The image above shows what happens when the television set is turned on and the grains of an electron gun effect are visible.

 

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A grain of a cathode ray tube (CRT) is a small, largely invisible, piece of glass or plastic that makes up the inside surface of a CRT screen tube. 

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The size of these glass or plastic particles ranges from 0.1 millimeter to 2 millimeters in diameter; typically finer grains are used in color TVs and thicker ones are used in black-and-white TVs.

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Grain size is controlled by the manufacturing process.A grain of a CRT screen is an array of many thousands (typically tens of thousands) or millions of tiny glass spheres. 

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These spheres are usually made from some form of glass, such as borosilicate or soda-lime type glasses, but are occasionally made from plastic. The purpose is to minimize interference effects (coloration and brightness shifts). 

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Such interference effects can be observed when two large flat surfaces are held parallel to each other, but not at the same distance from each other; this phenomenon is called gratings.

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Before a grain is added, the CRT screen is covered in a very thin (~1 micrometer) layer of phosphor. The purpose of this layer is to convert the electron beam into visible light.

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The phosphor used in CRTs usually contains some or all of the following elements: Ca, Ba, Mg, Mn, Sr and/or Pb. 

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The latter two are typically used in green-screen phosphors (sometimes called "Z", for yellow), for red screens Sr is used instead of Pb . For blue screens either an oxide of a rare earth metal such as Ce or Yttrium oxide can be used. In the case of white screens, the phosphor can be a mixture of a few different oxides.

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A thick plastic layer forms part of the front surface of the screen. It is made from polyvinylidene fluoride (PVDF). 

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This layer makes up about one-third to one-half of the total thickness of the screen. The rest is a thin (typically one micrometer) glass or plastic coating which makes up most of the surface area and functions as a light blocker, mirror and support layer for other components in CRTs (such as phosphor).

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The plastic used in front of a CRT screen is (or at least used to be) made from PVDF which is no longer available. Since it is a very important part, its properties are well-documented. It is resistant to water and some chemicals but for example ammonium hydroxide and sodium hydroxide can damage the plastic by dissolving it. 

 

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It can withstand small amounts of ammonia, alcohols, ketones and esters, as well as cleaning fluids intended for glass cleaning. Stronger cleaning fluids should be rinsed off after a minute. Acids can also damage the PVDF layer, especially in combination with organic solvents. Hydrochloric acid is the most corrosive.

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The plastic layer on the front of a CRT screen has a double role: it blocks some light to reduce the effects of interference and prevents the phosphor  TV layer from emitting direct light. 

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The latter effect is important because phosphors do not radiate white light; they all emit colored light, except for green-screen phosphors which do not emit light at all and serve to block unwanted, back-reflected light (the "green problem").

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