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Principles of fluorescent lights How do they compare with filament bulbs, in effectiveness and cost.

Principles of fluorescent lights How do they compare with filament bulbs, in effectiveness and cost.

Introduction: (Initial Observation)

Fluorescent lamps are different from regular light bulbs (or incandescent lamps) in many ways. They look different and produce a different color of light. Fluorescent lamps are the most commonly used commercial light source in North America. In fact, fluorescent lamps illuminate 71% of the commercial space in the United States. Most fluorescent lamps are in the form of long tubes. Newer version of fluorescent lamps are not that long and are known as compact Fluorescent or energy saving lamps.

This project is an opportunity to learn more about the principles of fluorescent lights and compare them with incandescent bulbs.


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 Fluorescent lights and how they work. Read books, magazines or ask professionals who might know in order to learn about the principles of a fluorescent light and how do they compare with incandescent lamps. Keep track of where you got your information from.

Project Guide:

Fluorescent light is very different from a regular light (also known as incandescent light). For example in an incandescent light electricity passes through a thin wire known as filament. Electric current will make the filament so hot that it glows and emits light. But in fluorescent lights, electricity passes through a gas (you will learn what type o gas soon) and gas radiates an invisible UV (Ultra Violet) light without getting hot. Then a white powder known as phosphor gets this invisible UV light and produces visible light.

Following are some general information about fluorescent lamps to help you for a quick start. Some of the questions related to fluorescent lamps and their comparison with incandescent lamps are:

  • For a certain amount of light, which lamp is more efficient (consumes less electricity), fluorescent or incandescent?
  • Which lamp creates more heat in order to generate certain amount of light?
  • How much do we save with energy saving fluorescent lamps?


If you like to see how does a fluorescent lamp work, read this section.

The design of the fluorescent lamp is really quite simple. This design remains the same whether the form is a straight tube, circular, or convoluted as in compact fixtures. A fluorescent tube is constructed in its most simple form with a filament similar to an incandescent light bulb at either end, and a coating of a fluorescent material on the inside of the glass envelope. The tube itself vacuum evacuated, a small amount of vaporized mercury is added to the tube, and then it is injected with a small quantity of argon gas. (So the gas inside the florescent bulb is a mixture of argon gas and mercury vapor).

When a current is applied to the filaments at both ends of the tube, the filaments become what are called “cathodes”, meaning that they provide an intense source of positively charged electrons. This energizes the argon gas to a ‘plasma state’ which “excites” the metallic mercury. At this point, the flooding of positive electrons cause the electrons in the shell of the mercury atoms to “jump” (move outward) from a neutral or “ground state” and become “excited”. This pushes electrons outward, filling an “empty” orbital ring with an new electron. The atom then releases its excess electron as the atom attempts to return to its neutral state, and through this process the mercury gas becomes “energized”, driving the excess energy off in the form of photons that lie within the ultraviolet range. The external ballast device serves to limit the amount of current which is delivered to the plasma in this process, maintaining a consistent and uniform source of electrical flow to the cathodes.

This same atom which has just released a photon then picks up another from the cathode flow, continuously repeating the process for as long as the cathode is attached to a source of current.

In the case of mercury, this element emits a very strong line at 2537 Angstroms, well into the far ultraviolet range (UVC). By its own properties, this wavelength is dangerous as none of it ever penetrates to the earth, and life is not prepared to deal with these wavelengths of radiation. But this frequency is helpful in two ways. If no phosphors were added to the tube, this lamp would be the kind that is found in UV sterilizing equipment (such as in barber shops for combs and scissors, and in bowling alleys for shoes), as it will kill all living organisms exposed to it after a time. But this is not the case in consumer fluorescent tubes. This spectral line of photons strike a suitably doped (meaning selected impurities are added) calcium halophosphate coating inside the tube itself, which causes two things to happen. First, the coating filters out the dangerous UVC radiation, and then converts the energy to a different spectral range, mostly that of the visible spectrum.

