Film Base(17)
The film base is the plastic support that carries the light-sensitive emulsion. Requirements for a suitable film base include optical transparency, freedom from optical imperfections, chemical stability, photographic inertness, and resistance to moisture and processing chemicals. Mechanical strength, resistance to tearing, flexibility, dimensional stability, and freedom from physical distortion are also important factors in processing, printing, and projection.

Two general types of film base are currently used-cellulose triacetate esters and a synthetic polyester polymer known as ESTAR Base. Cellulose triacetate film base is made by combining the cellulose triacetate with suitable solvents and a plasticizer. Most current KODAK and EASTMAN Motion Picture Films are coated on a cellulose triacetate base.

ESTAR Base, a polyethylene terephthalate polyester, is used for some KODAK and EASTMAN Motion Picture Films (mostly intermediate and print films) because of its high strength, chemical stability, toughness, tear resistance, flexibility, and dimensional stability. The greater strength of ESTAR Base permits the manufacture of thinner films that require less room for storage. ESTAR Base films and other polyester base films, cannot be successfully spliced with readily available commercial film cements. You can splice these films with a tape splicer or with a splicer that uses an ultrasonic or an inductive beating current to melt and fuse the film ends.

Antihalation Backing
Light penetrating the emulsion of a film can be reflected from the base- emulsion interface back into the emulsion. As a result, there is a secondary exposure causing an undesirable reduction in the sharpness of the image and some light scattering, called halation, around images of bright objects. See Figure 27. A dark layer coated on or in the film base will absorb and minimize this reflection, hence it is called an antihalation layer. Three methods of minimizing halation are commonly used:

Rem Jet: A black-pigmented, nongelatin layer on the back of the film base serves as an antihalation and antistatic layer. This layer is removed during photographic processing.

Antihalation undercoating: A silver or dyed gelatin layer directly beneath the emulsion is used on some thin emulsion films. Any color in this layer is removed during processing. This type of layer is particularly effective in preventing halation for high-resolution emulsions. An antistatic and/or anticurl layer may be coated on the back of the film base when this type of antihalation layer is used.

Figure 27
	Light Piping

Dyed film base: Film bases, especially polyester, can also transmit or pipe light that strikes the edge of the film. This light can travel inside the base and fog the emulsion (Figure 27). A neutral-density dye is incorporated in some film bases and serves to both reduce halation and prevent light piping. This dye density may vary from a just detectable level to approximately 0.2. The higher level is used primarily for halation protection in black-and-white negative films on cellulosic bases. Unlike fog, the gray dye does not reduce the density range of an image, because it, like a neutral- density filter, adds the same density to all areas. It has, therefore, a negligible effect on picture quality.

Edge Numbers
Edge numbers (also called key numbers or footage numbers) are placed at regular intervals along the film edge for convenience in frame-for-frame matching of the camera film to the workprint. The numbers are printed along one edge outside the perforations on 35 mm film and between the perforations on 35 mm film and between the perforations on 16 mm film. The numbers are sequential, usually occurring every 16 frames (every 12 inches) on 35 mm film and every 20 frames (every 6 inches) on 16 mm film. In a few instances, edge numbers on 16 mm films are located every 40 frames (12 inches).

All Kodak camera film is edge numbered at the time of manufacture in one of two ways:

Latent Image: The film edge is exposed by a printer mounted at the perforator to produce an image visible only on processed film. The five or seven digits are sequential and will change every 16 (35 mm) or 20 (16 mm) frames. The cluster of numbers and letters to the left of the sequential numbers are a manufacturer's code for the type of product, the perforator, and the equipment used to produce the product. All Kodak 16 mm and 35 mm camera color film is latent-image edge numbered (Figure 28).

Visible Ink Image: During manufacturing, the filrn stock is numbered with a visible ink. Again, this process is performed at the perforators. The ink, unaffected by photographic chemicals, is printed on the emulsion surface of the film. The numbers are visible on both the raw stock and the processed film. In Figure 29, the visible ink edge numbering will be more visible after processing. All 35 mm Kodak black-and-white motion picture camera films have ink edge numbers. The letter "C" is a manufacturer's product identification.

Figure 28
	Latent image edge numbering

A third method of applying edge numbering is very often used by commercial motion picture labs. There the film is numbered on the base side, generally with yellow ink. This numbering does not interfere with the manufacturer's edge numbers because the lab numbers are ordinarily printed on the opposite edge of the film. Normally, both the original camera film and the workprint are edge numbered identically for later ease in matching the two.

