Difference between revisions of "Measure Light Intensity"
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== Steps == | == Steps == | ||
===Measuring Light Intensity for a Room or Light Source=== | ===Measuring Light Intensity for a Room or Light Source=== | ||
| − | #Understand photometers that measure lux and foot-candles. These are units that describe the intensity of light on a surface, or ''illuminance''. Photometers that measure this are usually what people are looking for when they want to set up a photo shoot, or test whether a room is too bright or too dim.<ref>http://www.energybooks.com/pdf/D1150.pdf</ref> | + | #Understand photometers that measure lux and foot-candles. These are units that describe the intensity of light on a surface, or ''illuminance''. Photometers that measure this are usually what people are looking for when they want to set up a photo shoot, or test whether a room is too bright or too dim.<ref name="rf1">http://www.energybooks.com/pdf/D1150.pdf</ref> |
| − | #*Some light meters are specialized for different types of lighting. For instance, one may produce more accurate results when used to measure sodium lighting.<ref>http://www.bosworthinstrument.com/Lightmeterinfo.html</ref> | + | #*Some light meters are specialized for different types of lighting. For instance, one may produce more accurate results when used to measure sodium lighting.<ref name="rf2">http://www.bosworthinstrument.com/Lightmeterinfo.html</ref> |
#*You can even purchase a "light meter" in some mobile device's app stores. Check the reviews first, as some of these apps are inaccurate. | #*You can even purchase a "light meter" in some mobile device's app stores. Check the reviews first, as some of these apps are inaccurate. | ||
#*Lux is the modern accepted standard, but some devices still measure in foot-candles. Use [http://www.rapidtables.com/calc/light/lux-to-fc-calculator.htm this online calculator] to convert between them. | #*Lux is the modern accepted standard, but some devices still measure in foot-candles. Use [http://www.rapidtables.com/calc/light/lux-to-fc-calculator.htm this online calculator] to convert between them. | ||
| − | #Know how to interpret illuminance units. Here are a few examples of typical illuminance measurements, to help you determine whether your lighting needs to change:<ref>http://www.engineeringtoolbox.com/light-level-rooms-d_708.html</ref><ref | + | #Know how to interpret illuminance units. Here are a few examples of typical illuminance measurements, to help you determine whether your lighting needs to change:<ref name="rf3">http://www.engineeringtoolbox.com/light-level-rooms-d_708.html</ref><ref name="rf2" /> |
#*Most office work is comfortably done at 250 – 500 lux (23–46 foot-candles). | #*Most office work is comfortably done at 250 – 500 lux (23–46 foot-candles). | ||
#*Supermarkets or work areas that involve drawing or other detail work are typically illuminated to 750 – 1,000 lux (70–93 foot-candles). The upper end of this range is equivalent to an indoor area next to a window on a clear, sunny day. | #*Supermarkets or work areas that involve drawing or other detail work are typically illuminated to 750 – 1,000 lux (70–93 foot-candles). The upper end of this range is equivalent to an indoor area next to a window on a clear, sunny day. | ||
#Understand lumens and luminance. If a light bulb or lamp label or advertisement mentions "lumens," it is describing how much total energy is emitted as visible light, a concept called '''luminance'''. Here's what you need to know: | #Understand lumens and luminance. If a light bulb or lamp label or advertisement mentions "lumens," it is describing how much total energy is emitted as visible light, a concept called '''luminance'''. Here's what you need to know: | ||
| − | #*The "initial lumens" describes how much light will be given off once the light is stabilized. This takes about 100 hours of use for fluorescent and HID lights.<ref>http://www.ledtronics.com/TechNotes/TechNotes.aspx?id=13</ref> | + | #*The "initial lumens" describes how much light will be given off once the light is stabilized. This takes about 100 hours of use for fluorescent and HID lights.<ref name="rf4">http://www.ledtronics.com/TechNotes/TechNotes.aspx?id=13</ref> |
#*The "mean lumens" or "rated lumens" tells you the estimated average luminance over the life span of the device. The actual luminance will be brighter than this early on, and become dimmer than this near the end of the light source's recommended lifespan. | #*The "mean lumens" or "rated lumens" tells you the estimated average luminance over the life span of the device. The actual luminance will be brighter than this early on, and become dimmer than this near the end of the light source's recommended lifespan. | ||
| − | #*To figure out how many lumens you need, use the steps above to determine how many foot candles of illuminance you want in a room, and multiply by the square footage of the room.<ref>http://maximlighting.com/fpage_lighting_need.aspx</ref> Aim higher than the result for rooms with dark walls, and aim lower for rooms with other major light sources. | + | #*To figure out how many lumens you need, use the steps above to determine how many foot candles of illuminance you want in a room, and multiply by the square footage of the room.<ref name="rf5">http://maximlighting.com/fpage_lighting_need.aspx</ref> Aim higher than the result for rooms with dark walls, and aim lower for rooms with other major light sources. |
