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How can the strength of light be measured? (The effect on degradable materials)

How can the strength of light be measured? (The effect on degradable materials)

Introduction: (Initial Observation)

In the first few times that I was visiting art museums, I was wondering why it is so dark and why don’t they turn on more lights? I had this question until one day in discovery or learning channel I saw a program about preserving artworks. During this program I learned that light can degrade material such as paper and fabric. Now I knew why museums avoid having excessive light. They want to protect the art works.

Light has some benefits and some hazards. In tunnels and roads, sufficient light offers a safer driving condition, while low light creates hazards and accidents. Photographers need to measure the amount of light in order to adjust their camera and produce a clear picture. In this project we will study light measurement and the effect of light on degradable material.


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:

Use the Internet and search for keywords such as “lightmeter”, “Lightmeter + Photography”, “Light + Preservation”, “Light + Degradable”. Also search for “Light degradable” and study about light degradable plastic and light degradable pesticides.

Light and Life http://www.pol-us.net/ASP_Home/asp_bro2.html
Art work preservation from light damage http://aic.stanford.edu/treasure/paper.html
Conservation/ preservation http://palimpsest.stanford.edu/bytopic/genpub/

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.

The purpose of this project is to see how can we measure the strength of light. We will also study the effect of light on degradable materials.

Finally you may measure and compare the amount of light in different indoor and outdoor places.

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.

First part of this project about light measurement does not require defining variables. But for the second part about the effect of light on degradable material, we define variables as follows:

Independent variables are type of light, intensity of the light and exposure time.

Dependent variables are the rate of degradation. We use discoloration as a sign of degradation.


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.

My hypothesis is that certain lights (most likely Ultra Violet) are more effective in discoloration. My hypothesis is based on my observation that in museums they don’t use florescent lights, but printers use florescent light boxes for printing film on a printing plate. Also UV light is used in tanning because it has fast tanning effect on skin.

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:

For this experiment we want to measure the light in different areas. We can do it by using a device called the lightmeter.

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 using a photo resistor?

You can build your own lightmeter using a multimeter and a photo resistor. Photo resistor 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.

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.

How to make a light meter using a solar cell?

Solar cells produce electricity (DC voltage) when they are exposed to light. You may set your multimeter to read DC volts and connect it to a solar cell or a photo voltaic cell and use that as a light meter.

Calibrate your light meter

If you have access to a professional light meter, you can use that to see how many ohms resistance in your meter is equal to one Lumen. Then you can use yours to measure light by Lumens. If you don’t have access to a professional light meter, you may calibrate yours in a dark room using candles. A traditional unit of luminance is foot-candle. Foot-candle is the amount of luminance produced by a candle at 1 foot distance. In a dark room place your light meter one foot away from a candle and read the resistance (if you are using a photo resistor) or read the DC voltage (if you are using a solar cell or photo voltaic cell). Then repeat this with 2 and 3 candles. Using these data you will know how many ohms resistance or how many volts is equal to 1 foot-candle, 2 foot-candle and etc.

When you are testing light in different areas, be aware that what you get is usually a reflection of light, so it matters what direction you are holding your light meter to. Just make sure that you do it the same way for all areas that you test.

Experiment 2: Measure and compare the amount of light at different places.

Introduction: When you have a calibrated light meter, you can measure the amount of luminance at any place in foot-candles. In this experiment you will measure and record light in different places in foot-candles.

Procedure: Take your light meter to different outdoor areas or indoor places (rooms) and record the amount of light. Record your results in foot-candle in a table like this:

Place Luminance (foot-candle/ fc.)
Outdoor Sunny area
Outdoor shade
Living room in natural day light
Living room with synthetic light at night

To give you an idea of light levels, on a clear sunny summer day at noon, the light in the sun would be over 10,000 fc. At the same time of day on a overcast winter day, the light level may be less than 500 fc.

Make a graph:

You may use the above results table to make a bar graph and visually present your results. Make one vertical bar for each place you measure the light. Write the place under each bar. The height of each bar will represent the Luminance at that place.

Experiment 3:

Get a piece of black or dark color fabric, (Cheap cotton). Cut it in about 5 pieces. Place one piece in a dark room.
Place one piece in normal light.
Place one piece next to the window with maximum exposure to sunlight.
Place one piece on a Florescent light bulb.
Place one piece on a cold UV light bulb, or just expose it to UV light.

After 2 weeks compare discolorations in your samples.

Materials and Equipment:

Can be extracted from experiments.

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.


If you do any calculations for your project, write your calculations in this section of your report.

Summary of Results:

If you’ve bought light bulbs lately, you probably noticed plain old incandescent bulbs are in the minority. Today compact fluorescence and halogen bulbs are two popular alternatives because they last longer and produce much more light than their incandescent counterparts. Until recently, consumers have been used to buying a particular wattage bulb, say 60, 75 or 100 watts. But that measure really doesn’t apply for the newer super-efficient bulbs. Instead you should start looking at the number of lumens a bulb provides. Lumens are a measure of light. Watts measure the electricity used — and the more lumens you get with fewer watts is your clear winner every time. Energy-efficiency reduces electric bills and the whiter and brighter light of compact fluorescence and halogen bulbs is better for kitchens and bathrooms. It brightens home offices, too. Shop lumens and you’ll see the light.


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.


Search your local library for light and photochemistry and use them as your references.

Question: What is resistance?

Answer: When we say resistance, it means resistance against movements of electrons. As you might know by now, movement of electrons in a conductor is called electricity. Copper wire for example is a good conductor. So if you use a thick copper wire to connect the positive pole of a battery to the negative pole of the same battery, all the excess electrons in the negative pole start to travel toward the positive pole until the concentration of electrons in both poles becomes equal. Now if you would use a very thin copper wire, it was not that easy for the electrons to travel from one side to the other side. That is why we say a thin copper wire has more resistance than a thick copper wire. This is like you and 300 other students want to pass through a very narrow hallway. You cannot easily walk together and it takes a long time for all of you to go through this passage. So longer and narrower hall way has more resistance against passing students than a short and wide hallway.

The same way a longer and thinner wire has more resistance against movement of electrons.

When a large group of people try to pass through a narrow and long passage, they will create heat and they will all get hot. This often is caused by friction of people on each other and on the walls. The same thing happens to a large group of electrons trying to pass through a long and narrow wire. They create heat. This property is used to make electric heater and light bulbs. In the case of light bulb the heat is enough to make the metal glow.

Do you know how long is the filament of a light bulb? Do you know how thin it is? The filament of regular light bulbs is about 3 feet long and it is so thin that is practically invisible. In factories they wrap filament wire in a double coil so it will get shorter and becomes more visible.

Can a hallway be wide and short and still having resistance against passing students? Yes it can! Specially if this hallway is filled with furniture up to the roof. Students can go over the furniture and go through the gaps between furniture to pass, but it is hard and this is also a resistance. The same thing happens in conductors. Imagine instead of a pure copper wire you use a wire that is 95% sand and 5% copper. Sand is not conductive, so this wire will have a lot of resistance.

When the resistance is higher, it means that less electrons can pass through. When the resistance is lower, it means that more electrons can pass through.

Photocell is a special wire (formed as a small electronic component) that changes resistance based on the light. While in the dark it has a very high resistance, however it has very low no resistance when it is exposed to a bright light. It’s almost like the electrons need to see where they are going. In the dark they just bump on each other and get stocked in the photocell. But when there is light, they can walk through easily.