Figure 30 is a sample of EASTMAN EKTACHROME Video News Film 7240 (Tungsten), edge numbered by a laboratory in New York City.

With double-system sound, both the film and the magnetic tape are often edge numbered by the lab for ease of editing.

Visible ink edge numbering	Laboratory applied edge numbering
Figure 29	Figure 30

In 1990, Eastman Kodak Company introduced a new edge-numbering system that will eventually be included on all Eastman camera negative films, both black-and-white and color. The new system incorporates EASTMAN KEYKODE^tm; numbers which are machine readable in bar code. A variety of scanners can read this bar code in the same way that the bar code on most products in supermarkets is read by a scanner in the checkout line. The human-readable key numbers are similar to previous edge numbers, but are easier to read. In this improved format, the key number consists of 12 highly legible characters printed at the familiar one-foot, 64 perforation interval. The KEYKODE^tm; number incorporates the same human-readable number, but in a bar code. See Figures 31 and 32.

EASTMAN 16 mm Edgeprint Format
Featuring KEYKODE^TM Numbers

Figure 31 BASE UP

Eastman Kodak Company, 1990		Kodak, Eastman and Keykode are trademarks. H-41a

EASTMAN 35 mm Edgeprint Format
Featuring KEYKODE^TM Numbers

Figure 32

Dimensional Change Characteristics
Motion picture film dimensions are influenced by variations in environmental conditions. The film swells during processing, shrinks during drying, and continues to shrink at a decreasing rate throughout its life. These dimensional changes in film are either temporary (reversible) or permanent (irreversible). Temporary size changes are caused by a modification in the moisture content or the temperature of the film. The extent of both temporary and permanent size alterations is largely dependent upon the film support. However, since the emulsion is considerably more hygroscopic than the base, it also has a marked influence on dimensional variations caused by humidity. Permanent shrinkage of film on cellulose triacetate support is due to loss of residual solvents or plasticizer, and, to a slight extent, the gradual elimination of strains introduced during manufacture or processing. ESTAR Base has no residual solvent or plasticizer and absorbs less moisture than cellulose triacetate; consequently, its size changes are considerably less. Some permanent shrinkage occurs in aging of raw stock processing, and aging of processed film. Values for the dimensions change characteristics of current KODAK and EASTMAN Motion Picture Films are given in the table below.

Temporary Size Change
Moisture. Relative Humidity (RH) of the air is the major factor affecting the moisture content of the film, thus governing the temporary expansion or contraction of the film (assuming constant temperature). For camera films, the humidity coefficients are slightly higher than for positive print films. The coefficients given in the table above are averages for the range of 15- to 50-percent RH, where the relationship between film size and relative  humidity is approximately linear. For ESTAR Base films, this coefficient is larger at lower humidity ranges, and smaller at higher humidity ranges. When a given relative humidity level is approached from above, the exact dimensions of a piece of film on cellulose triacetate support may be slightly larger than when the level is approached from below. The opposite is true for ESTAR Base films, which will be slightly larger when the film is previously conditioned to a lower humidity than it would be if conditioned to a higher humidity.

Temperature. Photographic film expands with heat and contracts with cold in direct relationship to the film's thermal coefficient. The thermal coefficients for current KODAK and EASTMAN Motion Picture Films are listed in the previous table.

Rates of Temporary Change. Following a shift in the relative humidity of the air surrounding a single strand of film, humidity size alterations occur rapidly in the first 10 minutes and continue for about an hour. If the film is in a roll, this time will be extended to several weeks because the moisture must follow a longer path. In the case of temperature variations, a single strand of film coming in contact with a hot metal surface, for example, will change almost instantly. A roll of film, on the other hand, requires several hours to alter size.

Swell during Processing. All motion picture films swell during photographic processing and shrink during drying. The swell of triacetate films is initially rapid and depends upon the temperature of the processing solutions, time, and film tension. Acetate films swell more in the widthwise than in the lengthwise direction, and negative films swell more than print films. The change for ESTAR Base films is much smaller. The effects of drying upon the final dimensions are discussed in the section on permanent size change.

		Swell %
Film Type	Base	Length	Width
Negative	Triacetate	0.4	0.6
Positive- Black-and-White and Color	Triacetate	0.3	0.5
Reversal-Color	Acetate-Propionate	0.6	0.8
Positive-Color	ESTAR	0.05	0.05

SWELL DURING PROCESSING

MINUTES AT 21^oC (70^oF)
IN PROCESSING SOLUTIONS

Permanent Size Change
Permanent size change is the summation of the shrinkage of the raw film, the size change due to processing, and the shrinkage of the processed film.

