So I’m not too sure what I’m doing here. I what to know how many photons there are in the given wavelength of “light”. I have been digging for about two weeks in spare time. I am learning about plants and lighting and trying to make a project to build a light tailored to plants needs. At the moment I need to find out how to get total about of photons that are produced by a given light.
Terms I know “of”:
mole (unit)
einstein ( unit)
Irradiance
"PAR"and “PUR”
plus some others
I have some background in electronics and engineering as this is what i am going to school for. just two days ago I learned the differences between optics for the human eye and measurements and true power are different . lumens, candle, etc vs photons etc. I am wanting to learn more about light and need steered in the right direction .
the issue at hand is that spec sheets have ton of info but most are geared towared the human eye can we take the info they give use and come up with irradiance in photos or einsteins. As i learn about botany (hobby) I have learned they have done test to show that this is the max amount of light a plant can use in this wavelength.
sorry for the long post and thanks for your time. here is a sample of a few lights i am looking at atm. note one is in unit lm and other mW.
Congratulations! You are embarking on the study of irradiance and illuminance, which has more bizarre-sounding and confusing terms than any other branch of physics. You have
and so on. Don’t say them fast, or you’re liable to say some dirty words.
The subject is potentially confusing, and the terminology is haphazard, but the basic ideas are straightforward. irradiance deals with the flow of light of all wavelengths, while illuminance is concerned with light visible to the human eye. The latter are important to lighting engineers, but anyone dealing with optics problems or lighting of growing plants will be oncerned with the former.
All of this seems complex because you are dealing with the flow of electromagnetic radiation from one surface to another, over some given distance. You want to know about the flow through some solid angle, from a patch of a certain area, at a given angle, that is received by a patch of another given area a specified distance away, tilted at yet a different angle relative to the line joining the two surfaces. At, one might add, a particular wavelength. Since the ebergy of a photon varies with the wavelength, the energy carried by a given number of photons will vary with wavelength.
This is clearly a bit too long to be covered in a post, and it’s clumsy, when you consider that geometry is involved, and this is a rotten waty to draw pictures. You should get yourself a good book on radiometry. Wikipedia is good as a reference, but not the greatest for learning the topic:
Some good books are
Klein and Furtak’s Optics
Philip Baumeister’s sadly out-of-print Radiometria y Fotometria
Robert W. Boyd’s Radiometry and the Detection of Optical Radiation
or any of the books titled something like Introduction to Radiometry
Assume that the light source is monochromatic (it’s close).
The energy per photon is:
hv (v = greek nu) or hc/^ (^ = greek lambda)
Then, you simply need to figure out how much energy is being emitted (use the power dissipation of the LED * efficiency) and divide by the energy / photon.
Why you are doing this - well that’s a question I can’t answer.
Wavelength affects the *energy *of individual photons, but not their number. This non-intuitive fact causes the photoelectric effect, which Einstein gained early fame for explaining.
I don’t know how nonintuitive that is. Everyone knows that two light sources can be different brightnesses, despite being the same color. A halogen light bulb produces roughly the same mix of wavelengths that the Sun does, but I think it’s perfectly intuitive (and correct) that the Sun produces a lot more photons than the halogen bulb does.