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Study of formation of images on a TV tube

Study of formation of images on a TV tube

Introduction: (Initial Observation)

Since 1884 when the first mechanical television was invented until today that we have electronic and digital televisions in every home, there has been many changes and improvements to this technology. This project is an investigation on the technology of television and the way that images can be transmitted over wires.

Although we see televisions every day, it is still a world of wonders to the eyes of curious individuals. Initially, I was wondering what is behind the T.V. glass that makes such moving images. Later, I was asking myself how do images travel in wires or air? and in addition to all of these, I am now wondering how colors are formed in a television?

Any of these questions are a good start for my science project.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about televisions and how they work. Read books, magazines, or ask professionals who might know in order to learn about the main components of a television and their principles. Keep track of where you got your information from.

Visit the website of the Early Television Foundation at www.earlytelevision.org. Make sure you also look at the links page of that website. You can find many good links there including some educational movies. If you live close to Hilliard – Ohio, you may also decide to visit the museum.

Read the sound, light spectrum in order to learn about additive and subtracting color mixing.

Also read the basic principles of Television in order to learn about the main components of a television and learn some technical terms related to the television.

The following is some information about televisions:

In a black-and-white TV, the screen is coated with white phosphor and the electron beam “paints” an image onto the screen by moving the electron beam across the phosphor a line at a time.

Electron gun that produces the electron beam is an important component of every television.

Electron Guns

An electron gun, such as in a television picture tube, generates a beam of electrons. In this section we discuss how it works.

A diagram of an electron gun appears to the right. There are two vertical metal plates; the right hand plate has a small hole cut in it. A voltage source, indicated by V, maintains a voltage across the plates, with the left hand plate negative and the right hand plate positive.

When a metal plate is heated, a process called thermionic emission literally boils electrons off the surface of the metal. Normally, the electrons only make it a fraction of a millimeter away; this is because when the electron boiled off the surface of the metal, it left that part of the plate with a net positive electric charge which pulls the electron right back into the plate.

In the figure, we are heating up the left hand plate so thermionic electrons will be boiled off the surface. But because of the voltage difference being maintained across the plate, electrons that boil off between the two plates do not fall back into the plate, but instead are attracted to the right hand positive plate. Most of the electrons crash into the positive plate, as shown. However, the electron in the middle would have crashed into the plate except that we have cut a hole in that part of it. So we get a beam of electrons out of this “electron gun.”

In real electron guns, such as at the back of a TV picture tube, the negative plate is not heated with a campfire as in our figure. Instead, a small filament of wire has a current passed through it. The filament heats up, glows red, and heats up the negative plate. You may have seen that red glow in the back of a TV picture tube.

We control the speed of the electrons in the beam with the voltage, and the number of electrons by how hot we make the negatively charged plate.

One more small point. Because the hole in the right hand plate is not of zero size, electrons can emerge in directions slightly away from perfectly horizontal. Thus, the beam of electrons will tend to “spray” somewhat.


  • Traditional cathode is hot filament of wire that “boils” off electrons
  • 1 to 30 KV between cathode, grid, screen accelerates electrons (F = eE)
  • Raster Scan of Electron Beam in Traditional CRT
  • Electric and magnetic fields sweep beam
  • Screen coated with fluorescent phosphors (ZnS, ZnO, etc.)
  • Phosphors excited by electrons emit Red, Green, or Blue light

Mechanical TV:

The Nipkow disk was a device which its inventor, Paul Nipkow, thought could be used to transmit pictures by wire. The disk had a spiral of holes cut into it. These holes were positioned so that they could scan every part of an image in turn as the disk spun around. The light coming from each point would then be turned into in electrical current.

This electrical signal would light up a second light at the other end of the wire. The second light would flicker because the amount of current it received would depend on the brightness of the image being scanned. The light from this light bulb passing through a second disk spinning at the same speed, would then project the picture onto a screen.

Nipkow’s Mechanical Television System, 1884

In 1884, German inventor Paul Nipkow designed a way to transmit images electrically using a pair of spinning metal disks. He punched holes in a spiral pattern along these “Nipkow disks,” one of which was housed in a transmitter and the other in a receiver.

The first disk “scanned” an object as light shone through its holes onto selenium, changing the chemical element’s resistance and thus its electrical current. The current would move over a wire, causing a lamp in the receiver to flicker. Anyone looking at the lamp through the second spinning disk saw the original image revealed. Nipkow’s development led to the first public demonstration of television in 1926.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement. You have many different choices of question or purpose when you study about television. Following are some samples.

The purpose of this project is to study:

  1. How the images are formed on a TV tube
  2. How the images are transmitted in wires
  3. How colors are formed on a color TV

All the above proposed questions suggest that this is a display project. For the question number 3 however, you may propose it as a question in an experimental project. For example you may ask:

How does the intensity of different color lights affect the combination color?