Raw Stock Shrinkage. Immediately after slitting and processing, the unexposed motion-picture film is placed in cans that are sealed with tape. Until the film is removed from the can, solvent loss from triacetate film is extremely low. The lengthwise shrinkage will rarely exceed 0.5 percent during the first 6 months in a 1000-foot can of 35 mm film. ESTAR Base films will not shrink more than 0.2 percent while in a taped can.

Processing Shrinkage. The net effect of processing triacetate base film is normally slight shrinkage (see table) unless the film has been stretched. Some commercial processing machines have sufficiently high tension to stretch the wet film (particularly 16 mm film); consequently, a lower net processing shrinkage or even a slight permanent stretch may result. Because of its greater strength and resistance to moisture, the overall size change of ESTAR Base films is much less.

Aging shrinkage. It is important that motion picture negatives, internegatives, and color originals have low aging shrinkage so that you can make satisfactory prints or duplicates even after many years of storage. With motion picture positive film intended for projection only, shrinkage is not especially critical because it has little effect on projection.

The rate at which aging shrinkage occurs depends upon the conditions of storage and use. Shrinkage is hastened by high temperature and, in the case of triacetate films, by high relative humidity which aids the diffusion of solvents from the film base.

The potential aging shrinkage of current motion-picture films is given in this table. In the case of processed negatives made on stock manufactured since June 1954, the potential lengthwise shrinkage of about 0.2 percent is generally reached within the first two years and almost no further shrinkage occurs thereafter. This very small net change is a considerable improvement over the shrinkage characteristics of negative materials available before 1954 and permits good printing even after long periods of keeping.

The lengthwise shrinkage of release prints made on triacetate supports is about 0.1 to 0.3 percent for 35 mm film and 0.1 to 0.4 percent for 16 mm film during the first two years. Higher shrinkage can occur over a longer period, as indicated in this table. Shrinkage of films on ESTAR Base is unlikely to exceed 0.04 percent.

Although aging shrinking of motion picture films is a permanent size change, humidity and thermal size changes can either increase or decrease the observed size change. 

Other Physical Characteristics
Aside from image quality considerations, other factors can affect the satisfactory performance of motion picture film.

Curl
Photographic-film curl is defined as the departure from flatness of photographic film. Curl toward the emulsion is called positive while curl away from the emulsion is termed negative. Although the curl level is established during manufacture, it is influenced by the relative humidity during use or storage, processing and drying temperatures, and the winding configuration.

Figure 33

At low relative humidities, the emulsion layer contracts more than the base generally producing positive curl. As the relative humidity increases, the contractive force of the emulsion layer decreases and the inherent curl of the support becomes dominant.

Film wound in rolls tends to assume the lengthwise curl conforming to the curve of the roll. When a strip of this curled film is pulled into a flat configuration, the lengthwise curl is transformed into a widthwise curl.

Buckling and Fluting
Very high or low relative humidity can also cause abnormal distortions of film in rolls. Buckling, caused by the differential shrinkage of the outside edges of the film, occurs if a tightly wound roll of film is kept in a very dry atmosphere. Fluting, the opposite effect, is caused by the differential swelling of the outside edges of the film; it occurs if the roll of film is kept in a very moist atmosphere. To avoid these changes, do not expose the film rolls to extreme fluctuations in relative humidity.

Aditional reading on "Physical Characteristics of film."

Adelstein, P. Z. and Calhoun, J. M., "Interpretation of Dimensional Changes in Cellulose Ester Base Motion Picture Films," Journal of the SMPTE, 69:157-63, March 1960.

Adelstein, P. Z. Graham, C. L., and West, L. E., "Preservation of Motion Picture Color Films Having Permanent Value," Journal of the SMPTE, 79:1011-1018, November 1970.

Calhoun, J. M " "The Physical Properties and Dimensional Behavior of Modon Picture Films," Journal of the SMPTE, 43:227-66, October 1944.

Fordyce, C. R., "Improved Safety Motion Picture Film Support," Journal of the SMPTE, 51:331-50, October 1948.

Fordyce, C. R., Calhoun, J. M., and Moyer, E. E., "Shrinkage Behavior of Motion Picture Film," Journal of the SMPTE, 64:62-66, February 1955.

Miller, A. J. and Robertson, A. C., "Motion Picture Film-Its Size and Dimensional Characteristics," Journal of the SMPTE, 74:3-1 1, January 1965.

Neblette, C. B., "Photography-Its Materials and Process," Chapter 11, D. VanNostrand Co., Inc., 1962