Depending on the blend of doping materials combined with the calcium halophosphate phosphor, the output range of the tube will vary, and thus a fluorescent device can be custom tailored to produce certain ranges or specifications of light output. This is how various manufacturers produce lighting devices which have different output characteristics. While there are several thousand doping materials which have the potential to change the spectral output of a lamp, only a hundred or so produce useable wavelengths. Characteristics and properties of pertinent lighting devices may be found in the Fluorescent Tubes section.

Fluorescent lamp construction consists of a glass tube with the following features:

  • filled with an argon or argon-krypton gas and a small amount of mercury
  • coated on the inside with phosphors
  • coated on the inside with phosphors

Fluorescent lamps provide light by the following process.

  • An electric discharge (current) is maintained between the electrodes through the mercury vapor and inert gas.
  • This current excites the mercury atoms, causing them to emit non-visible ultraviolet (UV) radiation.
  • This UV radiation is converted into visible light by the phosphors lining the tube.

Discharge lamps (such as fluorescent) require a ballast to provide correct starting voltage and to regulate the operating current after the lamp has started.

How does a fluorescent light bulb work?

A fluorescent lamp consists of a glass tube that is filled with mercury vapor at low pressure. The inside of the tube is coated with a phosphorous substance. Two coiled metal (tungsten) filaments are at each end of the tube. When an electric current flows through the filaments they start to get hot and glow (like a regular light bulb). When we apply a voltage between the two filaments and electrons get sucked from one filament to the other. While zipping through the tube electrons crash into mercury atoms, which start to glow and send out ultra violet (UV) light.

UV-light is very, very violet. Actually it is so violet that you can’t see it, but you can get a sunburn from it. So on its own UV-light wouldn’t make a useful lamp, that’s why there is a phosphorous substance in the inside of the glass tube. When UV-light hits the phosphor atoms, they absorb the UV light and send out the white light that illuminates your room. The conversion of light from one type to another is called fluorescence, which gave the fluorescent lamp its name.

Fluorescent lights conserve energy. For the same amount of light they need less power than usual light bulbs. By the way, the funny shaped light bulbs made of bend glass tubes in the supermarket are actually fluorescent lamps. Please don’t play and break fluorescent lamps they contain chemicals that are poisonous. When disposing of a fluorescent lamp, you should call the Recycling Center!

Inside a fluorescent light is low-pressure mercury vapor. When ionized, mercury vapor emits ultraviolet light. Human eyes are not sensitive to ultraviolet light (although human skin is). Therefore the inside of a fluorescent light is coated with a phosphor. A phosphor is a substance that can accept energy in one form (for example, energy from a high-speed electron as in a TV tube) and emit the energy in the form of visible light. In a fluorescent lamp the phosphor accepts the energy of ultraviolet photons and emits visible photons.

The light we see from a fluorescent tube is the light given off by the phosphor coating the inside of the tube (the phosphor fluorescence when energized, hence the name).





More about Fluorescent Lamp

The fluorescent lamp is a gas discharge tube whose output of light is so increased by special means that it can be used for lighting purposes. The inner surface of the wall of the tube is coated with light-emitting substances – usually fluorescent or phosphorescent metallic salts (calcium tungstate, zinc sulfide, zinc silicate). The tube is filled with mercury vapor at extremely low pressure. The electrons ejected from the incandescent collide with the mercury atoms and cause these to emit radiation which consists for the most part of ultraviolet rays, which are invisible. The visible portion of the mercury vapor rays is situated in the green and blue range of the spectrum and gives a pale light. The ultraviolet light strikes the fluorescent substance with which the wall of the tube is coated and causes this substance to emit radiation with a longer wavelength in the visible range of the spectrum – i.e., the coating transforms the invisible rays into visible light. By suitable choice of the fluorescent substance, this light can be given any desired color. The lamp has to be operated with a choke, which prevents a harmful rise in voltage and serves to ignite the lamp. For this purpose a small auxiliary glow lamp provided with a thermal contact is connected in parallel with the main lamp. When the current is switched on, the glow lamp first lights up (the bimetallic thermal contact is now open). This causes the bimetallic strip to warm up and close the contact, with the result that the glow lamp is short-circuited and the cathodes of the main lamp receive the full current that makes them incandescent. The bimetallic strip cools and breaks the contact. Through the agency of the choke this interruption of the circuit produces a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself. Because it is bypassed by the main lamp, the small auxiliary lamp then ceases to function. The bimetallic strip keeps the contact open. The cathodes of the main lamp are kept glowing at white heat by the impingement of positive mercury ions, and the lamp thus continues to function and emit light in the manner described. The light of a fluorescent lamp is not produced by an incandescent body (such as the filament of an ordinary electric lamp), but is emitted as a result of the excitation of atoms (namely, those of the mercury vapor and the fluorescent coating) and is extremely economical. Because of the large light-emitting area, a fluorescent lamp gives a pleasant light which produces only soft shadows.

