Light Intensity Unit Measurement? (Solved)

ProdigyGenius · Post by **ProdigyGenius** » Mon Oct 20, 2014 6:58 am

Quick question. I am alternately conducting the The Joly Photometer: Measuring Light Intensity Using the Inverse Square Law, but what unit of measurement would I use for relative light intensity? It has to be the universal SI unit of measurement.

theborg · Post by **theborg** » Mon Oct 20, 2014 4:55 pm

ProdigyGenius,

Thanks for the great question. Luminous intensity has the SI unit, candela, and has the symbol (cd). It is a measure of candlepower. However, candlepower and candela are measurements of light at the source and don't say much about the light at some distance. Instead, we measure the amount of light illuminating a surface area. This is called illuminance and is measured in lumens, with 1 lumen = 1 candela x steradian. We can think of this in terms of an area. If you take a transparent sphere with radius 1 meter, its surface area will be given by 4 pi r^2 = 12.57 m^2. If you place this sphere around a 1 candela candle, the energy passing through 1 m^2 of the sphere is 1 lumen, and total luminous flux is 12.57 lumens. In SI units the standard is 1 lumen per square meter and is called 1 lux.

so, 1 lux = 1 lumen/m^2 = 1 (candela (steradian))/m^2

ProdigyGenius · Post by **ProdigyGenius** » Sun Oct 26, 2014 1:10 pm

Yea, I don't mean to be rude, but the only thing I understood was that I am suppose to use the lumen (lux) unit. How would I measure that with this project? Also, I am using (compact) fluorescent, incandescent, and light-emitting diode (LED) light bulbs and to find which has the highest relative light intensity and efficiency. My question is would I need to keep all their wattage the same, or would that not affect my results. Furthermore, if they don't need to be the same wattage could I therefore also use a candle as another light source? Any help is appreciated, thanks.

norman40 · Post by **norman40** » Wed Oct 29, 2014 6:16 am

Hello ProdigyGenius,

In the “Joly Photometer” project the light intensity that you measure for a test light source is relative to the intensity of a standard light source. With the Joly photometer, you measure the distance from your test light source to the photometer that gives the same light intensity as the standard light source. The ratio of the square of the test source distance to the square of the standard source distance is the relative intensity. There are no units for this relative measurement.

But if you know the intensity (in lumen or lux) of the standard source you can calculate the intensity of the test source in the same units. For example, say that you use an incandescent bulb rated at 400 lumen as your standard. You might test another bulb with your Joly photometer and find that this bulb needs to be twice as far away from the photometer to see the same light intensity as the standard bulb. The square of the distance ratio would be 4. That would mean that the test bulb has 4 times the light intensity as the standard bulb, or 1600 lumen.

I think testing compact fluorescent, LED and incandescent bulbs of the same wattage would be a nice experiment. You could easily compare relative intensities at the same wattage. But it may be difficult to find all these types of lights with the same wattage. If you can't find bulbs of the same wattage you could test bulbs of different wattage and divide the relative intensities by the wattage values to compare efficiencies.

I hope this helps. Please post again if you have more questions

A. Norman

ProdigyGenius · Post by **ProdigyGenius** » Wed Oct 29, 2014 6:44 pm

I somewhat understand, so does this mean I am not using any SI unit of measurement because that is a requirement. Also, I can afford the identical wattage of different light bulbs, but can the wattage still be different and work the same way, or would that have to also be controlled? Could I do the same with a candle light to vary the results?

norman40 · Post by **norman40** » Thu Oct 30, 2014 1:13 pm

Hello ProdigyGenius,

I think you can test light bulbs of different wattages. You would expect different light intensity depending on both the wattage and the type of bulb (LED, CFL, incandescent). To make a good comparison of light intensity from different bulb types, you'd need to account for the wattage. One way of making this accounting would be to divide your relative intensity values by the wattage values. The resulting ratios would represent efficiency ratings (light intensity per watt) for the different bulbs.

