 Welcome to the series, Photographic Chemistry, presented by the Foundation of the American Institute for Conservation of Historic and Artistic Works. This program was made possible by grants from the National Endowment for the Humanities and the Andrew W. Mellon Foundation. Each program in this series is presented as a short video. Depending on your video viewer, you should be able to pause, return to a previous section, or skip ahead to a later section by using a scroll bar or on-screen icons. You will find an outline of the course and short quizzes to test your understanding on the course webpage. The illustration on this slide shows the spectral absorption differences or the light sensitivity differences between the different silver halide compounds. On the top of this slide is a graphic which shows the visible region of the electromagnetic spectrum, which extends from 400 to 700 nanometers. This is the photopic range. This is the range by which our eyes are able to detect radiation. And we see scenes in our world only because our eyes can respond to reflective wavelengths that exist within this very narrow range between 400 and 700 nanometers. So if we compare the response to our light-capturing material, starting with silver chloride, what we see that only a very small portion of the electromagnetic spectrum that falls in the visible range is captured by the silver chloride grain. The range of wavelengths that the silver chloride grain responds to extends from about 400 nanometers to 420 nanometers. Now when I say that, actually the silver halide grain also responds to wavelengths in the ultraviolet portion of the electromagnetic spectrum. But these are not wavelengths we can see. So what really matters most to us are only the responses of our materials to the range by which our eyes are sensitive. That is the visible spectrum from 400 to 700 nanometers. So as we see, silver chloride only captures a very, very small portion of that range in the blue portion of the electromagnetic spectrum, or again, 400 to 420 nanometers. Because of this very small sliver of response, this is why in the very early days when all the salt of papers and recording materials were primarily silver chloride, the exposure times were considerably long. And this is because that only light striking these materials that would be recorded was just again a sliver of the total amount of light coming from our scene. Silver chloride was incapable of recording subject matter in the scene, which fell in the green or the red portion of the electromagnetic spectrum. So the sensitivity, when we talk about silver halide grain sensitivity, we're talking about those wavelengths that the grain can respond to, and we need to correlate that with what our eyes see. So starting, silver chloride, again, only captures a very, very small portion from about 400 to 420 nanometers. So the natural next step is to broaden the range of sensitivity to be able to modify our materials to be able to capture more and more light coming from various objects from my scene. So to be able to do that, more experimentation was conducted and it was found that if we included bromide into this mix, that we, in fact, do see an extension of the sensitivity range. So when we make silver halides that contain a mixture of both chloride and bromide, that is oftentimes called a silver-chlorobromide grain, we get an extension of the sensitivity out to about 480 nanometers. So that's an improvement. We've gone from 420. We have extended that out to about 480. We're starting to get into that yellow portion now of the visible range. And so it's an improvement, but it's not still enough. Most outdoor scenes are predominantly greens and reds. So our photographic material still lacks the full range of sensitivity required to capture all information in the scene. So let's now pretend it's you and I back in the day seeing and working on this problem of extending the range of our photographic materials. Sure, the history of how this was done is found in the literature, but let's see if we comply our own knowledge of chemistry to see what the natural next step would be. Remember, we're working our way down the halides that we have used already silver chloride and we know its properties, and we've just learned that if we add silver bromide, we get a response that is going in the right direction for us, extending the range out further into the visible spectrum. So look at the halide group and what would be the very next step in the experimentation that you would do to extend this range further. And if you thought about it, of course, you would now pick the iodide. So what if you made a silver chloride grain, I'm sorry, a silver halide grain, which contained a composite of all three halides. So we now build a chlorobromoiodo, that is a silver chlorobromoiodo grain that's just a silver halide grain with all three halides in it at various proportions. Do some experimentation. And what we find is, in fact, we get an extension. So again, we're going in the right direction. By adding iodide to our grain, we have now further extended our range out to 520 nanometers. We're getting there, but we're still not quite there. The most sensitive part of the visible spectrum for the human eye is around the 550 range. So we're approaching that, but we're still not quite there. So with all this development, we were able to capture the full blue spectrum of everything in our scene. We're starting to get into the green portion of the spectrum, but we're just not quite there at this moment using the halides in the periodic table. Let's review where we are in our development of a photographic material that responds to wavelengths of light that extend throughout the visible spectrum from 400 to 700 nanometers. Using the silver halides alone, even using all three silver halide salts, the chloro with the bromo, with the iodide together, and a silver halide grain, that gives us blue sensitivity, extending from a range from 400 nanometers out to about 520 nanometers. Now this blue sensitivity is capturing all that portion of our scene, but what we're missing is, again, most of the content of a lot of scenes, the greens and the reds. If we were outdoors, the green leaves and the red roses, we're not going to be able to capture those types of objects in our scene using just the silver halide technology because silver halides alone, even used in combination, lack the sensitivity to record green and red light. It would seem like at this point that we'd be stuck, that we've already used all the silver halide technology at our disposal to extend the range as far as we could. But now just imagine, what would you need to be able to make your photographic material sensitive to the entire range of the visible spectrum? You'd want to be able to find something now outside of the silver halide grain that would still, in part, spectral sensitivity of that grain to wavelengths of light that it normally would not have otherwise. So we're going to look for now something that extends the sensitivity, and we're going to call that a spectral sensitizer because what we're looking for is some type of a material that in fact extends our sensitivity range throughout the blue, the green, and the red portions of the visible spectrum. A clue for increasing this limited response of our photographic halide material came from a chance observation by the German photographer and scientist Hermann Vogel back in 1873. You see, while he was testing dry colloidium plates, which the manufacturer at that time had been secretly treating with a yellow dye to reduce halation, Vogel noted that the plates were green sensitive. After some clever detective work, Vogel established that that yellow dye must have been that responsible material that we've been looking for to be able to extend the spectral sensitivity further out into the visible range. Through a lot of work, these dye photographic materials were said to be orthochromatic, ortho and chroma, meaning the correct color, but now sensitive to a broader range, the blue, the green, and now including the greenish yellow light, but still no sensitivity to the red portion of the spectrum which contains visible content in our scenes. With this chance observation and some fine work by Hermann Vogel, now photographers and photographic chemists had a clue, an idea of what they needed to do to add new material to photographic emulsions to extend their spectral sensitivity. These spectral sensitizers were dyes which would respond to the light that the silver halide grain could not respond to but yet still provide a recording medium for those wavelengths that were blind to the silver halide grains themselves. Through a better part of the early 20th century, a whole range of spectral sensitizers were discovered and developed and incorporated with silver halide crystals in order to fully incorporate the entire range of the visible spectrum so that it panned out through the full range from 400 to 700 nanometers. This spectral sensitizing extension was called panchromatic technology or pan plus chroma, meaning all of the universal colors would be sensitive to and respond to the entire visible spectrum. So now with this development of incorporated dyes that would adsorb onto the silver halide grains intimately, we would extend the range from 400 nanometers all the way out to 700 nanometers and naturally what this means is we're now capable of recording the blues, the greens and the red objects in our scenes. You have completed this unit. Depending on your video viewer, you should be able to scroll back to any point in the video as desired. The short quiz found in the course materials on the website may help you confirm your understanding of the concepts introduced here. Many thanks to the instructor, production editor, coordinator and the collaborative workshops in Photograph Conservation Committee for their work to make this program possible.