What combination of Red, Green and Blue lights create White color light?


What combination of Red, Green and Blue lights create yellow color light?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

As a display project, you will not need to define variables. You just gather information and then design an experiment or make a model to show how something works.

For the question number 3 however, if you propose it as a question in an experimental project, the variables can be defined as follows:

Independent variables are the rate or intensity of the main color lights of Red, Green and Blue.

Dependent variable is the resulting color light.

Controlled variables are experiment procedures and other lights in the environment.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

Following are sample hypothesis for question number 3 as an experimental project.

  • The combination of red, green and blue lights with equal intensity produces white color.
  • The combination of green and red lights with equal intensity create yellow light.

Note that the hypothesis do not have to be correct. Your experiment results may show that your hypothesis have been wrong.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: How the images are formed on a TV tube?

Introduction: Pictures on magazines and printed material can not change and can not move!, how come images on a TV screen can move? how are they formed?

In this experiment we use magnifier to make close observation and see the details of image formation on a TV.


  1. Turn on the TV and look at it from an average distance of about 10 to 15 feet (or 2 to 4 meters). Notice the quality of images.
  2. Get closer to the TV and look at it from a distance of about one foot (12 to 15 centimeter). How does the quality of images change when you get closer to the TV?
  3. Use your glass magnifier and closely examine parts of the image? What are the components that form the images?
  4. Repeat this experiment with your computer monitor? What are the differences?
  5. How is black color formed on a TV or monitor?
  6. How is white color formed on a TV or monitor?
  7. How is yellow color formed on a TV or monitor?

Experiment 2: How the images are transmitted in wire?

As seen in the previous experiment, images are formed from a collection of dots or pixels. In a video camera each pixel is a light sensor and in TV each pixel is a light spot.

Large outdoor electronic lamp video screens use small light bulbs or light emitting diodes (LEDs) to form images.

In this experiment you will build a device that shows how changes of light on a pixel of a video camera transmits and simulates the light condition on a pixel of television or outdoor video screens.

You will use a photocell as a light sensor and a small light bulb as a pixel. The light in the light bulb will change based on the light on the photocell. In addition to video and television, this system has many other applications as well. You can use it to see if there is any light inside a device such as a furnace. Flame creates light so your device can detect flames. In many heating devices such a system is used to automatically disconnect the fuel if for some reason the flame goes off. Such a mechanism contributes to the safety of heating devices.

You may think that such a device can simply be built by connecting a light bulb in sequence with the battery and photocell. But this will not work because photocell limits the current in the circuit, so the electric current will not be enough to turn on the light bulb. You will need to use an amplifier or transistor for this device.

Transistors are a kind of switch. The emitter will get connected to the collector if we apply a small electric charge on the base.


  1. Get a photocell and a general purpose NPN transistor such as BC337-25 or PN2222A.
  2. Identify the 3 pins of your transistor (emitter, base, collector)
  3. Connect the emitter pin of the transistor to the negative pole of a 6 volt battery.
  4. Connect the collector pin of the transistor to a light bulb and then to the positive pole of the same battery.
  5. Connect another wire from the base of the transistor to the photocell and then to the positive pole of the battery.

Picture on the right shows this setup. Red wire is connected to the positive pole of the 6 volts battery.

White wire goes to the negative pole of the 6 volts battery.

3 pins of the transistor are identified by letters E=emitter, B=base and C=collector.

Here we cover the photocell by a black plastic cap, so the light goes off.

In the dark electrons from the negative pole of the battery get to the emitter (E), but they can not pass through the transistor to get to collector (C). So the circuit will remain open and the light stays off.

In the light, a small positive charge goes through the photocell and gets to the base (B) and that small charge will make the transistor to pass electrons from E to C to build a closed circuit and the light goes on.

As you see the transistor is like a switch. E will be connected to C if a small charge is applied on the base.

We used a board and some screws to connect wires and components.

Experiment 3: How colors are formed on a TV tube?

Introduction: As described in the technical literature about TV and as observed in experiment number one, all colors and images are formed by changes in the intensity of color spots or pixels on televisions and monitors. The main color lights that form the images are red, green and blue. In this experiment we test to see how different color lights can combine to form a new color.


  1. Cut a round disk of about 10 cm (4 inches) in diameter from a cardboard.
  2. Cut equal size pies of red, green and blue papers and paste them side by side on the round disk such that the entire disk is covered.
  3. Place the disk on a small electric motor and spin it fast so you will see the additive combination of three color lights.
  4. See what color you get and how you can change the ratio of 3 colors to get other colors such as white and yellow.

In this image I just taped the pies of color paper to each other. I did not use any cardboard disk.

I also used a small battery operated fan to spin the color disk.

