Measurement of light intensity is very important in designing a room lighting or preparing for photography. The term “intensity” is used in a variety of ways so you should take the time to learn which units and methods of measurement are appropriate for your purposes. Professional photographers and lighting professionals usually use digital meters, but you can also make a simple comparative light meter called the Joly photometer.
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Method 1 of 2: Measuring the Light Intensity of a Room or Light Source
Step 1. Understand photometers that measure lux and foot candles
These units describe the intensity of light at the surface, or "illuminance" (illuminance). Photometers that measure illumination are commonly used to set up a shooting session, or to test whether a room is too bright or too dim.
- Some light meters are specially designed for different types of lighting. For example, measurement results can be more accurate when used to measure sodium exposure.
- You can even buy a “light meter” in the mobile app store. Check application reviews first because some of these applications are not very accurate.
- Lux is the standard commonly used today, but some devices still measure in standard foot candles. Use this online calculator to convert between the two standards.
Step 2. Know how to interpret the unit of illumination
Here are some commonly used illumination measurements to help you determine changes to exposure:
- Most office work can be done within 250–500 lux (23–46 foot-candles).
- Supermarkets or work areas that involve drawing or other detail work are typically lit at 750–1,000 lux (70–93 foot-candles). The upper limit of this range is equivalent to the indoor area next to the window on a sunny day.
Step 3. Understand about lumen and luminance
If a light bulb, lamp label, or advertisement mentions the word lumen, the number describes the total amount of energy emitted by visible light. This concept is named lighting. Here's what you need to know:
- The initial "lumen" describes the amount of light that will be emitted when the lamp is stabilized. Usually fluorescent or HID lamps require 100 hours of use to stabilize.
- "Mean lumen" or "mean lumen" describes the approximate average luminance over the lifetime of the device. The actual lighting will be brighter at first, and dimmer towards the end of its useful life.
- To find out how many lumens you need, use the steps above to determine the number of foot candles of lighting you want in the room, and multiply by the area (square meters) of the room. It's a good idea to increase the results for rooms with dark walls, and lower them for rooms with lots of large light sources.
Step 4. Measure the beam (highlight) and field angle (corner of the room)
Flashlights and other devices that emit light in certain directions can be described using these two new terms. You can find this yourself using a photometer that measures lux or foot candles, and with a straightedge beam or protractor:
- Hold the photometer directly in the path of the brightest beam. Move until you find the point with the highest intensity (lighting).
- Keep the distance from the light source unchanged, and move the photometer in one direction until the light intensity drops to 50% of its maximum level. Use a tight thread or another straightedge to mark a line from the light source to this point.
- Walk in the opposite direction until you find a spot on the reverse side of the spotlight with an intensity of 50% of maximum exposure. Mark a new line from this point.
- Use a protractor to measure the angle between the two lines. This is the spotlight angle, and describes the angle at which the light source is shining brightly.
- To find the field angle, repeat this step, but mark the point where the intensity reaches 10% of its maximum level.
Method 2 of 2: Measuring Relative Intensity with Home Devices
Step 1. Use this method to compare light sources
This device can be made easily at home. The device is named the “Joly Photometer” after its inventor, and can be used to measure the relative intensity of two light sources. With a little knowledge of physics and the materials below, you can find light bulbs that emit more light, as well as bulbs that are more efficient for the amount of power they use.
Measurement relatively does not return results in units. You will clearly know the ratio of the intensities of two lights, but you cannot relate them to a third light source without repeating the comparisons one by one.
Step 2. Cut a paraffin bar in half
Purchase paraffin wax from a hardware store or supermarket, and take as much as 0.55 kg. Use a sharp knife to cut the paraffin stick into two equal pieces.
Cut the stems gently so the wax doesn't break
Step 3. Place the foil between the two pieces of paraffin
Cut out a sheet of foil and lay it down so that it covers the top of one of the strips of foil. Place the other piece of paraffin on top of the aluminum.
Step 4. Place the paraffin “bread” vertically
In order for this device to work, you need to stand it vertically at one end so that the aluminum sheet in the center stands upright as well. If your candle can't stand on its own, feel free to let it lie horizontally for now. Don't forget, the box that will be made must be able to accommodate the candle blocks that stand vertically.
