Converting Lumens to Joules for 1cm Plastic/Glass Sphere

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In summary, the conversation revolves around calculating the amount of energy needed to raise the temperature of a 1cm diameter sphere made of plastic or glass. The speaker is considering using a bright LED flashlight for this purpose, but is unsure how to convert lumens to candelas and then to watts. They have found a converter online, but are seeking further insight. Another suggestion is to measure the power of the flashlight and calculate the absorbed energy based on that. The difficulty in converting lumens to watts is also mentioned.
  • #1
OpenLaszlo
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(Note that this is not a homework question, but rather something I'm pursing in my leisure)

I'd to do a quick calculation to see what sort of power I would need to raise the temperature of a small sphere (1cm diameter, made of plastic or glass) with an absorption coefficient of about .5. I was thinking about using a bright LED flashlight (1200-2000 lumens) to do this. However, I'm having trouble computing how much energy I would be depositing on the sphere (I assume just the front surface). Essentially, I'd like to convert a certain exposure time (maybe 15s or so of illumination) into the units of Joules so I can roughly estimate the temperature change via Q=m*c_p*delta T.

I realize that the first step should be to convert lumens to candelas, which could be done if I can measure the beam angle of the flashlight. Once I have my number in candelas, however, I'm not really sure where to go. I've found a converter online at http://www.unitconversion.org/unit_converter/luminance.html which converts candela/m^2 to Watts/cm^2 per steradian (at 555nm). I'm unsure as to how they arrived at this conversion factor and was hoping someone might be able to lend a little insight.

Thanks!
 
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  • #2
That seems complicated. Why not just measure the power of your flashlight (say it emits a few mW), estimate how much of that is absorbed by the object (if you held the light directly against the sphere, it would absorb a mW or so), and from that you can calculate how much energy is absorbed after a given amount of time.

Note- converting from lumens to watts is not trivial. And I see that consumer LED specs don't distinguish between power draw and emitted power. So maybe you have to slog through a conversion of lumens to watts, which is nontrivial except for narrowband sources (like lasers). FWIW, a 100W incandescent bulb emits only 1750 lm

http://members.misty.com/don/bulb1.html

Sounds like the easiest thing is to go backwards- shine the light on the sphere, measure the increase in temperature, and infer the light output.
 

1. How do you convert lumens to joules for a 1cm plastic/glass sphere?

To convert lumens to joules for a 1cm plastic/glass sphere, you can use the formula: Joules = Lumens x (4π x r2). This formula takes into account the surface area of the sphere (4π x r2) and multiplies it by the amount of light emitted (lumens) to determine the total amount of energy (joules).

2. What is the importance of converting lumens to joules for a 1cm plastic/glass sphere?

Converting lumens to joules for a 1cm plastic/glass sphere is important because it allows us to accurately measure and understand the amount of energy being emitted by the sphere. This information can be useful in various scientific experiments and applications.

3. Can the conversion factor for lumens to joules differ for different materials or sizes of spheres?

Yes, the conversion factor for lumens to joules may differ for different materials or sizes of spheres. This is because the formula takes into account the surface area of the sphere, which can vary depending on the material and size. It is important to use the correct conversion factor for the specific sphere being measured.

4. How can I measure the lumens of a 1cm plastic/glass sphere?

To measure the lumens of a 1cm plastic/glass sphere, you will need a light meter which measures the amount of light emitted in lumens. You can then place the sphere in front of the light meter and record the reading. This reading can then be used in the conversion formula to determine the amount of energy (joules) being emitted.

5. Are there any other factors that may affect the conversion of lumens to joules for a 1cm plastic/glass sphere?

Yes, there are other factors that may affect the conversion of lumens to joules for a 1cm plastic/glass sphere. For example, the temperature and humidity of the environment may impact the accuracy of the measurement, as well as the quality and type of light source being used. It is important to take these factors into consideration when performing the conversion.

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