Measuring the intensity of light is important when designing the lighting system for a room or when taking a photograph. The term "intensity" is used in different ways, so it is advisable to stop for a moment to learn the meaning of the various units and methods of measurement. Professional photographers and electricians use a digital photometer, but you can also use a simple comparison tool called a Joly diffusion photometer.
Steps
Method 1 of 2: Measure the Light Intensity of a Room or a Light Source
Step 1. Know that photometers measure light intensity in lux and "foot-candle"
Both of these units of measurement (the second strictly American) describe the intensity of light on a surface or illuminance. Photometers that measure illuminance are the indispensable tool for people who need to set up a photo shoot or check if a room is too bright or too dark.
- Some instruments are built for different types of light. For example, a model could only be very accurate when used with sodium lamps.
- You can even purchase a "photometer" as a mobile app. But check the reviews first, as many can be inaccurate.
- Lux is the international unit of measurement, but it is not uncommon to find some instruments still calibrated in foot-candles. If you are interested in converting lux into this unit of measurement, you can use an online calculator. It must be said that in Italy it is not at all easy to find foot-candle photometers. Furthermore, it must be emphasized that it is necessary not to confuse with the unit of measurement "candle per square meter (cd / m2)", since this defines the luminance and not the illuminance.
Step 2. Learn to interpret the units of measurement of illuminance
Here are some typical examples to help you understand if your lighting needs need to be changed:
- Good workplace (office) lighting should generate an illuminance of 250-500 lux.
- Supermarkets or areas where drawing or detailing work is carried out are usually illuminated at 750-1000 lux. The upper end of this range is equivalent to the area of a room near a sunny window.
Step 3. Learn what lumen and luminance are
The first is the unit of measurement of the luminous flux, a concept different from luminance which is instead the ratio between the light intensity emitted by a source in the direction of the observer and the area of the emitting surface. Sometimes, on bulbs or lamps you can find a value expressed in "lumen" that describes the amount of visible light emitted:
- The "initial lumens" describe how much light a source emits once it has stabilized. For example, fluorescent and HID bulbs need 100 hours of use to stabilize.
- The "average lumens" or "theoretical lumens" express the estimated quantity of light that the source should emit in the ideal conditions of use for its entire duration. In reality this value is higher in the early days and then decreases as the light source “ages”.
- To understand how many lumens you need, consider the list of lux that was described above and multiply the value by the area of the room (in square meters).
Step 4. Measure the light beam and the angle of illumination
Flashlights and other devices that are capable of emitting a beam of light can also be described with these two characteristics, which you can find using a photometer that measures lux with a ruler and a protractor:
- Hold the photometer directly in the brightest part of the beam. Move it until it detects the highest value.
- Try not to move the photometer away from the light source, but move it in one direction only until the intensity drops by 50% compared to the maximum value. Use tight twine or a straight line to draw a segment from the light source to this point.
- Now move the photometer in the opposite direction, inside the light beam, until you notice a second 50% decrease in intensity. Draw another segment.
- With a protractor, it measures the angle between the two lines. This is the "beam angle" of light and describes the width of the sector well illuminated by the light source.
- To find the lighting angle, proceed with the same operations, but draw the lines at the points where the light intensity reaches 10% of the maximum value.
Method 2 of 2: Measuring Relative Intensity with a Craft Device
Step 1. Use this tool to compare different light sources
It is a device that can be easily made at home, after purchasing some essential materials. It is called the "Joly photometer", named after its inventor, and can be used to measure the relative intensity of two light sources. With a little knowledge of physics and the tools described below, you will be able to understand which of the light bulbs taken into consideration emits a higher or lower amount of light and which is the most efficient in relation to the energy consumed.
Relative measurements do not offer values expressed in units of measure. You will be able to quantify the light intensity emitted by one source in relation to that emitted by a second source, but you will not be able to compare them to a third without repeating the experiment
Step 2. Cut a paraffin block in half
Buy a piece of solid paraffin at a hardware store, a half-kilo pack is enough. With a sharp knife, cut it into two equal parts.
Work slowly to avoid creating small fragments
Step 3. Insert a sheet of aluminum foil between the two pieces of paraffin
Tear off a piece from the roll and lay it on top of one of the two blocks until the surface is completely covered. Place the second block on the aluminum foil.