You can use two rubber bands to hold the paraffin rods and foil together. Place a rubber band near one end of the bar and one near the other
Step 5. Cut out three windows on the cardboard box
Choose a box large enough to hold the candle blocks. You can use a purchased candle packaging box. Use a ruler and scissors to cut out three windows on the box:
- Cut two identical-sized windows on opposite sides. Each window will display a different paraffin bar once the wax block has been inserted.
- Cut a third window of any size on the front of the box. This window should be centered so you can see the two pieces of paraffin holding the foil.
Step 6. Put the paraffin into the box
Keep the foil between the two candle sticks in a vertical position. You can use masking tape, strips of cardboard, or both to keep the wax block upright and parallel to the opposite window, and touching the foil in between.
If the box is open at the top, cover it with another piece of cardboard or other light-blocking material
Step 7. Determine the "reference point" of the light source
Select one of the light sources to compare as a “standard candle”. You will use it as a measure of light intensity. If you are comparing more than two light sources, this “standard candle” will always be used.
Step 8. Arrange the two light sources so that they are in a straight line
Place two lamps, or other light sources on a flat surface in a straight line. The distance between the two should be much larger than the width of the box you created.
Step 9. Place the photometer between the two light sources
The height of the photometer should be exactly the same as the light sources so that the light completely shines through the candle block through the side window. Remember, the distance from the light source to the photometer must be far enough so that the illumination is evenly distributed.
Step 10. Turn off all the lights in the room
Close all windows, blinds, or curtains so that only light from light sources shines through the beams.
Step 11. Arrange the squares until both paraffin blocks appear equally bright
Move the photometer toward a dimmer source of light to illuminate the paraffin beam. Watch through the first window as you adjust the position of the squares, and stop when the two candlesticks appear equally bright.
Step 12. Measure the distance between the photometer and each light source
Use a tape measure to measure the distance between the foil to the selected “reference point” light source. Now we name it as d1. Take notes, then measure the distance from the foil to the other light source (d2).
You can measure distance using any unit, but it must be consistent. For example, if you measure in centimeters or meters, change the result so that the units are only centimeters (cm)
Step 13. Understand the laws of physics involved
The brightness of the beam decreases with each square of distance from the light source because we measure the amount of light falling into the two-dimensional "area", but light radiating through the three-dimensional "volume". In other words, when the light source travels twice as far (x2), the resulting light scatters four times (x22). We can write brightness as I/d2'
- I is the intensity and d is the distance, as we used in the previous step.
- Technically, the "brightness" we describe, in this context refers to "illumination".
Step 14. Use this knowledge to solve for relative intensities
Both beams have the same “illumination” when they both appear equally bright. We can write it down as a formula, then construct it to produce I2, or the relative intensity of the second light source:
- I1/d12 = I2/d22
- I2 = I1(d22/d12)
- Since we are only measuring the relative intensity, or the ratio of the two, we simply write I1 = 1. Thus, the formula becomes simpler: I2 = d22/d12
- For example, say the distance d1 to a reference point light source of 0.6 meters, and a distance d2 to the second light source is 1.5 meters:
- I2 = 52/22 = 25/4 = 6, 25
- The second light source has an intensity 6, 25 times bigger from the first light source.
Step 15. Calculate efficiency
If you count a light bulb that lists the wattage, for example "60 W" which means "60 watts," that's how much power the bulb uses. Share the lamp's relative intensity with this power to find out how efficient it is, relative to other light sources. For example:
- A 60 watt lamp with a relative intensity of 6 has a relative efficiency of 6/60 = 0.1.
- A 40 watt lamp with a relative intensity of 1 has a relative efficiency of 1/40 = 0.025.
- Because 0.1 / 0.025 = 4, a 60 W lamp is four times more efficient at converting electricity into light. Note that this lamp still uses more power than a 40 W lamp and therefore costs more. Efficiency simply states how efficient a lamp is in utilizing electricity and converting it into light.
