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The camera obscura, an ancient device that captures light through a lens or pinhole and projects it onto a surface, tantalized artists, scientists and inventors with the dream of capturing its fleeting images. Eventually, scientists discovered that certain chemicals react to light with visible and persistent changes. Inventors developed devices and techniques for capturing images using silver halide and other photosensitive chemicals. Photography revolutionized the magic lantern show, made stereoviews practical and movies possible.
The use of photochemistry is not limited to storing images: optical soundtracks store sound on movie film and phototypesetting discs store fonts as photographic negatives, for example. Photochemical systems are also used in photolithography, photo-etching and other industrial processes.
Thermal printing and write-once optical discs also depend on chemical reactions. When certain dyes are heated, they become opaque, which can be used to print labels or to put marks on an optical disc.
Photographs store information as photochemical changes that alter the reflectance or transmittance of an emulsion layered on a substrate like celluloid, glass or paper. The chemicals used for photography are, in most cases, silver halides in the form of microscopic crystals suspended in a gel. Silver halide crystals become opaque metallic silver in proportion to the amount of light falling on them. The initial exposure creates a latent image, which becomes visible with the application of additional chemicals during developing. The result is a negative, from which a separate positive print is generated in a second photochemical process. Reversal film is a variation often used for slides in which developing the film exposed in the camera creates a positive on the same strip of film.
The first photographic processes were black and white. Daguerreotype, introduced by Louis Daguerre in 1839, produced high-resolution images, with the commercial disadvantage that making duplicates required taking the photograph from life again (not unlike the situation with early cylinder records). In 1841 Willam Henry Fox Talbot introduced the calotype process, in which positive images could be repeatedly printed from negatives, an approach that established the modern photographic process. The use of glass plates coated with silver-bromide in albumen, collodion and finally gelatin increased the sensitivity of photography until by the 1880s exposures of a fraction of a second were possible.
A black and white photograph captures light across the entire visible spectrum but reduces it to a single variable, intensity, rather than the mix of wavelengths that we sense as color. Capturing that mix occupied scientist and inventors for decades after the invention of photography.
A solution began to emerge with the trichromatic theory proposed by Thomas Young in 1802 and proven experimentally by Hermann von Helmholtz in 1850. The Young-Helmholtz theory asserted that the retina has three types of receptors sensitive to ranges of the visible spectrum centered on red, green and blue. In 1861, James Clerk Maxwell demonstrated the theory using color separations: separate black and white photographs taken through red, green and blue filters. The separation images were then combined in an additive color process by superimposing them using three projectors provided with red, green and blue filters. In the later Ives Krömsköp device, separations could be viewed without the projectors. The images could also be combined in a subtractive color process by creating a gel relief from the three images, dyeing the gels greenish-blue, pink and yellow, then layering them on a glass substrate (see Sanger-Shepherd and Lumiere Trichrome below).
Commercialization of color photography required something simpler than special viewers, multiple projectors or complex layering procedures. Screen processes, an alternative first suggested by Ducos du Hauron in 1868, are additive color processes that intermingle the three separation images in a single image that can be viewed directly. The photograph is taken through a screen consisting of tiny red, green and blue filters distributed in a grid or random pattern. The resulting underlying image is actually black and white, but color emerges when viewed through an identical screen. Just as with a pointillist painting or computer monitor, the tiny regions of pure color are fused by the brain.
Subtractive color processes merge color separations into a single image by superimposing them physically. With Kodachrome and most subsequent color processes, the film consists of three layers on a single base, an arrangement known as integral tripack. The top layer is sensitive to blue, the next to green and the last to red. During development the layers are dyed complimentary colors through the action of dye couplers: the blue layer is dyed yellow, the green dyed magenta and the red dyed cyan. When light passes through the finished image, the layer dyed yellow subtracts blue, the magenta layer subtracts green and the cyan layer subtracts red. The color at any point in the image is the combination of colors that are not subtracted. For example, red is seen when blue and green are subtracted from white light (Pénichon 2013, 128–131).
There are also subtractive processes that are not based on tripack film: Sanger-Shepherd, Lumiere Trichrome and Technicolor, for example, start by exposing three physically separate plates or strips of film through color filters. The separations are then dyed and superimposed to form the final image. The processes differ in how that is accomplished, but the result is the same as for tripack-based processes: three layers dyed in complementary colors. See those items below for more detail.
Photography has been applied to a wide variety of applications aimed at a broad range of users—professionals, amateurs, children, educators, scientists. The result is a profusion of materials, form factors, sizes and packaging.
Magic lanterns, the ancestor of slide projectors (themselves relics), were invented in the mid-1600s. Until photography was available, slides were hand painted or transfer printed—less than ideal for images that would be enlarged by projection. Hyalotype, the printing of photographs on glass, was invented by William and Frederick Langenheim in 1848 and immediately applied to making lantern slides. Photographic slides could provide far more detail than existing techniques and vastly increased the range of subjects available. Interest in magic lanterns grew and the industry expanded rapidly in the second half of the 19th century.
Celluloid 35mm slides began to replace glass magic lantern slides after Kodak introduced Kodachrome color slides in 1936, although magic lanterns remained in use for theater announcements, classroom instruction and business presentations into the 1950s. Celluloid slides were lighter, less fragile and less expensive than glass. Smaller slides led to smaller and lighter projectors. Reversal films like Kodachrome, which create a positive image when developed instead of a negative, simplified the process—slides could be cut out from the developed roll and placed directly in cardboard or plastic mounts. Home slide shows documenting vacations and family events became practical, affordable and ubiquitous.