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.

  • For a certain amount of light, which lamp is more efficient (consumes less electricity), fluorescent or incandescent?
  • Which lamp creates more heat in order to generate certain amount of light?
  • How much do we save with energy saving fluorescent lamps?

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.

Independent variables that we will study in this project are the type of the light bulb (Fluorescent or incandescent) and the rate of energy consumption (Watts).

Dependent variables are the amount of light and the amount of heat produced by the lamp.


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.

Sample Hypothesis:

My hypothesis is that fluorescent lamps consume less electricity for certain amount of light. They also create less heat which is often just a waste of energy. About the amount of saving by fluorescent lamps, I think the saving is about 50% in comparison to incandescent lights. My hypothesis is based on my observation and collected information.

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.”

At this point we have gathered information and studied about fluorescent lights and incandescent lights. We know that in an incandescent light electricity passes through a thin wire known as filament. But in fluorescent lights, electricity passes through a gas (Mercury vapor) and gas radiates an invisible UV light that changes to visible light up-on contact with fluorescent material coated inside the fluorescent tube.

Now we want to perform experiments to find an answer to our other questions.

Experiment 1:

For a certain amount of light, which lamp is more efficient (consumes less electricity), fluorescent or incandescent?


Although we can visually compare the amount of light in similar bulbs, for this experiment we will use a light meter to test the amount of light.

How to get a lightmeter:

There are two ways to get a light meter. The fastest is to find a camera store that has new or used light meters. These are light operated meters that require no batteries, and are quite portable. They are also reasonably well calibrated. I have a Weston Master 6, but any inexpensive meter will do for a start. If you don’t want to buy a light meter (about $30 or so for a new one), go to Radio Shack and find one of their circuit design books for photocells. Assemble it from the design.

How to make a light meter:

You can build your own lightmeter using a multimeter and a photocell. Photocell is a small electronic component that changes resistance based on the light. It normally (in the dark) has a very high resistance. When you expose it to light it’s electrical resistance reduces. (The sample that I used has about 30000 ohms resistance in the dark, but it has only about 200 ohms resistance in the sunlight. (Ohm is the unit of electrical resistance)

Set your multi-meter to ohms (to measure resistance) and connect the probes to the legs of your photocell. It will show the resistance of photocell at your environment light. Use your finger to cover the photocell to see how does the resistance change. You can use any digital or analogue multimeter for this experiment.

This simple light meter is perfect for comparing light from different sources. This instrument will not show the light by Lumens or any other unit.

Where to test?

You need to test several light bulbs at the same test conditions. For example you may perform your tests in a place with no external light. A box painted white may be a good choice. the distance of light source to your light meter should be the same in all experiments. Also it is good if you place a screen on that side of the light bulb that is toward the lightmeter. Screen can prevent un-even direct light to hit the photocell.

How to test?

Mount the light meter on one of the walls of the box in a way that you can read the amount of light from outside. With the help of an adult, place bulbs (one at a time) under the box and turn them on to read the light amount in your lightmeter.

Note: Use a box similar to a shoe box or larger. This box has only 5 sides, so you can use it to cover the light. The light is connected to the electricity using a cord (That is not shown in the diagram).

Record the results in a table and use them for your analysis and report. Results table can look like this:

Light type Watts Light / brightness Price

Watts is the amount of electricity that a light bulb consumes per hour. It is written on the bulb or on it’s packaging.

Light is what you will measure. If you use a professional light meter, larger number means more light, but if you make your own light meter using a photocell and an Ohm meter, you will need to convert the numbers to a number between 0 for dark and 100 for direct sunlight. One way to do that is using the following formula.