Step 4. Return the "sandwich" to an upright position
For this tool to work, the paraffin must rest on one side, so that the aluminum is vertical. If you can't get it in this position, leave it horizontally for now. But remember that the box you are going to build requires the block to be vertical.
You can use two rubber bands to keep the block compact. Place one near the top and one on the bottom
Step 5. Cut out three openings in a cardboard box
Choose one that is large enough to hold the paraffin. Often the same packaging as the block is the best solution. Help yourself with a ruler and a pair of scissors to cut out three windows:
- Open two identical ones on opposite sides of the box. Each of them allows you to see one side of the paraffin block once it is inserted.
- Cut a third opening, in the size of your choice, in the front of the box. However, it should be well centered in order to be able to see both halves of the paraffin with the aluminum in the middle.
Step 6. Insert the block into the box
Remember that the paper must remain inside and upright. You could be helped by pieces of cardboard, a little adhesive tape or both to keep the paraffin vertically and parallel to the two opposite windows without the aluminum slipping out.
If the top of the box is open, cover it with another piece of cardboard or similar material that blocks the light
Step 7. Choose a light source as a "reference point"
This will be the unit of measurement with which you will compare the other lights and will be the guideline for evaluating the intensity. If you are going to compare more than two lights, then you will need to use this source for each experiment.
Step 8. Arrange two light sources in a straight line
Place two light bulbs, two LEDs, or some other type of light source in a straight line, on a flat surface. The distance between them must be significantly greater than the width of the box you built.
Step 9. Put the photometer between the two lights
It must be exactly at the same height as the light sources, so that the paraffin block is fully illuminated through the two side windows. Remember that the two light sources must be quite distant from each other to ensure uniform lighting.
Step 10. Turn off all other lights in the room
Close any windows, shutters or blinds so that the only lights hitting the paraffin are the ones in the test.
Step 11. Adjust the position of the box until the two halves of the block are illuminated in the same way
Move the photometer to the less lighted side of the paraffin. Check through the front window so you can adjust the position of the box; stop when both halves seem equally illuminated to you.
Step 12. Measure the distance between the photometer and each light source
Use a tape measure and place the zero on the line of the aluminum foil, stretch it towards the light you have chosen as a "reference". This distance is called d1. Write down the value and then repeat the procedure for the other light source. The distance that separates it from the aluminum foil is called d2.
You can measure this distance with any unit of measurement, the important thing is to stay constant. For example, if you used meters and centimeters, convert the values to use centimeters only
Step 13. Understand the physical concept underlying the experiment
The brightness of the paraffin pieces decreases with the square of the distance that separates them from the light, because we are considering the light hitting a two-dimensional surface (i.e. an area) even if, in truth, the light radiates in all directions and hits a space (i.e. a volume). In other words, when the light source moves twice (x2) farther, it is distributed over an area four times larger (x22). We can therefore write the brilliance as "I / d2
- I is the intensity and d is the distance from the light source, precisely the values we used in the previous steps.,
- Technically, what we are describing as brilliance is said illuminance.
Step 14. Use these notions to calculate the relative intensity
When both paraffin halves are equally bright, their "illuminance" is identical. You can write the formula and solve it for I2, i.e. the relative intensity of the second light source:
- THE1/ d12 = I2/ d22.
- THE2 = I1(d22/ d12).
- Since you are only measuring a relative intensity, i.e. the ratio of two light sources, you can establish that I.1 = 1. This greatly simplifies the formula which becomes: I2 = d22/ d12.
- For example, suppose the distance d1 to the reference light source is 2 meters and the distance d2 at the second light is 5 meters:
- THE2 = 52/22 = 25/4 = 6, 25
- The second light source has an intensity 6, 25 times greater than the reference one.
Step 15. Calculate the efficiency
If you are using bulbs that have the power indicated on them, for example "60 W", which means "60 watts", then you know how much electricity they consume. Divide the relative intensity of the bulb by the power and you will find its efficiency, in relation to the reference light source. For instance:
- A 60 watt light bulb with a relative intensity of 6 has a relative efficiency of: 6/60 = 0.1.
- A 40 watt light bulb with a relative intensity of 1 has a relative efficiency of 1/40 = 0.025.
- Given that 0, 1/0, 025 = 4, the 60 W light bulb is 4 times more efficient in transforming electricity into light. Note that you can still use a 40W more powerful bulb, but it will cost you more; efficiency allows you to know how much yield you can get from your "economic investment".