I hope this helps. Please post again if you have more questions

A. Norman

ProdigyGenius · Post by **ProdigyGenius** » Fri Oct 31, 2014 6:13 pm

Thanks for the help that the wattage don't have to be necessary the same but you have to divide by the wattage. So would this also work with a candle? What would be an average candle's flame wattage be, or is that impossible to calculate?

norman40 · Post by **norman40** » Sat Nov 01, 2014 8:59 am

Hi ProdigyGenius,

Great question! Yes, you could measure the light intensity from a candle and calculate an efficiency in terms of intensity per watt.

Candles produce light (and heat) by burning paraffin. In the burning process (also called combustion), oxygen from the air combines with carbon and hydrogen from the paraffin to form carbon dioxide and water. The total heat produced by the combustion reactions (in KJ/g) multiplied by the rate at which the paraffin is burned (in g/s) is the wattage of the candle. An average candle produces about 80 watts.

Here are a couple of links that discuss candle wattage in more detail:

http://www.thenakedscientists.com/HTML/ ... tion/1857/

http://en.wikipedia.org/wiki/Candle

I hope this helps. Please post again if you have more questions

A. Norman

ProdigyGenius · Post by **ProdigyGenius** » Thu Nov 20, 2014 6:19 am

Okay I have decided to utilize 40 watt equivalent light bulbs since they are the cheapest and I couldn't find all of them that had 80 watts without it being super expensive. One change I had to made was to purchase a Halogen incandescent light bulb which is pretty the "new" incandescent type of light bulb. So my question is how would I graph this experiment. I know I can do an easy bar graph but my educational instructor recommends we do a line graph, so how would I set that up? If there isn't really a way I will tell my teacher but any ideas would help. Also if I were to include the candle light in my experiment would I have to divide everything I do with it because 80/2=40? Thanks to anyone who helps.

norman40 · Post by **norman40** » Fri Nov 21, 2014 7:03 am

Hi ProdigyGenius,

I think you're on the right track with the bar chart idea. Bar charts are good for comparing numerical data collected for different items or categories, like different kinds of light sources.

My suggestion is to display your light intensity measurements on one axis and your light source types on the other axis. Another display could be light intensity divided by wattage consumption (efficiency) for the different light source types.

Please note that LED and CFL bulbs advertised as “40 watt equivalent” produce the same amount of light as a 40 watt incandescent bulb. But the LED and CFL bulbs consume much less electrical energy.

Wattages used by “40 watt equivalent” LED and CFL bulbs will be about 5 watts and 13 watts, while the incandescent bulb uses 40 watts. You should use the wattage consumption ratings for your bulbs when you compare their efficiencies. The wattage rating should be shown somewhere on the light bulb package.

For some more information see the following link showing a comparison of “60 watt equivalent” bulbs:

http://www.designrecycleinc.com/led%20comp%20chart.html

When calculating the efficiency of the candle, I would divide the measured light intensity by 80 watts. I think this represents a fair comparison with light bulb intensity divided by wattage consumption rating.

I hope this helps. Please post again if you have more questions

A. Norman

ProdigyGenius · Post by **ProdigyGenius** » Fri Nov 21, 2014 2:52 pm

Thanks for your reply norman. I asked mt teacher about the graph and she said it was fine because she didn't really see a way to put it in a line graph either. I don't know why but I really feel the urge to present at least one of my research collected data in a line graph because I just think they look professional. So is it possible to do a line graph with my ten trials for each type of light bulb or should I just do a chart/table to save space? That wasn't really my essential question but I just wanted to ask that too. My real concern is the second part of your message. I didn't really understand what you meant because one of my controlled variables was the wattage of each light bulb, so wouldn't the wattage have to be "equivalent." If not does that mean I have to buy more light bulbs (my parents wouldn't like that)? Other than that thanks for all the help and replies you have given me thus far it has been very helpful.

norman40 · Post by **norman40** » Sat Nov 22, 2014 2:14 pm

Hi ProdigyGenius,

I don't think you need to buy more light bulbs! Your selection of “40 watt equivalent” LED and CFL bulbs is OK.

