Priya- Polaroid | Scientia project

How do Polaroids Work?

Recently I was gifted a Polaroid from my friends (which I now absolutely cherish), but I found myself curious as to how exactly the camera may work, the mechanism behind the slow development of an image on the iconic Polaroid film.

Whilst researching, I found that understanding the way a black and white film camera works is necessary to understand the mechanism in which a colour polaroid works (understandable as this was the initial invention). I shall try to explain this to the best of my understanding.

BLACK AND WHITE PHOTOGRAPHY

To start off, one should know about the film in the Polaroid camera. In black and white film, you ave a top layer of plastic and a light sensitive layer. This light sensitive layer is made of silver halides (for example silver bromide) which react and decompose when exposed to light (essentially it breaks down into silver and halogen ions). This means that silver atoms form in the areas exposed to light, making the image darker.

Now, for the part where the image is shot, and its development. When the shutter of a camera opens, light enters the camera and hits the film creating a map of the image. The shutter only opens for a fraction of a second to ensure that it capture (more or less) a frozen picture. The areas of the scene where there is more light exposure will be darker on the film, whilst the areas with less light will be lighter. This is as more silver atoms are formed where there is more light hitting, and the accumulation of metallic silver atoms will render that part darker. This forms an inverted image known as a negative image: the areas which should be light are dark, and the areas that should be dark are light. This would be seen as the step between the first and second image below.

Image taken by Priya Shah in Bali Indonesia

To generate the positive Black and White image, one would simply need to expose the negative black and white image to light again: the dark areas would not have as much light exposure, whereas the light areas would have less exposure. Thus silver atoms would accumulate at the now light areas, which were the initial dark areas and vice versa. It's kind of like a reverse UNO card.

So the mechanism in which black and white film works is just some basic chemistry: the decomposition of silver halide atoms to silver and the halogen.

COLOR PHOTOGRAPHY

Coloured film works similarly, but is more complex. I'll start off by an image of a diagram of the film I found on many websites that explained how Polaroids work.

Image taken from https://www.scienceabc.com/wp-content/uploads/2018/05/Polaroid-picture-3-coating-layers.-1.jpg

Here's a brief explanation of each layer.

Image layer: Where the inks travel up to and where the image forms

Reagent layer: where the reagent is applied

Light sensitive layer (blue, green, or red): the layer sensitive to blue, green or red light.

Base: A black layer that ensures that no light is reflected back through the film, which could cause distortions to the image.

The dyes are pretty self explanatory (different ink colours). There are 2 other layers that aren't displayed that I will touch on later: the timing layer and the acid layer.

Just like the black and white film, the light sensitive layers are also made of silver halides, however each layer is only sensitive to some wavelengths of light, as noted above. Differing from the black and white film, coloured film have different colours of dye underneath each layer.

You may have noticed that the coloured dye below each light sensitive layer are opposing colours as shown in the light color wheel below ie. the red sensitive layers have cyan dye underneath.

This is necessary to the function of the film, it works the same way exposing film to light again reverses the negative image (the reverse UNO card).

For example, when green light hits the green sensitive layer, metallic silver will form on those areas. If you had green dye under there, it would be stopped from moving up to the image layer by the silver atoms formed. By using magenta dye (greens opposite color in light) under the green layer, that dye is prevented from moving up to the image layer. The formation of silver atoms on the green parts of the film acts like a wall stopping magenta dye from moving up to the image layer. However cyan and yellow dyes do make it up to the image layer in the green areas forming... well green. The same thing happens for all the other colours- and as you see in the complex color wheel on your computer, red green and blue colors of light can form any other color.

Image taken from http://lucaskrech.com/blog/index.php/2010/01/22/color-theory-basics-additive-and-subtractive-color-mixing/

The reagent is a mixture with alkali and opaque white pigments, as well as other chemicals. It is stored in the white pockets you see on a Polaroids white border, and is released into the reagent layer when the shutter opens. The reagent creates an opaque layer preventing further light from entering the image and distorting it. It also then moves down the light sensitive layers and reacts with the dyes. The dyes and reagent then diffuse upwards to the image layer.

However, at this point you still won't see an image. This is as there is an acid layer above the image layer that covers your view of the image. Once the reagent reaches this layer (this process is slowed down by another layer suitably called the timing layer), the alkali and opaque white pigments in the reagent reacts with the acid clearing up that layer. This is when you finally see the Polaroid image slowly come into view- the best part of taking a Polaroid picture.

Sources

"How do instant cameras work?" (2 April 2001)
HowStuffWorks.com Available at: https://electronics.howstuffworks.com/question605.htm

Tom Harris (14 February 2002). "How Instant Film Works"

HowStuffWorks.com. Available at: https://science.howstuffworks.com/innovation/everyday-innovations/instant-film.htm

Akash Peshin (14 May 2018). "How do Polaroid Pictures Work"

ScienceABC.com. Available at: https://www.scienceabc.com/innovation/how-do-polaroid-pictures-work-instant-color-films.html

Charles Woodworth (1 April 2000). "How Photographic Film Works"
HowStuffWorks.com. Available at: https://electronics.howstuffworks.com/film.htm

Helmut Erich Robert Gernsheim and L. Andrew Mannheim (February 10, 2017). "Technology of photography".

Encyclopædia Britannica, inc. Available at: https://www.britannica.com/technology/technology-of-photography/Instant-picture-photography

Nichole Marie Witten (2016). "The Chemistry of Photography"

Available at: https://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=1085&context=senior_theses

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