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I see this term used in many places. It appears to be some side of the film but how would I be able to identify this? One side is sensitive to light and the other one isn't (light can still go through it).

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Film base is the transparent medium on which the photo sensitive emulsion is applied. It used to be made of nitrate (highly combustible), later acetate and polyester.

The main functions of base are:

  • provide support for the emulsion. The base is about 0.110 to 0.180 millimeter thick (roll film thinnest, large format thickest and the 35mm in between), making most of the total thickness of the film.
  • keep the emulsion flat when exposed; otherwise parts of the picture would be out of focus. This is helped by a pressure plate on the back of the camera, but some fancy cameras actually used vacuum.
  • not interfere with the positive process. Base for transparency films has to be clear (obviously), base for B&W films can be lightly tinted (it helps to counter halation) but not interfering with multigradation (no yellow, green or magenta - blue and gray is OK) and base for color films has amber tint.
  • provide archival stability. Polyester works the best, nitrate is risky because of the combustion issues.
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Photographic film is a light sensitive coat atop a material that is flexible and transparent. The first successful base material was made of cellulose nitrate made from purified cotton treated with nitric acid. This made an ideal base material. Cellulose nitrate was used until just after World War II. It has a major flaw in that it will burst into flames at a temperature of about 300°F. Worse, it will continue to burn vigorously even if drenched with water. Likely a movie house, somewhere in the world burned down every day. Thankfully “Safety Film” was introduced in the early 1950’s. It is cellulose acetate which will burn but it will self-extinguish. Today many different plastic film bases are in common use.

Undeveloped photographic is relatively opaque but strong light can travel completely through it. Images of bright object are sometimes spoiled by an unwanted halo that seems to surrounding them. This is called halation. It is light that passed through during the exposure and re-enters the film from the rear. To prevent, a special deposit called an “annihilation coat” is applied to the base film before the light sensitive coat is applied.

Darkroom workers are required to identify the emulsion side (side with light sensitive goodies) and the base side of roll film. When working under a safelight, the emulsion side likely appears dull by reflected light and the base side appears to have a luster. It can be difficult to make this determination especially when working in total darkness. Some tricks of the trade: All roll film tends to curl with an inward twist. The emulsion side is the innermost side.

Sometimes we wet a finger and touch a tiny area at a corner. The emulsion side will feel tacky. Sheet film is individual pre-cut films that we loaded into film holders. These films all had notches on one edge. The notches we could feel in total darkness, they identified the film type. We orientated the sheet till the notches were felt in the upper right corner. Holding the sheets this way, the emulsion side was facing us.

You can identify the emulsion side of developed film by looking at it via light reelected off its shiny surfaces. The emulsion side is dull compared to the base side. You can also look at the edge printing which will be numbers, letters and film manufacturer. These words and numbers read correctly when you are looking at the base side of the film.

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  • \$\begingroup\$ Not to be too nitpicky: the opaqueness of undeveloped film is due to emulsion, not base. The same for antihalation layer. Both go away during the development process (mainly fixer). The base itself is clear. It is usually transparent for B&W film and orange tinted for color film. The amber tint makes the paper in positive process blind to a tiny part of the spectrum that can be used as a safelight. \$\endgroup\$
    – Jindra Lacko
    Apr 20, 2017 at 6:22
  • \$\begingroup\$ The opacity of undeveloped film is a combination, the emulsions are salts of silver naturally off white yellowish. To this mix are added sensitizing dyes that alter the color of the crystals to force them to react to frequencies other than violet and blue. The anti-halation coat is opaque, generally dyed the color the film is less sensitive too. In some films it is carbon black dispersed in an acid plastic. Most 35mm film base is dyed gray to mitigate light piping likely when the film is not rolled adjacent to a black paper backing that is common for the larger sized rolls films. \$\endgroup\$
    – Alan Marcus
    Apr 20, 2017 at 6:59
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    \$\begingroup\$ The orange tint of color negative films comes from un-developed dye in the cyan and magenta emulsions. Together they give the orange tint. The un-developed dye creates two positive image overlayed on the three negative color images. This is an auto-mask system used to bolster the cyan and magenta dye. Both are slightly wrong as to color because of the complexity of making the three dyes lecuo (Greek for hidden) until they receive a single missing ingredient from the developer. The yellow is perfect. We have yet to find three dyes that can be lecuo and blossom correctly when developed. \$\endgroup\$
    – Alan Marcus
    Apr 20, 2017 at 7:17
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I am not writing about film.

Some of the earlier bases to put the sensitive material were metal plates, and glass. Inclusive your own skin is photosensitive, so you have a skin based photosensitive material.

Well, then someone used film, which had obvious advantages over metal and glass, mainly rolling it.

You identify it if

  1. It is film.
  2. It is photographic... (Sorry I could not hold myself)

As it is transparent you could say that does not matter if you expose it on the wrong side, it will be exposed, but yes, all this bases have a jelly of sensitive emulsion, that needs to be exposed on this side, because if not, the light is distorted by the base itself, and you get a blurry image.

The way to diferentiate it is normally the matt side, because the glossy side is due the acetate nature of it.

And normally, as the film comes in rolls, the emulsion is on the inner side, so it protects the emulsion when it is rolled.

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  • \$\begingroup\$ Would using the wrong side not mess your focusing up by the film base's thickness, even if there was no distortion? \$\endgroup\$
    – rackandboneman
    Apr 20, 2017 at 9:17
  • \$\begingroup\$ The 35mm and medium format films are packaged in a way that you can not expose them the wrong way. It can happen with large format films, which come in sheets that you have to load yourself in total darkness. The sheets have notches on a side to help you determining the emulsion side + all film manufacturers remind you to practice loading in daylight. \$\endgroup\$
    – Jindra Lacko
    Apr 20, 2017 at 11:04
  • \$\begingroup\$ On a second thought: you actually can expose 35mm film with emulsion backwards. It involves a lot of effort - you need to unwind the film and respool it in total darkness. It does not mess up the focus, but it messes up the color rendition. It is used for artistic effect and the technique is called redscaling (you can guess why). \$\endgroup\$
    – Jindra Lacko
    Apr 20, 2017 at 11:16
  • \$\begingroup\$ Most feature a robust anti-halation layer to preclude exposure should the film get loaded with the emulsion side away from the lens. Kodachrome and most movie film have an opaque coat on the obverse called rem-jet (removable jet-black). This is lamp black dispersed in a binder that relaxes in the presence of an alkaline solution. It guards against exposure from the rear that can occur if the photographer should remove his eye from the viewfinder. Nevertheless, amateurs frequently loaded movie film backwards, by mistake. When severely overexposed, weird images resulted. \$\endgroup\$
    – Alan Marcus
    Apr 20, 2017 at 15:29

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