# What makes an X-ray machine's white photographic film go black?

I read on a bbc gcse bitesize webpage "In older X-ray machines, white photographic film" [edit- I now know that bbc webpage is wrong, and it's not a white film. I'll leave the rest of the post after the next sentence, as is, but note that from the next paragraph, the rest of the post is based on the misunderstanding from that bbc link that there is white film (And I guess it's talking of any pre digital when it says 'older' - and anyhow prior to film they used photographic plates - glass rather than plastic/film ).]

I understand that X-ray images are black and white. You have the X-rays themselves, the white film, and an object/person in-between.

The white parts of the image are bones and/or things the X-rays couldn't get through, the white film remains white, and the black part is where the X-rays did pass through, and it blackens the white film.

I've also heard that they have a cassette/casing, and in that is fluorescent material or coating and film. The X-rays hit the fluorescent material and (I guess by absorption and reflection, or absorption and emission) converts it into regular light, and there is a phosphorescent material/coating there, too, so the light remains for a while, and the light, I guess, blackens the film.

I understand they don't work like that anymore and now you can have other types of X-ray detection mechanisms. But I want to understand at least how the traditional ones are black and white.

I understand that X-ray machines used to use a white photographic film.

What is the white photographic film made from that distinguishes it from a black photographic film? Can I buy white photographic film?

I have seen old-fashioned camera film before.

In black and white photography, was it a black photographic film that went white when exposed to light?

• Answer not worth putting as answer: en.wikipedia.org/wiki/Photographic_processing Aug 12 '15 at 17:12
• @ssavec I don't see the answer there. It mentions about film that is black and goes white on exposure. But white film used in xrays is white and goes black on exposure.. and i'm asking about that white film.. how its composition is different Aug 12 '15 at 18:24
• @barlop Normal B&W film is transparent ("white") if not exposed and becomes dark where exposed and developed, same as x-ray film. This is known as a negative image. ssavec's link mentions a reversal process for making positives (for making projection slides), but apart from a few changes to make them work better with x-rays, the film behaviour and chemistry is no different than normal silver halide B&W film. (incidentally, x-ray film is still very commonly used, especially in things like dentistry where the cost of digital imagers is a concern) Aug 12 '15 at 18:53
• @MichaelD.M.Dryden interesting.. it doesn't seem to be white it seems to be like this(photo of a black and white film selling on ebay) i.imgur.com/fkT2jK8.png which is this colour #8c866c backgroundcolor.net/hex/8c866c whereas xray photos seem to have proper white. Would it have been white like white paper until it was yanked out of the roll? if so then e.g. how soon after being taken out of the roll would it have changed from paper white to that hex #8c866c colour? Aug 12 '15 at 19:54
• @barlop That's before it's developed and fixed. Once developed and fixed, if not exposed, the silver halide is dissolved and you only see the transparent plastic base of the film (they come in a few colours from grey to purple). The silver halide that has been exposed to light is converted into elemental silver by the developer and thus appears dark. Aug 12 '15 at 20:01

X-ray film is very similar to standard black and white negative film. It's basically a transparent plastic base coated with an emulsion of silver halide particles (a salt of $\ce{Ag+}$ with a halide, usually $\ce{Cl-}$ or $\ce{Br-}$). As shown below, when light hits one of these particles, it reduces a small amount of the halide to elemental silver (1). When this is exposed to a photographic developer (essentially a mild reducing agent), more elemental silver is produced around the initial sites (2).$$\ce{Ag^+ +e- -> Ag}$$

Finally, a fixing agent is added which solubilizes the unexposed silver halide so it can be removed from the emulsion, preventing further light exposure from fogging the film.

Thus, the regions that are exposed to light appear dark when developed as the tiny silver particles ($\ce{Ag}$) block light passing through the film and the unexposed regions are transparent, taking on the colour of the plastic base (often blue or grey). X-ray films are typically viewed on an illuminated white background, making the unexposed areas appear white (or off-white, coloured by the plastic base).

Normal B&W film is actually slightly sensitive to x-rays but, the emulsion of dedicated x-ray film contains other components to facilitate the silver reduction from x-ray exposure. Source