In the case of LED and CFL bulbs, “40 watt equivalent” means that these types of bulbs produce the same light intensity as a 40 watt incandescent bulb.

A 40 watt incandescent bulb uses 40 watts of electricity to produce light. A “40 watt equivalent” LED bulb uses about 5 watts of electricity to produce light. A “40 watt equivalent” CFL bulb uses about 13 watts of electricity to produce light.

So, when you compare intensity of light produced per watt of electricity used, you should account for the wattage used by the bulbs, not the “wattage equivalence”.

I think you could display your data with a line chart if you like. To do this you could construct your chart as you would for a bar chart. Instead of bars, just plot your data points and connect them with lines. If you are using spreadsheet software (like Excel) you can easily convert from a bar chart to a line chart format.

I hope this helps and good luck. Please post again if you have more questions.

A. Norman

ProdigyGenius · Post by **ProdigyGenius** » Mon Nov 24, 2014 2:28 pm

Quick question. I have already recorded my independent and dependent variables, but my teacher also requires us to identify our controls and constants. I have looked up the difference between the two but would just like someone to double-check, and currently its Thanksgiving Break so my teacher can't check it. So... Would the constants of this experiment be wattage (which is 40), qualities of the testing room (i.e. absolutely no light), and lamp distance? (if there are any other important ones I missed please tell me) I couldn't really figure out the control on this particular experiment but I guess it would be the relative intensity of two identical light bulbs compared to one another would always be 1.0 using the formula I2=I1d22/d12. Also since I have to compare each light source to each other then there are 12 choices and I I have to do each ten times so 120 trials in total? I can do all these trials but I'm just asking. Furthermore I saw you said divide the candle's relative light intensity by 80 but wouldn't it be 40 since one of my constants is wattage which is 40. Last, one of removable top lamps I was going to use is not working so could I conduct my experiment using two identical desk lamps. They both have a radius of 2.5, diameter of 5, and therefore a circumference of 15.70. Please correct me if I'm wrong and Happy Thanksgiving!

norman40 · Post by **norman40** » Wed Nov 26, 2014 7:44 am

Hi ProdigyGenius,

From the information you've posted I think you've correctly identified wattage and qualities of the testing room as constants (or controlled variables). I think that your desk lamps represent another constant (controlled variable). I agree that you should use identical desk lamps because different lamp configurations could affect your results.

I don't think lamp distance (from the Joly photometer) can be constant because the distance for one of the light bulbs must be varied to make the measurement.

I think the test of two identical bulbs is the control in this experiment. If everything is working OK, you should find that the lamps are at equal distances from the photometer.

I'm glad to see that you've planned repeat trials in your work. Repeated measurements give you an idea of the reliability of your results and conclusions from them. This is missing in many of the science fair projects that I've seen.

Comparing all of your light sources with each other as you've outlined does require many trials. I think it would be OK to choose one light source as a constant reference and compare all of your other light sources with it. This would reduce the number of trials considerably even with 10 tests of each comparison.

The efficiency comparison is relative light intensity per watt used to produce the light (intensity divided by wattage). You want to use 80 for the candle's wattage because that is the energy used to make the light. Your choice of 40 watts for your light bulbs has nothing to do with the wattage from the candle.

I hope this helps and good luck. Please post again if you have more questions.

A. Norman

ProdigyGenius · Post by **ProdigyGenius** » Thu Nov 27, 2014 5:20 pm

Thanks norman for all your help. I have finished conducting my experiment and here are my results. Candles were of course the least efficient and light intense, then came Halogen, CFL, and first place went to LED. I just have one more question. You said I should account for the "Actual Wattage" used for each light source, so I did (LED was 6, CFL was 9, Halogen was 29, and I believe the Candle was 80W). I would like to ask, does this mean the dependent variable of my experiment the whole time was wattage not light source type, or is that associated together?