| − | #Measure beam and field angle. Flashlights and other devices that emit light in a certain direction can be described using these two additional terms.<ref>http://www.schorsch.com/en/kbase/glossary/beam-angle.html</ref> You can find these yourself using a photometer that measures lux or foot-candles, and with a straightedge and protractor: | + | #Measure beam and field angle. Flashlights and other devices that emit light in a certain direction can be described using these two additional terms.<ref name="rf6">http://www.schorsch.com/en/kbase/glossary/beam-angle.html</ref> You can find these yourself using a photometer that measures lux or foot-candles, and with a straightedge and protractor: |
#*Hold the photometer directly in the path of the brightest beam. Move it around until you find the spot with the maximum intensity (illuminance). | #*Hold the photometer directly in the path of the brightest beam. Move it around until you find the spot with the maximum intensity (illuminance). | ||
#*Staying the same distance from the light source, move the photometer in one direction, until the light intensity drops to 50% of the maximum level. Use a taut string or other straightedge to mark the line from the light source to this point. | #*Staying the same distance from the light source, move the photometer in one direction, until the light intensity drops to 50% of the maximum level. Use a taut string or other straightedge to mark the line from the light source to this point. | ||
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#*''Relative''' measurements won't give you a result in terms of units. You'll know exactly how two light intensities compare, but won't be able to relate them to a third intensity without repeating the experiment. | #*''Relative''' measurements won't give you a result in terms of units. You'll know exactly how two light intensities compare, but won't be able to relate them to a third intensity without repeating the experiment. | ||
#Cut a slab of paraffin wax in half. Purchase a package of paraffin wax from a hardware store or grocery store, and pull out one ¼ pound (0.55 kilogram) slab. Using a sharp knife, cut the slab into two equal pieces. | #Cut a slab of paraffin wax in half. Purchase a package of paraffin wax from a hardware store or grocery store, and pull out one ¼ pound (0.55 kilogram) slab. Using a sharp knife, cut the slab into two equal pieces. | ||
| − | #*Cut through the slab slowly to avoid breaking off pieces.<ref>http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p031.shtml#procedure</ref> | + | #*Cut through the slab slowly to avoid breaking off pieces.<ref name="rf7">http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p031.shtml#procedure</ref> |
#Sandwich aluminum foil between the paraffin pieces. Tear off a sheet of aluminum foil and lay it on top of one of the two paraffin pieces, completely covering the top surface. Lay the second piece of paraffin on top of the aluminum. | #Sandwich aluminum foil between the paraffin pieces. Tear off a sheet of aluminum foil and lay it on top of one of the two paraffin pieces, completely covering the top surface. Lay the second piece of paraffin on top of the aluminum. | ||
#Turn the "sandwich" vertically. For this device to work, we'll need to stand it on its end, so the foil sheet in the middle is vertical. If your wax doesn't stand up on its own, you can keep it horizontal for now. Just remember that the box you'll be constructing should be designed to hold the wax in position vertically. | #Turn the "sandwich" vertically. For this device to work, we'll need to stand it on its end, so the foil sheet in the middle is vertical. If your wax doesn't stand up on its own, you can keep it horizontal for now. Just remember that the box you'll be constructing should be designed to hold the wax in position vertically. | ||
| − | #*You can use two rubber bands to hold the block together.<ref>http://mysite.du.edu/~etuttle/electron/elect45.htm</ref> Put one near the top of the sandwich and the other near the bottom. | + | #*You can use two rubber bands to hold the block together.<ref name="rf8">http://mysite.du.edu/~etuttle/electron/elect45.htm</ref> Put one near the top of the sandwich and the other near the bottom. |
#Cut three windows into a cardboard box. Choose a box just large enough to hold your wax block. The packaging that the wax was sold in often works well. Use a ruler and pair of scissors to cut three windows into the box: | #Cut three windows into a cardboard box. Choose a box just large enough to hold your wax block. The packaging that the wax was sold in often works well. Use a ruler and pair of scissors to cut three windows into the box: | ||
#*Cut two windows on opposite sides, exactly the same size. Each window will view a different half of the paraffin, once the block is placed inside. | #*Cut two windows on opposite sides, exactly the same size. Each window will view a different half of the paraffin, once the block is placed inside. | ||