(LampResistance – SunResistance)*100 / (DarkResistance-SunResistance)

For example if the resistance shown by a certain light is 5000 ohms and DarkResistance is 30000 ohm and SunResistance is 200 ohms then

(5000-200)* 100 / (30000-200) = 16

So the number in the light column of table will be 16.

After collecting your data and the above calculation you will have a table like this. (Numbers in this table are fake!, You need to do your own test)

Light type Watts Light / brightness Price
Fluorescent 40 25 7
Incandescent 60 30 2

Now you need to do some calculations and find out at what price and what watt do these lights produce the same amount of light.

Experiment 2:

Which lamp creates more heat in order to generate certain amount of light?


In this experiment you will also place a thermometer inside the box to measure the heat increase after certain amount of time (For example one hour).

Everything else is the same as the previous experiment. Instead of price you will write the temperature increase in the box. Your final table may look like this:

Light type Watts Light / brightness Temperature increase
Fluorescent 40 25 3
Incandescent 60 30 9

Again the above temperature increase is not for the same amount of light. You need to calculate how much the temperature would increase if both lights had the same amount of light (For example if both had the brightness of 50).

To do this calculation you must multiply the temperature increase by 50 and divide it by the measured brightness. In other words you multiply 50 by 3 and divide it by 25. You can do the same with watts; so you multiply the 40 watts by 50 and divide it by 40.

In other words you increase all items in one row with the same ratio.

The new results table may look like this:

Light type Watts Light / brightness Temperature increase
Fluorescent 80 50 6
Incandescent 100 50 15

Now you have the temperature increase and wattage (electrical consumption) for the same amount of brightness.

Experiment 3:

How much do we save with energy saving fluorescent lamps?


The results table of experiment 1, also can be used to calculate the saving. To do that you need to know how much is the price of 1000 watt electricity in your area. If you don’t know that, use an estimated amount of 15 cents per kilo watts. Your calculation should show how much you will save per hour if you use a fluorescent light of 40 watts instead of an incandescent light with equivalent amount of light. When you know the savings for a 40 watts fluorescent light, you can easily calculate the saving for other lights. (I used 40 watts in this example to match the type of fluorescent light used in the previous example. You can use any other size for your experiments and calculations.)

Note: Try to use a compact fluorescent light for your experiments. Compact fluorescent bulbs have a screw base and will be mounted like regular incandescent lights. So they will not need additional fixtures and attachments such as ballast and starter (Everything is built in the base)

Additional Experiment 1: Make a Blinking Light

In this experiment we use a starter of a fluorescent light to make a regular light bulb blink. We can do this because the starter of Fluorescent bulb is actually a time switch.

Warning: This experiment can only be performed using live electricity with obvious risk of electric shock and death. Only students who are supervised by someone with knowledge and experience of electrical safety may perform this experiment.


  • Regular small light bulb (15 watt is O.K.)
  • Starter (15 watts or 30 watts)
  • Socket or base for the light bulb
  • Electric insulated wire
  • Plug (Consult an electrician on selecting the plug and wire to make sure they match)


Connect one end of the wire pair to the light socket and the other end to the plug. Screw the light bulb and then insert the plug to an electric outlet to make sure that the light comes on. Unplug the wire and then separate the pair of wires. Cut one of the two wires somewhere in the middle, remove the insulator and connect a starter like the image.

At this time your setup has a few bare spots with no insulation. Warn everyone to stay away while doing this experiment. Unplug the your setup as son as you made your observations.

If you are planning to use this setup as a part of your display, consult an electrician for proper insulation of all connections. Place your setup inside a clear plastic cage to make sure that no one will touch it.

Additional Experiment 2:

This experiment is mostly a demonstration. In this experiment you will light up a fluorescent light bulb with no electricity. Instead you will use microwave to excite the gas.


Get a small round fluorescent light and place it in a microwave. Start the microwave and watch to see if the bulb creates any light.

Note: We think this experiment is safe for microwave, specially if you try it only for a short time. However do it at your own risk.

Materials and Equipment:

List of material depends on your final experiment design.

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.


Write your calculations in your reports.

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.