented in 1838.  So the light sensitivity of silver salts, such as silver bromide, was recognized for a very long time.*  However, it was not until 1938 that a real theoretical-mechanistic explanation of this sensitivity began to be developed.+  We have already discussed the concept of valence and conduction bands in silver halides and how light can raise electrons between these two bands.  Recognize however, that when the electron is elevated in energy to the conduction band it leaves behind a positively charged region in the crystal lattice, referred to as a “hole.”  In fact, what light actually creates is an electron-hole pair.

In a perfect and pure crystal the electron will hang out a bit in the conduction band and eventually will recombine with one of the available holes.  So there should be this continuous up-down, creation-recombination process going on in the presence of light.  However, our photographic grains are not perfect crystals.  First, of all they are finite in size.  Second, they have impurities.  And third, there can be breaks or physical dislocations in the crystal lattice – like cracks in a driveway.

All of these imperfections can lead to the process shown in Figure 1.  Here, I’ve shown it as a single bond between a silver ion and a bromine ion, but we know that in fact the ionic interactions are three dimensional and more complex.  We have, for starters, a silver ion (Ag+) in close association with a bromine ion (Br-).  Light comes in and liberates the bromine’s electron.  So now we have a silver ion (Ag+) and bromine atom (Br) and an electron (e-).  The electron then combines with the silver ion to create atomic silver (Ag).  So we are left with silver (Ag) and bromine (Br).  It is the free atomic silver that we refer to as the latent image.

In the emulsion there are thousands of randomly distributed silver bromide crystals or grains.  These are the pixels of analogue photography.  Where light struck the emulsion some of these grains now contain free silver.  The more intense the light the more likely the grain is to contain free silver. The distribution of silver reflects the distribution of light and is what we refer to as “the latent image.”  You can’t see it and if you turn the lights on to try to see it, you will over expose the film – “Catch 22!”

The latent image is there waiting to be developed.  That will be the subject of my next technical blog.

*Perhaps the earliest reference to the concept of silver-based black and white photography is that of J. H. Schulze who observed in 1727 that a mixture of silver nitrate and chalk darkened on exposure to light.

+Gurney, R. W.; Mott, N. F. (1938). “The theory of the photolysis of silver bromide and the photographic latent image”. Proc. Roy. Soc. A164: 151–167.

Figure 1 – Women separating egg whites and egg yolks as the first step in the manufacture of silver albumen photographic paper from Josef Maria Eder’s Ausführliches Handbuch der Photographie, Book IV, part 1, 1898 edition. In the public domain in the US. See hyperlink in text for more information on the process.

A photographic emulsion is a fine suspension of insoluble light-sensitive crystals, for example silver bromide, suspended in a colloidal solution.  Basically, this means that while the crystals are not themselves soluble in water, they can be suspended in a material, such as gelatin, which is then allowed to harden thereby suspending the crystals permanently.

The emulsion is layered onto a substrate such as glass or a film, e.g. nitrocellulose or plastic.  In the nineteenth century the emulsion material was most often egg albumen, and this created an entire industry of manufacture that started with the separation of thousands of egg yolks and whites as shown in Figure 1.  Modern photographic emulsions are made from gelatin, which is partially hydrolyzed (boiled) extract of animal cartilage and bone – hence the classification of silver-gelatin print.

The light sensitive emulsion is generally manufactured, as suggested in my previous blog by dissolving silver nitrate and potassium bromide in a hot gelatin solution and allowing the crystals of silver bromide to precipitate out in the gelatin.

So now, we have the fundamental elements of a photosensitive material.  You can do this yourself at home, if you are industrious, and there are even commercial kits to help you.  If you want to do albumen printing you can purchase kits from Bostick and Sullivan. As should be obvious from our discussion of color photography and color movies, the fundamental silver halide process is ubiquitous.

Curiously, how it works, what the fundamental mechanism of light sensitivity is, was largely a mystery for the first century of photography.  That mystery is the subject of my next technical blog.

Figure 1 – The Periodic Table of the Elements.  The halide group is shown in yellow. From the Wikicommons and in the public domain.