#*Cut a third window of any size in the front of the box. This should be centered, so you can view both halves of the paraffin block, on either side of the aluminum foil. | #*Cut a third window of any size in the front of the box. This should be centered, so you can view both halves of the paraffin block, on either side of the aluminum foil. | ||
| − | #Place the paraffin inside the box. Keep the aluminum foil between the two paraffin wax blocks, in a vertical position. You may need to use tape, small pieces of cardboard, or both to keep the wax blocks upright and parallel with the opposing windowed sides, and touching the foil between them.<ref | + | #Place the paraffin inside the box. Keep the aluminum foil between the two paraffin wax blocks, in a vertical position. You may need to use tape, small pieces of cardboard, or both to keep the wax blocks upright and parallel with the opposing windowed sides, and touching the foil between them.<ref name="rf7" /> |
#*If the box is open at the top, cover it with another piece of cardboard or other light-blocking barrier. | #*If the box is open at the top, cover it with another piece of cardboard or other light-blocking barrier. | ||
#Decide on a "reference point" light source. Choose one of the light sources you'll be comparing as a "standard candle," which you'll use as a baseline for intensity. If you're comparing more than two light sources, you'll use this light source during every comparison. | #Decide on a "reference point" light source. Choose one of the light sources you'll be comparing as a "standard candle," which you'll use as a baseline for intensity. If you're comparing more than two light sources, you'll use this light source during every comparison. | ||
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#Measure the distance between the photometer and each light source. Use measuring tape to measure the distance from the aluminum foil to your chosen "reference point" light source. We'll call this '''d1'''. Write this down, then measure the distance from the aluminum foil to the light source on the opposite side, '''d2'''. | #Measure the distance between the photometer and each light source. Use measuring tape to measure the distance from the aluminum foil to your chosen "reference point" light source. We'll call this '''d1'''. Write this down, then measure the distance from the aluminum foil to the light source on the opposite side, '''d2'''. | ||
#*You can measure the distance using any unit, but make sure not to mix them. For example, if your measurement is in feet and inches, convert the result to use inches only. | #*You can measure the distance using any unit, but make sure not to mix them. For example, if your measurement is in feet and inches, convert the result to use inches only. | ||
| − | #Understand the physics involved. The brightness of the blocks decreases with the square of the distance, because we're measuring the amount of light that falls onto a two dimensional ''area'', but the light is radiating through three dimensional ''volume''.<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/vision/isql.html#c1</ref> In other words, when a light source moves twice as far away (x2), the light it produces is spread out across four times the area (x2<sup>2</sup>). We can write the brightness as "I / d<sup>2</sup>''' | + | #Understand the physics involved. The brightness of the blocks decreases with the square of the distance, because we're measuring the amount of light that falls onto a two dimensional ''area'', but the light is radiating through three dimensional ''volume''.<ref name="rf9">http://hyperphysics.phy-astr.gsu.edu/hbase/vision/isql.html#c1</ref> In other words, when a light source moves twice as far away (x2), the light it produces is spread out across four times the area (x2<sup>2</sup>). We can write the brightness as "I / d<sup>2</sup>''' |
#*I is the intensity and d is the distance, just like we used them in previous steps, | #*I is the intensity and d is the distance, just like we used them in previous steps, | ||
| − | #*Technically, what we described as brightness is referred to as '''illuminance''' in this context.<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/vision/areance.html#c1</ref> | + | #*Technically, what we described as brightness is referred to as '''illuminance''' in this context.<ref name="rf10">http://hyperphysics.phy-astr.gsu.edu/hbase/vision/areance.html#c1</ref> |
| − | #Use this knowledge to solve for the relative intensity. When both blocks appear equally bright, their "illuminance" is equal. We can write this as a formula, then rearrange it to solve for I<sub>2</sub>, or the relative intensity of the second light source:<ref | + | #Use this knowledge to solve for the relative intensity. When both blocks appear equally bright, their "illuminance" is equal. We can write this as a formula, then rearrange it to solve for I<sub>2</sub>, or the relative intensity of the second light source:<ref name="rf7" /> |
#*I<sub>1</sub>/d<sub>1</sub><sup>2</sup> = I<sub>2</sub>/d<sub>2</sub><sup>2</sup> | #*I<sub>1</sub>/d<sub>1</sub><sup>2</sup> = I<sub>2</sub>/d<sub>2</sub><sup>2</sup> | ||
#*I<sub>2</sub> = I<sub>1</sub>(d<sub>2</sub><sup>2</sup>/d<sub>1</sub><sup>2</sup>) | #*I<sub>2</sub> = I<sub>1</sub>(d<sub>2</sub><sup>2</sup>/d<sub>1</sub><sup>2</sup>) | ||