ProdigyGenius · Post by **ProdigyGenius** » Sat Nov 29, 2014 7:12 am

Would the I1 and I2 of the inverse square law formula for the experiment, The Joly Photometer: Measuring Light Intensity Using the Inverse Square Law, be the lumens or wattage for the light source? If lumens, what would be the average or standard lumen of a candle flame?

rmarz · Post by **rmarz** » Sat Nov 29, 2014 9:27 am

ProdigyGenius - You are attempting to measure the efficiency of various light sources, therefore the ultimate comparison is light intensity measured by the photometer and it's associated measured distance, then comparing that value to the power consumed by the light source. This will give you data to make comparisons. The attribute measured is light intensity, or lumens, measured by the photometer. You vary the distance of one of the light sources until the measured intensity by the photometer is equal (left and right aperatures appear equally bright). Then, use the inverse square law to calculate the intensity of your source. It doesn't matter what the specific value of a lumen is for this experiment. You are using your eyes as the sensor to judge when the light values in the photometer are equal. Then measuring the two distances to use in your equation.

There is a standard definition for the luminous output of a candle flame, and it is expressed as:

Mark's Standard Handbook for Mechanical Engineers (10th Edition) page 12-99 defines "candela, cd (formerly candle) is the unit of luminous intensity of a light source. One candela is defined as the luminous intensity in a given direction, of a source that emits monochromatic radiation of a frequency 540 x 10(12)HZ (approx. 55 nm) and of which the radiant intensity in that direction 1/683 W per steradian (W/sr)." It goes on to show in table 12.5.1 that a single candle flame is rated at 6.45 cd/in2.

A bit complicating, but inviting you to do some more research on the fascinating subject of light.

Rick Marz

ProdigyGenius · Post by **ProdigyGenius** » Sat Nov 29, 2014 12:29 pm

So I was thinking to complement my project I should include a chart of what people believe is the most energy efficient light bulb, because part of my research I found not many people are educated on this subject. Simple post a reply saying Halogen, CFL, LED, or IDK (I don't know). I intend this to be for mostly people who don't know about this, so no experts on this topic telling the answer. Also, don't just look it up it because you want to sound smart, this is simply a poll and I am not recording any names or anything, so try not to ruin the results. Last, don't just look at previous posts and go with the majority, just go with what you think, and if you don't know it, IDK it there is no shame. I need at least 30 participants, so anyone who replies I give you my gratitude.

ProdigyGenius · Post by **ProdigyGenius** » Sat Nov 29, 2014 12:48 pm

Thanks norman for all your help on the previous post, I moved this question to a new topic because I think you didn't see it because you didn't reply for three days. I have finished conducting my experiment and here are my results. Candles were of course the least efficient and light intense, then came Halogen, CFL, and first place went to LED. I just have one more question. You said I should account for the "Actual Wattage" used for each light source, so I did (LED was 6, CFL was 9, Halogen was 29, and I believe the Candle was 80W). I would like to ask,would this mean I would have to divide their relative light intensity all by the "actual wattage," instead of 40? Does this mean the dependent variable of my experiment the whole time was wattage not light source type, or is that associated together? This question is directed to norman40 who has helped me with my project this far, but any expert may answer if you are willing to read all of the discussion on the prior post.

deleted-71603 · Post by **deleted-71603** » Sun Nov 30, 2014 11:11 pm

ProdigyGenius, please keep all questions to one thread only, so that our experts can best help you based on what has already been discussed. We have merged all of your threads into one. Please understand that our experts are all volunteers, so you may not always get an immediate response.

Good luck, and thank you for using Science Buddies!

norman40 · Post by **norman40** » Mon Dec 01, 2014 8:24 am

Hi ProdigyGenius,

Congratulations on completing your experiment!

Light source type (halogen, CFL, LED, candle) is the independent variable in your experiment. An independent variable is one that is changed by the scientist.

The light source types you chose consume different amounts of electrical energy (“actual wattage”) by design. I think you are correct in saying that light source type and “actual wattage” are associated.

Your dependent variable is the light intensity measured with your photometer. A dependent variable is one that changes in response to an independent variable. When you changed light source types (you independent variable), you observed differences in light intensity (your dependent variable).

I hope this helps. Please let us know if you have more questions.

A. Norman

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