Slide strips consist of multiple images in a single mount. These are often called slides or filmstrips, but I've adopted this term to differentiate them from "slides," which consist of a single frame in a mount and "filmstrips," which are unmounted.
A sequence of photographic images mounted on a disc for projection or viewing through a hand-held viewer.
A filmstrip is a sequence of photographic stills on unmounted celluloid film—typically 35 mm or 16 mm with or without sprocket holes. The images are from photographs from life or of artwork that may include text, cartoons or illustrations. The first filmstrips were produced on 55 mm film around 1918 by the Underwoods of New York. Production was soon taken over by the Stillfilm Company. The familiar 35 mm filmstrip emerged in the mid-1920s.
In 1853, J. B. Dancer, an English optician and microscopist, used the newly-invented wet collodion process to create photographs mounted on slides for viewing under a microscope. They were popular as curiosities and sold well. The broader potential of microphotography was first demonstrated in 1870 when a French microphotographer named Rene Dagron used it to send messages by carrier pigeon into Paris while it was under siege during the Franco-Prussian War. In the twentieth century, microphotography, in the form of microfilm and microfiche, became a standard way of archiving magazines, newspapers and other documents. It also found use in the military and espionage during World War II and the cold war.
In addition to a transparency, a photograph can be reproduced as a print: an opaque image reproduced from a negative on photopaper.
Phototypesetting emerged in the early 1950s. At the time, most typesetting was done with linotype machines, a technique known as hot type. Linotype machines were large, highly complex machines in which type was cast on the fly from a lead alloy. Phototypesetting, or cold type, took an entirely different approach. The shapes of characters for a particular font were stored photographically as negative images on a transparent disc or plate. Characters on the disc were exposed to light one at a time by positioning the negative image over photosensitive paper.
The soundtrack for the first talkie (a film with spoken dialog), The Jazz Singer in 1927, was stored on a 16 inch, 33⅓ RPM record disc, which was played concurrently with the movie. Sound-on-disc systems were used in theaters for several years, but synchronization was an issue and the records wore out quickly. Editing a film when the soundtrack was on a shellac record was also extremely difficult.
A competing approach, sound-on-film, stored the sound on the film itself as an optical waveform—literally a picture of the sound. The waveform was recorded photographically using either light reflected by a vibrating mirror or light from a source whose intensity varied with the sound. The audio was played back by shining a light through the soundtrack onto a light sensitive vacuum tube (originally developed for an optical signaling system used by the U.S. Navy). The sound was thus inherently synchronized and editing the film simultaneously edited the soundtrack. Although audio recording was eventually separated from film recording, particularly with the introduction of magnetic tape recording, an optical soundtrack is still added when films are printed today.
Before digital synthesizers, optical organs like the Optigan stored analog waveforms photographically. The concept was the same as an optical sound track for a movie, except that the waveforms were stored as circular tracks on celluloid discs, with each track storing a different pitch. The sound wasn't great and the mechanisms weren't that robust. As a result, optical organs weren't very successful, although they are still sometimes used by recording artists in search of unusual or retro sounds. Those sounds have also lived on, ironically, in the form of digital samples for synthesizers.
Developed in the late 1980s by the Drexler Corp., the LaserCard could store 2.6 megabytes of data. A photosensitive strip is bonded to the card is exposed to ultraviolet light passing through transparent areas in a master. When developed, the exposed spots, which are darker than the unexposed areas of the strip, can be read by reflected laser light. Originally intended to hold medical data, the technology, now marketed by HID Global, is apparently still in use in multiple countries for government ID cards.
Certain dyes become opaque when heated by a laser or by direct contact with a heating element. The dye can be applied to a variety of substrates including plastic, metal or paper. The process is irreversible, limiting it to applications like write-once optical discs and thermal printing.
Standard CDs and DVDs are manufactured by injection molding and are thus inherently read-only. The high quality of prerecorded audio and video they offered, at least relative to tape, played a large part of in their rapid ascendence. But cassette audio and video tape had accustomed consumers to recording their own music and video. You couldn't make a mixtape with a CD. You couldn't record a television show on a DVD to watch later. In the case of data applications like backup or scientific instrumentation, the capacity, speed of access and durability of optical media was attractive, but, again, the ability to record was the missing piece. This began to change in the mid-1980s with the introduction of recordable CDs.
The discs in this section are Write Once Read Multiple (WORM). WORM discs consist of a metal layer, a layer of organic dye, and the usual protective plastic. Data is written to a blank disc using a laser at a high enough power to heat the dye layer and cause a chemical change that makes the dye opaque in selected spots—the origin of the phrase to "burn a CD". During reading, the laser, at a lower power, directs light at the dye layer. Where it's still transparent, the light passes through and is reflected back by the metal layer. Where the dye is opaque, light is absorbed. The chemical change is permanent, which means the disc can only be written once. For many applications, such as recording, backup and data collection, this limitation is no problem. Discs that can be written multiple times use a different technology (see Phase).
In 1981, British Telecom introduced a prepaid phonecard that used thermal marking to track usage. The card was read by reflected infrared light. The reflective coating was burned away by a heating element as the card was used, allowing the phone to determine how much time was left.
Barcodes and mailing labels are often printed using direct thermal printing. In direct thermal printing, the print head contains heating elements that cause a chemical change that blackens a specially coated paper. Unlike thermal transfer printing, a direct thermal printer uses no ink or ribbon.
