Before I go to talk more about film structure and how film works, I would like to have a brief introduction of simple color theory that is used in film system.

Visible light has a wavelength range from 400nm to 700nm in electromagnetic spectrum. We see the light from the sun as white light because it contains the full spectrum. White light can simply be classified into Red, Green and Blue light.

Visible light can be classified into red, green and blue light

An object either reflects or absorbs a certain wavelength of light. For example, when white light shines onto a object that reflects red light and absorbing green and blue light, we will see it as red color. A white object will be the one reflect all red, green and blue light. A green object will be relecting green light but absorbing red and blue light as the diagram below:

If we shine red light onto these objects again, the red object will reflect red light so it appears red, the white object will reflect red light but as there is no green and blue light, the white object will appear red color only. Finally, the green object will absorb all red light but since there is no green light from the light source so it cannot reflect green light, the green object will appear black color in this case.

Same theory applies when we shine green light onto these 3 objects. The white and green objects will appear white and green color respectively, while the red object will appear black in color.

Red, green and blue lights are additive colors.

Red, green and blue lights can be added (mixed) into more colors:

Additive Colors can be mixed

If red, green and blue lights are shined on a white surface, the overlapping areas:

  • Red + Blue = Magenta color
  • Green + Blue = Cyan color
  • Red + Green = Yellow color
  • Red + Green + Blue = White color

Let’s make a series of color filters based on these 3 new colors – magenta, cyan and yellow filters.

So you can see yellow filter absorbs (controls) blue light, magenta filter absorbs (controls) green light, cyan filter absorbs (controls) red light.

Therefore, by using different strength and different combination of yellow, magenta and cyan filters, we can control the amount of red, green blue lights and thus, reproducing the original color of an object.

That is also why magenta, yellow and cyan are called subtractive color.

In summary, remember these rules:

  • Yellow filter controls blue light
  • Magenta filter controls green light
  • Cyan filter controls red light

Film emulsion uses the subtractive color system to reproduce the original images. A processed film is in fact a combination of different strength of yellow, magenta and cyan filters.

I will discuss in later posts on how a piece of film is able to record an image and then be turned into a combination of different strength of yellow, magenta and cyan filters.

To be continued……

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