• Thanks. When you write " facilitate the silver reduction from x-ray exposure." do you mean to lessen or prevent the silver from being reduced due to xray exposure? (I understand that what happens with xrays is they hit a fluorescent material which lights up with normal light and hits the film - or I guess, hit the silver) Aug 12 '15 at 21:25
• Other way around—facilitate = make easier. You want the silver to be reduced when it's hit with an x-ray. Mixing it with a fluorescent dye would be one way of doing it. I don't know if there are any other methods in use. Aug 12 '15 at 21:27
• I see, so when the developer gets the film, the parts that got exposed to light, the black parts, have no silver, and the parts that remain clear, are the ones covered in silver. But then what's the point of the silver in the first place? If the silver disappears when the light hits.. e.g. I presume the film is protected in an environment of blackness/darkness(with the exception of the parts that intentionally got exposed to light when the picture was taken), and the film remains in an environment of darkness until after the developer develops it. Aug 12 '15 at 21:37
• Other way around again. The parts that are dark are where the silver halide has been reduced to elemental silver. The transparent parts were not reduced and the silver halide is washed away in the fixing step. The film does have to be protected from light except when intentionally exposed, though this is easy with x-rays since they will pass through materials that keep visible light out. Aug 12 '15 at 21:42
• thanks.. I understand that now. So why should elemental silver on a clear film , after development, make blackness? Aug 12 '15 at 21:48

Michael's answer is very good, i'll just simplify

I understand that X-ray images are black and white.

No, it's clear, then black and clear. Not white. When they are on a white background e.g. white table or white wall, they show as white and black.

If they were on a purple background then they'd show up as black and purple (purple bones). Because the clear part of the xray takes the colour of the background the xray image is on. But it's clear and black.

And re what makes the film go black, the film itself doesn't, but the exposed silver on it does.

You have the X-rays themselves, the white film, and an object/person in-between.

The X-rays themselves, the clear film, and an object/person in-between

Apparently clear/transparent film may come in a few background colours can be grey, can be purple, can be amber.. i've seen amber transparent colour film (that's the case with colour films used by photographers, and may or may not be the case with a photographers films for black and white photos, and the films used in xrays). The ones for black and white photos, and for xrays, may be grey

And yeah.. The CLEAR parts of the image are bones and/or things the X-rays couldn't get through (e.g. tumours), the clear parts of film remain clear,

and the black parts [on the film] is where the xrays hit

The film is covered in a kind of silver particles, silver halide.

The silver can be blackened by xrays or light..

It seems that technically, most of the rays hitting the silver are light, because in order to require less dosage of radiation, the scientists thought of using a fluorescent material, the xrays pass through that and the fluorescent material converts the xrays into visible light, it absorbs the xrays and emits visible light.

Fluorescent material does that(material with fluorescent phosphors). For example some "invisible ink" is florescent, it lights up when UV light is shone on it. It takes the high energy photons/short wavelength waves and absorbs it and emits lower energy photons/longer wavelength waves/visible light. Wikipedia says most fluorescent material converts that way, high energy to low energy.

In addition to flurescent material, there is also phosphorescent material, it has phosphorescent phosphurs,which keeps the light stored for some time.

The parts of the silver metal on the film that will be darkened, will be darkened mostly by light, and partly by xrays, since there's more light than xrays hitting it.

The clear parts of the image are bones and/or things the X-rays couldn't get through, the clear film remains clear, and the black part is where the X-rays did pass through, and it blackens the clear film.

It blackens the silver on the film, not the film itself. The film itself is a clear film with silver on it.

The film though is predevelopment.. And predevelopment, the entire film is covered in silver.

The parts of the silver that got hit with xrays or light, will go black. And change in form from silver halides to metalling silver, and they won't get washed away in development. The parts of the silver that didn't get hit, will get washed away during the development.

I've also heard that they have a cassette/casing, and in that is fluorescent material or coating and film. The X-rays hit the fluorescent material and (I guess by absorption and reflection, or absorption and emission) converts it into regular light, and there is a phosphorescent material/coating there, too, so the light remains for a while, and the light, I guess, blackens the film.

yeah I suppose so. And the light blackens the silver on the film not the film itself.

You ask about the materials used for xray film and regular photographic film.. Apparently the differences are minor.. And in both cases the film is clear.

He showed a colour photographic film that was amber/orange, and transparent.

The black and white one maybe is clear like regular glass is clear..I don't know if it has any slight colour at all but even if it did, it's transparent no question about it.

I think maybe the term "negative" doesn't apply with xrays. Because in regular photography the rays bounce off the subject rather than through the subject, and with a normal camera and film, what is light comes out dark and what is dark comes out light, hence it's called a negative.

Another thing that happens, (at least with regular photopgraphy, perhaps not with xrays), is the negative is turned into a positive.. And that's done by taking a picture of the negative (A), making a negative of the negative, which is a positive (B). I believe that's done with an enlarger that also enlarges it. The silver is dense and blocks the light. The fact that silver on A is black is irrelevant, since it's the denseness that blocks the light to produce B.

Michael mentions about the denseness of the silver, but I don't think it's relevant to the xray..in making a new image, but I think he means you'd get little clear areas / specks that aren't black in the xray if the silver wasn't dense, but it is dense.. So the denseness and the blackness helps there in the black parts of the xray image appearing black.