EU7.1. Comparing Light Sources

Assortment of different kinds of lights (incandescent and compact fluorescent)

Many types of lights can be found in a hardware store. There are energy-saver incandescent bulbs, fluorescent bulbs, long-life bulbs, low-cost light bulbs, LED lights and specialty lights of all kinds. Work with a team to examine a collection of light bulbs. Find the wattage printed on them. If the packaging of the bulbs is available, see what it says. Do any of them claim that they are better light bulbs than the others?

This investigation focuses on brightness and electric power consumption.

Decide on a question comparing the brightness of two bulbs. Here are some examples:

  • Are all 75-watt incandescent bulbs alike? A reflector bulb sends out its light differently from a standard light bulb; but they use energy at the same rate. Do they provide the same amount of illumination?
  • According to the packaging, a 15-watt compact fluorescent light is supposed to replace a 60-watt incandescent bulb. Is it really just as bright?

When you have decided on a question, build an oil-smudge photocomparator so you can compare brightness of lights. Here’s how ….

Assemble the Oil-Smudge Photocomparator

Parts of an oil spot   phtocomparator


  • 1 sheet of white typing paper, 8 1¦2″ x 11″
  • 1 sheet of cardboard, 10″ x 12″
  • 1 sheet of cardboard, 14″ x 44″
  • Small amount of oil (mineral oil, baby oil, or cooking oil)
  • Scissors
  • Masking tape

The exact size of the parts is not critical. You may need to tape a few pieces of cardboard together for the larger piece.

  1. Cut a six inch square hole in the center of the 10″ x 12″ cardboard. 
  2. Fold the 14″ x 44″ cardboard to fit around the 10″ x 12″ cardboard as shown. Tape it together so it forms a box with open ends. Tape the smaller piece of cardboard inside so it divides the box into two equal sections.
  3. With your finger, smear some oil onto the center of the typing paper in the shape of an “O” about two inches across. Use just enough oil so it soaks into the paper without running. Tape the smudged paper to the cardboard so the oil smudge is in the center of the box.

Test the Photocomparator

Student testing a photo comparator

Hold the photocomparator so it is between you and a bright light source. Look at the oil smudge. It should appear bright against a darker background. Now, hold the photocomparator so you are between it and a bright light source. Make sure your shadow is not blocking the light. Look at the oil smudge. It should appear dark against a lighter background. Can you explain why?

Whenever the sheet of paper is being illuminated by the same amount on each side, the smudge will be neither lighter nor darker than the background. The smudge may hardly show up at all.

In your experiments, you will shine lights from different sources onto opposite sides of the photocomparator. When the smudge hardly shows up at all you know the photocomparator is receiving equal amounts of light from each side. The distance of each light to the photocomparator will help you determine which light is brighter. You will need to shade your oil smudge from all other lights not involved in this experiment. 

Set Up Your Workspace

Additional Materials

  • Measuring tape or meter stick
  • Oil smudge photocomparator
  • Light bulbs to compare
  • Fixtures and clamps to hold the bulbs

The room you work in should be nearly dark. Be sure the light from one team’s bulbs is not shining directly into the photocomparator of another team. It may be necessary to put objects around your work area to block the light. 

Set up your two lights at least two meters apart. If there is space, it is better to have the lights three or four meters apart. You will be measuring the distances between the photocomparator and each of the lights.

Gathering the Data

1. On a blank sheet of paper prepare a data table like the one shown below. Fill out the data table with information from the packaging, including cost, wattage, and any comments about the bulbs. If you have no information about the lifetime of a bulb, you can estimate 1,000 hours for a typical incandescent bulb, and 10,000 hours for a compact fluorescent bulb.

white space
Description of BulbDistance to PhotocomparatorCost of bulbWattage of bulbLifetime (in hours)Other information
compact flourescent
soft light
Measuring distances from the two  light sources to the photocomparator

2. Have one team member hold the photocomparator between the two lights, A and B. Other team members should stand near lights A and B. They should tell the person holding the photocomparator to move it back and forth between A and B until they agree the oil smudge hardly shows at all. Then measure the distance from each light source to the oil smudge, and write the distances in the table.

3. If there is time, exchange experiments with another team. Make a new data table, and repeat the measurements to confirm the accuracy of each team’s data. 

Interpreting the Data

Equation relating light sources distances with brightness

1. Which light source illuminated the photocomparator the brightest? By noting which distance is greater, you can tell which light is brighter. The dimmer light will be closer to the photocomparator. 

It is possible to compare quantitatively the brightness of two lights, A and B, with this formula:

Use your data to compute the brightness ratio. If one light is considerably brighter than the other, how many times brighter is it?

Ratio to compare power use with brightness

2. Which light source is more energy efficient? Find the ratio of the wattages of the two bulbs by dividing the wattage of bulb A by the wattage of bulb B. You can find out which bulb is more efficient by comparing the brightness ratio with the wattage ratio. 

If the bulbs are equally efficient the ratios will be equal. If the brightness ratio is higher than the wattage ratio, then light A is more efficient. If the brightness ratio is lower than the wattage ratio, then light B is more efficient.

In general, you can express the efficiency as the Wattage Ratio divided by the Brightness Ratio. If Wr is the Wattage Ratio and Br is the Brightness Ratio, then the Efficiency Ef would be

Ef = Wr/Br

If A is 100W and B is 20W, the wattage ratio is 100W/20W = 5. If bulb A is five times brighter than bulb B, the brightness ratio is also 5, so A and B are equally efficient. But if bulb A is just twice as bright as bulb B, the brightness ratio is 2.
That means that bulb B is 2.5 times more efficient than A.

3. Did the package make any claims you can evaluate from this investigation? If so, did the bulbs live up to the advertising on the packaging? 

4. How do these lights serve your needs? Is there anything about the look of a light bulb, its size, the color of its light, or the direction it casts its light that makes it suitable for some purposes but not for others?

5. Summarize what you did, what you found, and your recommendations in a report.