In considering silver halide chemistry, the first question is exactly what is a silver halide.  A silver halide is a compound formed from silver and a halogen.  That didn’t get us very far.  What’s a halogen?  If we look at the period table of the elements (be sure to click on Tom Lehrer’s version) in Figure 1, the yellow group 17  are the halogen elements: Fluorine, Chlorine, Bromine, Iodine, and Antimony.

Figure 2 – The crystal structure of silver bromide. The silver atoms are the red spheres the bromide atoms are the pale blue spheres. Form the Wikicommons and in the public domain.

A silver halide then is a crystal formed between silver and one of these Group 17 elements.  Without getting into too much detail the silver metal gives away some of its electrons to become positively charged.  These are scarfed up by the halogen to become negatively charged.  The halogens ions all have a charge of -1 (electron’s worth of charge).  That’s why they are grouped together. Since opposites attract the silver and halogen ions attract one another and form very compact and structurally well-defined crystal structures.  These crystals typically take on a cubic form as shown in Figure 2, for the important photographic material silver bromide, where the red spheres are the silver atoms and the pale blue ones are the bromine atoms.  The other common photographic silver halides are silver chloride and silver iodide.

Silver bromide is not soluble in water.  This is convenient.  If you mix a solution of silver nitrate in water with potassium bromide in water, the silver and bromine switch partners to form silver bromide and potassium nitrate.  The potassium nitrate stays in solution.  The silver bromide precipitates into fine silver bromide crystals, ready to be washed and put into a photographic emulsion.  What an emulsion is will be the subject of my next technical blog.

Figure 1 – Valence and conduction energy bands in a crystal (c) 2013 DE Wolf

How do we classify matter?  One of the ways, one that most of us are familiar with, is in terms of “the state of matter.”  Is it a solid, a liquid, or a gas?  If you take some ice cubes, a solid, and stick them inside a bottle they maintain their ice cube form.  They are unperturbed by the fact that they are in the bottle.  If you melt the ice cubes, they become water, a liquid, and take on the shape of the bottle, but remain confined by gravity to the bottom of the bottle.  If you vaporize the water into steam, a gas, the steam fills the entire bottle and conforms to its shape completely.  Actually there is a fourth state of matter to consider.  If you ionize the water by ripping off its valence electrons and maybe even some of the lower energy ones, it become a charged gas, referred to as a plasma.

At an atomic level, the atoms in a gas are largely oblivious of one another.  This is because they seldom come in contact with one another.  Their interactions, with say light, are single atom events and independent of what the other atoms are doing.  In a liquid the atoms start to interact with one another.  In a solid the interaction is complete.  The atoms form well defined structures called crystals.  The crystal now interacts with light as a unit.  As a result, we talk about the energy levels of the crystal, no longer of the individual atoms.

Because of this cooperative effect, the energy states start to blur out and become a continuum.  However, there is still an energy gap between the states of bound valence electrons, called the valence band [of energy levels], and the energies of liberated or free electrons, called the [conductance band].  I’ve illustrated this schematically in Figure 1.

(BTW Figure 1 gives you the opportunity to thumb your nose at your high school or physics college teacher, who always insisted that you label axes of a graph and marked you down if you didn’t.  The y axis is energy, but there is no x axis label.  This diagram is called a Jablonski diagram in physics.)

How does an electron gain the energy to escape from the valence band to the conductance band in a crystal.  It can be done with heat, or electrical energy, or with light.  A photon of light can be absorbed and give the electron enough energy to escape.

You can define three types of solid materials on the basis of how easy it is to liberate the valence electrons.  In conductors, typically metals, the band gap energy is so small that it occurs very easily at room temperature.  Many electrons go into the conductance band, and if you attach the conductor across the terminals of a battery a current readily develops.  In insulators, the band gap is huge and precious few electrons are in the conduction band.  The material does not conduct electrons.  Finally, there are in between materials, called semiconductors, which need a boast to get the valence electrons into conduction band.

One such boast, as we’ve already described, is light.  This kind of semiconductor/ light interaction is the basis of both analogue and digital photography.  As we will see, it explains how silver halide films work, how camera light meters work, and how digital camera sensors work.

