How does a Colour Photocopier Really Work?
A colour xerographic copier isn't all that much more complicated than a black and white one. It simply combines four different coloured toners on a single sheet of paper to create full colour images. The fact that only four toners are needed to make us see all the possible colours is a consequence of our simple colour vision; our eyes really only detect three different types of light (red, green, and blue) and our brains interpret various mixtures of those three lights as different colours. To make use of this fact, three of the toners are designed to block particular types of light (one toner blocks red light, one blocks green light, and one blocks blue light). The fourth toner is simply black and helps to improve the contrast of the finished copies.
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The Physics of Colour in Colour Copiers
We can examine colour vision more carefully when we look at Television and Fluorescent Lamps. For the present problem, you only need to know that the colour photocopier is trying to detect how much red light, how much green light, and how much blue light there is coming off the original document. The cheapest colour xerographic system exposes the same photoconductor drum to light from the document four separate times: once through a filter that passes only red light, once through a filter that passes only green light, once through a filter that passes only blue light, and once without any filter at all. The first pass determines where to place red-absorbing toner, the second pass places green-absorbing toner, the third pass places blue-absorbing toner, and the last pass places black toner. These four toner images are superimposed on the paper and create a full colour image.
Photocopier Drum Charge - Technical Question and Answers - The Physics
Q1. Before the first pass (the one that uses red-filtered light to control the placement of red-absorbing toner), the photoconductor drum is covered with positive electric charge. This charge comes from a thin wire (a corotron) that is held at a high positive voltage of 10,000 volts. Charges leave this wire via a corona discharge and stick to the nearby drum surface. As these positive charges stick, the voltage of the photoconductor surface increases. However, it never exceeds 10,000 volts. Why not?
Answer: For the voltage on the photoconductor surface were to exceed 10,000 volts, something would have to do work on the (positive) charge to transfer it from the thin wire to the photoconductor surface and there isn't anything to do that work.
How: (Positive) charge naturally flows from higher voltage to lower voltage and releases electrostatic potential energy in the processes. Charge can't spontaneously flow from lower voltage to higher voltage because that would mean that its electrostatic potential energy rises. Without a source of energy, such a rise would violate conservation of energy.
Q2. The photoconductor surface is supported by a metal cylinder that's connected by a wire to the earth. Since the voltage of the earth is zero, the cylinder and the inside surface of the photoconductor drum are also always at zero volts. That's true even though negative charges flow through the cylinder and onto the inner surface of the photoconductor as positive charges land on the outer surface of the photoconductor Once the photoconductor is fully charged, its outside surface has a voltage of 10,000 volts and its inside surface has voltage of 0 volts. A small patch of the photoconductor is then exposed to red light from a document and 0.000001 coulomb (one millionth of a coulomb) of charge crosses from the outside surface to the inside surface of the photoconductor. How much potential energy is released when this charge moves? (This question involves a simple calculation and a quantitative answer. For simplicity, assume that the voltages of surfaces aren't changed by the transfer of charge.)
Answer: 0.01 joules of energy are released.
How: Voltage measures the electrostatic potential energy per unit of charge. In this case, there are 0.000001 coulombs of charge having 10,000 volts (10,000 joules per coulomb) of voltage. If that quantity of charge is permitted to release all of its electrostatic potential energy and drop to 0 volts, then 0.000001 coulombs times 10,000 joules per coulomb will be released. That product yields 0.01 joules, which is the amount of energy that will be released when the charge is permitted to cross through the photoconductor.
Q3. A colour photocopier's photoconductor must respond to red light while the photoconductor in a black & white photocopier can and usually does ignore red light. What must be different about the arrangement of quantum levels in the colour photocopier's photoconductor as compared to that in the black & white photocopier?
Answer: The colour photocopier's photoconductor must have a small energy difference between the filled valence levels and the empty conduction levels. In the black & white photocopier, the energy difference can be larger.
How: For the colour photocopier's photoconductor to respond to red light, with its low photon energy, the energy required to shift an electron from a valence level to a conduction level must be relatively small. The bands of levels must be relatively close in energy. But since the black & white photocopier's photoconductor doesn't have to respond to red light, it can have a wider energy separation between its valence levels and its conduction levels.
Q4. A photocopier inevitably places some toner where it doesn't belong or omits it from where it does belong. This effect is partly the result of thermal energy. The hotter the environment, the more mistakes the photocopier will make. Fortunately, it has to be pretty hot before the thermal mistakes are noticeable. (a) How does thermal energy cause the photocopier to make mistakes and (b) why is the photoconductor used in a colour photocopier more vulnerable to these thermal mistakes than the photoconductor used in a black & white photocopier?
Answer: (a) Thermal energy can shift an electron from a valence level to a conduction level and allow the photoconductor to conduct charge. (b) The small energy separation between the valence and conduction bands in the colour photocopier's photoconductor makes it easier for thermal energy to shift an electron from a valence to a conduction level and cause conduction.
How: Both light and thermal energy are capable of shifting electrons from valence levels to conduction levels in a semiconductor. The closer the valence levels are to the conduction levels, the more easily thermal energy can cause this sort of shift. Once it occurs, the photoconductor is able to conduct a small amount of electric current. That's why overheating a photoconductor/semiconductor is usually a bad idea.
About Colour Photocopiers - A History and Technical overview of How Colour Photocopiers Work