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                                                                         Your Position: Home > Articles > LED/Lighting Knowledge > How to Measure Light Intensity

How to Measure Light Intensity

Monika / 2013-03-13

Building the Joly Photometer

  1. Cut a 1/4-pound (0.55 kg) slab of paraffin into equal halves. Cut through the slab slowly so you don't break or chip it.
     
  2. Cut a length of aluminum foil the same size as each of the wax blocks.
     
  3. Place the foil between the wax blocks.
     
  4. Cut windows into 3 sides of a small cardboard box. The 2 end windows should be the same size as each other, as these windows will allow light to hit the wax blocks when placed inside the box.
     
  5. Place the blocks inside the box. You may need to use tape, small pieces of cardboard, or both to keep the wax blocks parallel with the opposing windowed sides and the foil between them.
     

Using the Joly Photometer

  1. Arrange 2 light sources in a straight line. You should first choose 1 of the test bulbs as a "standard candle" to compare bulbs of different wattages against. Your standard candle will be assigned a comparative light intensity of 1.
     
  2. Place the photometer between the light sources. The photometer should be at the same height as the light sources so that the lights completely illuminate the wax blocks. Likewise, allow enough distance between the light sources initially so that the distance between them is significantly greater than the size of the wax blocks to allow even illumination.
     
  3. Turn off all other lights in the room. Close any window, shades, or blinds so that only light from the test light sources is hitting the blocks.
     
  4. Move the photometer toward or away from 1 of the bulbs until both wax blocks appear equally bright.
     
  5. Measure the distance between the photometer and each of the sources. Measure from the bulb to the aluminum foil sandwiched between the wax blocks, including the thickness of the block facing the bulb.
     
  6. Calculate the relative light intensity of the second bulb against your standard bulb. Light intensity falls off according to the square of the distance between the light source and the receiving device (the comparative photometer). At the point where each of the wax blocks in the photometer is illuminated equally, the intensity of the standard candle bulb divided by the square of its distance from the photometer is equal to the intensity of the second bulb divided by the square of its distance. You can thus find the relative intensity of the second bulb by dividing its distance squared by the distance squared of the standard candle bulb and multiplying by the standard candle bulb's intensity.
    • If the distance between the second bulb and the photometer is 4 m and the distance from the standard candle bulb is 2 m, the relative intensity of the second bulb to the standard candle bulb is 4 squared, or 16, divided by 2 squared, or 4, multiplied by the standard candle bulb's relative intensity of 1, or 4.
       
    • If you know the actual light intensity of the standard candle bulb, you can use that value instead of 1 to find the actual light intensity of the second bulb. Light intensity as measured by the comparative photometer is measured using a unit called the lux, defined as 1 lumen (a unit of light output) per square meter. If your standard candle bulb has an intensity of 500 lux, the second bulb in the example would have a light intensity of 2,000 lux.
       
    • You can figure the bulb's efficiency by dividing its intensity by its wattage.
       
     

Tips

  • You can use the Joly photometer to compare the light intensities of incandescent bulbs against compact fluorescent and LED (light-emitting diode) bulbs, as well as to candles. (The use of a candle for measuring light intensity led to the terms "standard candle" and "candlepower," as well as to the unit name "candela," a measure of luminous intensity.)
  • After using the Joly photometer to measure comparative light intensities, you can measure light intensity using an analog or digital light meter. Newer digital light meters measure light intensity in lux, while older analog light meters usually measure light intensity in an older unit called the foot-candle, defined as 1 lumen per square foot. One foot-candle equals 10.76 lux.

 Warnings

  • While making a Joly photometer is suitable for either a classroom or home school science project, an adult should supervise cutting the paraffin blocks for safety's sake.

Things You'll Need

  • Box of paraffin wax (available in a 1-pound/2.2-kg box at a grocery or hardware store)
  • Knife sharp enough to cut 1/4-pound (0.55 kg) slab of paraffin
  • Aluminum foil
  • Small cardboard box (the size of the box the wax comes in)
  • Scissors
  • 2 Light fixtures of the same kind, such as clamp-on lamps
  • Measuring tape
  • Test light bulbs (3 or more different ratings in wattage or lumens)

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