# light, photons, irradiance **URL:** \ **Category:** Factual Questions\ **Created:** [September 19, 2011, 7:45am UTC](https://boards.straightdope.com/t/light-photons-irradiance/596801 "2011-09-19T07:45:13Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![kadimkris](https://avatars.discourse-cdn.com/v4/letter/k/b782af/32.png) [@kadimkris](https://boards.straightdope.com/u/kadimkris)\ **Post date:** [September 19, 2011, 7:45am UTC](https://boards.straightdope.com/t/light-photons-irradiance/596801/1 "2011-09-19T07:45:13Z")
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. cree xlam xr-c spec sheet [http://www.cree.com/products/pdf/XLampXR-C.pdf](http://www.cree.com/products/pdf/XLampXR-C.pdf) royal blue wavelength range 450-465 min radiant flux 300 mW (+/- 7%) 350mA @ 3.6 volts dc ( this = .35\*3.6= 1.225 watts ) red wavelenght 620-630 luminous flux 39.8 lm (+/- 7%) 700 mA @ 2.4 volts dc ( this = .7\*2.4= 1.68 watts ) ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [September 19, 2011, 1:43pm UTC](https://boards.straightdope.com/t/light-photons-irradiance/596801/2 "2011-09-19T13:43:39Z")
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 \*lumens lux phots stilbs candles candelas foot candles watts per square centimeter \* 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: > **[Radiometry](https://en.wikipedia.org/wiki/Radiometry)** > > Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye. The fundamental difference between radiometry and photometry is that radiometry gives the entire optical radiation spectrum, while photometry is limited to the visible spectrum. Radiometry is distinct f... > **[Radiance](https://en.wikipedia.org/wiki/Radiance)** > > In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiation, and to quantify emission of neutrinos and other particles. The SI unit of radiance is the watt per steradian per square metre (W·sr−1·m−2). It is a directional quantity: the radiance of a surface depends on the direction from which it is being observed... > **[Luminance](https://en.wikipedia.org/wiki/Luminance)** > > Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. > The procedure for conversion from spectral radiance to luminance is standardized by the CIE and ISO. > Brightness is the term for the subjective impression of the objective luminance measurement standard (see Objectivity (science) § O... 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_ ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 19, 2011, 1:44pm UTC](https://boards.straightdope.com/t/light-photons-irradiance/596801/3 "2011-09-19T13:44:40Z")
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. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [September 19, 2011, 4:48pm UTC](https://boards.straightdope.com/t/light-photons-irradiance/596801/4 "2011-09-19T16:48:28Z")
> [@kadimkris](#): > > I [want] to know how many photons there are in the given wavelength of “light” Wavelength affects the \*energy \*of individual photons, but not their _number_. This non-intuitive fact causes the [photoelectric effect](http://en.wikipedia.org/wiki/Photoelectric_effect), which Einstein gained early fame for explaining. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 19, 2011, 5:14pm UTC](https://boards.straightdope.com/t/light-photons-irradiance/596801/5 "2011-09-19T17:14:48Z")
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.