A ring of clear adhesive tape is used to connect the color disk to the electric fan.

This electric fan had plastic propellers that came off easily by pulling.

By starting the fan I could see the additive combination of the colors of the disk.

You need to make different disks with different ratios of 3 colors in order to record and compare the results. In my example in the right, the disk has a light green/blue color while spinning. In order to get white color, I will need to reduce the ratio of green and blue and increase the ratio of red.

Find out what combination of Red-Green-Blue colors created the closest match to white color. Note that the ratio that you find may be different from the ratio used in televisions. The reason is that the color tone of your papers may be different from the fluorescent colors used in television.

Experiment 4: How colors are formed on a TV tube?

Another method for testing additive color combinations is using three flashlights or desk lamps covered with red, green, and blue glasses. It is best if desk lamps come with a dimmer switch. In this way, you will also be able to change the intensity of different color lights to see how they affect the combination of color lights.

Face the lights to one spot on a white wall. Change the lights order or intensity to see the combination color.

Experiment 5: Can magnets bend or change the direction of an electron beam?

Introduction: An important part of each television picture tube is the deflector. A deflector is an electromagnet assembled on the nock of picture tube and is responsible to move electron beam. In this experiment we can see if magnets really change the direction of electron beams.

Warning: This experiment may cause permanent discoloration on a monitor or TV screen. Do not use strong magnets for this experiment. Do not hold the magnet close to the sides of the screen where other metal components may magnetize permanently. Do not do this test on valuable televisions. I just bought a television from a garage sale for $7.00. Such inexpensive equipment are perfect for science experiments.


  1. Turn on the television or monitor.
  2. Move a small magnet toward the center of the screen.
  3. Record your observation.

Additional Experiment: Left-Right Reversing Television

This change of wiring inside a television set gives the ability to see the pictures of the screen reversed, as in a mirror. Right is left and left is right.

In the entry-level version, it’s just fixed with the picture reversed. In the advanced version, a switch is added so it’s possible to switch between normal mode and reversed mode.

Safety warning: Televisions can be dangerous if you open them up, and opening up a television set without making the correct precautions first can be hazardous. So, before opening up the television you should check that you own it or have the permission from whoever owns it, or it could be risky as the owner of the television might get cross about it! Electricity is also dangerous, so you should be sensible about that too. This experiment must only be conducted by adults familiar with handling electrical circuits. Children can observe from a safe distance.


1. Locate the deflector coil assembly. This is a big coily thing which is all around the neck of the television tube. Or to put it another way, the tube neck is right in the middle of the television, and the deflector coil assembly is around it and further forward.

2. Locate the wires going to the deflector coil assembly. Ideally, there should be four of them. Two of these are the horizontal and two of them are the vertical. It is the two which are the horizontal which are of interest here.

3. Find out which are which. (Worth taking time to get right rather than guessing, as an unlucky guess can blow the frame output!). To find out which are which, you need to snip the wires one at a time and reconnect them one at a time in turn and see what happens. If either of the horizontal are disconnected, then there’s no horizontal, so you see a vertical line. Special care should be taken not to electrocute yourself.

When I did this experiment, I turned off the TV every time that I needed to cut or connect any wires. I also had my leather gloves on. I don’t like electric shocks even if it is from a capacitor in an unplugged television.

Having found which two wires are horizontal, you SWAP THEM! This reverses the picture left-right. Very curious.

Now the advanced version:

1. Get a double-changeover switch. This is of the type with six connectors. Two of these are “common”, two of them are “position 1” and the other two are “position 2”.

2. Connect the two wires from the deflector coil assembly to the two “common” on the switch.

3. Connect the other two wires to the two “position 1” terminals.

4. Connect an extra pair of wires from the “position 1” terminals to the “position 2” terminals but swapped over.

5. Test the whole arrangement in both positions and make sure it all works.

If someone looks at this television, they will probably not notice much difference in the beginning. Unfamiliar landscapes and people look the same with the picture left-right reversed. Then you start to spot the exceptions…

# They’re driving on the wrong side of the road. Easily mistaken for being in a different country.

# Geography looks VERY STRANGE. Weather reports showing reversed geography with alien land-formations, etc.

# That which is familiar becomes unfamiliar. Scenery which you have become accustomed to being that way round sometimes looks near unrecognizable the other way around.

# Various other symmetry conventions are reversed. These suddenly become noticed because the assumptions are at odds with the observations, and the differences then show up as if magnified. For example, clocks, the preference for putting a stamp on the top-right of an envelope, which side coats fasten, left-handedness with eating utensils, etc.

This experiment is adapted from http://www.zyra.org.uk/iscope.htm

Materials and Equipment:

List of material can be extracted from the experiment section. Make up this list for each experiment after you complete your experiment.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


No calculations are required for this experiment.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.