In the next few technical blogs, I’d like to explore the inner workings of the silver halide film process.  Breaking it all down, we need to sequentially consider:

• What is a silver halide?
• What is a photographic emulsion?
• How is the latent image formed?
• How is the latent image developed?
• How is the developed image fixed or prevented from further interaction with light?

Today, I’d like to begin a discussion of the physics behind light detection. By light detection I mean both how analogue films work and how digital camera sensors work. Surprisingly, the answers are closely related because both work as a result of semiconductor physics.

In discussing these topics, we are fundamentally talking about how light interacts with matter and we already know the answer. Light interacts with matter in complex, varied, and often beautiful ways. Consider a rainbow, which is the interaction of light with spherical water drops inside clouds. That is a process called refraction and spectral dispersion. Consider the mobius strip photographs in my “Manmade gallery.” Those spectacular colors are created by a process called optical birefringence.

Figure 1 – The potential well or Curve of Binding Energy (c) DEWolf 2013

But let’s keep things reasonably simple here. We need to understand our subject qualitatively, not quantitatively, which means, yay, no equations!

OK, so we know that atoms are made up of protons, which are positive charges, and electrons, which are negative charges. Opposites attract. So given half a chance an electron will rush towards a proton. That’s how lightening works! Ka-boom!

Now let’s hold an electron some distance away from the proton. You will feel the force of attraction in your fingers. This is referred to as potential energy, because if you let go the electron will move very quickly towards the proton, that is there is the potential for motion, aka kinetic or motion energy. By convention, because of the attractive nature of the force, the energy is said to be negative. We can plot the energy as a function of the distance between the electron and proton. In general the electron will seek the lowest energy. That is, it will try to go to the bottom of the well. The bottom of the well here is at minus infinity, which is a very cozy place for our electron to hide out.

The well of Figure 1 is what the electron sees. An electron can only be at the bottom of the well if it has no kinetic energy. If it has kinetic energy it will be a bit higher in the well. Next, it turns out that the electron cannot have any energy value that it wants. Only very discrete and well defined levels exist. This statement about discrete states is the foundation of the science of quantum mechanics. While it may seem bizarre, it is never-the-less borne out by countless supporting experiments. It is on a very sound theoretical and experimental proof-of-the-pudding foundation.

Figure 2 – Atomic spectrum of the plasma glow of a HeNe laser (c) 2013 DEWolf

In Figure 1 I’ve shown some energy levels where the electron can reside. The electron can move between these states, but only if you give it more energy, because energy has to be conserved in physics. Where does that energy come from? One possibility is light. If the photon of light has exactly the right amount energy, is exactly the right color, it can be absorbed and elevate the electron to the next allowed energy level. You can also accomplish this by heating the system up, or bombarding it with a spray of electrons. Always a precise amount of energy is absorbed, called one quanta. Often the electron falls back to the previous state. To do so it has to emit light of the precise color again. Consider Figure 2, which shows the “plasma glow” of a Helium Neon laser. The plasma glow emission is caused by transitions between lots of states and is created by showering the gas with a beam of electrons. The emitted light is a set of very sharp discrete wavelengths.

The one last critical point is that if you give the electron more than a certain amount of energy, referred to as the ionization potential, it can escape the proton all together. It becomes a free electron!  I’ve shown the ionization potential (change in energy required to take the electron from it’s lowest energy level to the top of the binding curve) in orange.

In atoms that have more than one electron, the electrons sequentially fill up the energy levels.  Two electron cannot exist in the same state.  The term state is not quite the same as the term energy level.  But that doesn’t really matter for our discussion.  What is important is that the electron with the highest energy, the one that is nearest the top is the most active electron.  It is referred to as “the valence” electron and is most likely to become ionized and escape from the atom or, in fact, to interact with neighboring atoms.

The Bottom Line

The important points for our discussion of light detection in photography are:

• electrons in atoms occupy specific energy levels
• the most active electrons are those with the highest energy, referred to as valence electrons
• light can cause electrons to move between energy levels
• given sufficient energy an electron can be ionized and escape from the atom