Dear all,

Lamp spectra measured with a spectroradiometer produce the physical spectral power distribution. In case one wants to measure the effectiveness of a lamp for a particular photochemical or photobiological process, one can apply a so-called action spectrum as a spectral weighting function upon the physical spectrum. If the phenomenon of interest is e.g. human vision then the human eye spectral response curve is applied as a weighting function and the result is expressed in LUX. By definition a good lux meter hence CANNOT see infrared or ultraviolet light, because a good lux meter only responds to visible light. A meter for PAR heavily takes into account the spectral regions that contribute to photosynthesis and disregards the rest. A dermatologist that wants to dose UV-therapy uses a radiometer with the standard erythema action spectrum, etc.

With a well calibrated spectroradiometer and a library of specific action spectra, one can derive all these specific responses like LUX, PAR, erythema, etc. Unfortunately a good spectroradiometer is expensive, which is why there are many field portable meters using a wide band detector in combination with spectral filters to shape the various desired response curves for various purposes. As long as one uses the right light probe for the right purpose, that is a very practical approach in daily routine use.

Within the EN 12455-series of standards pretty detailed information can be found on how to perform reliable optical exposure measurements for specific photobiological processes. And for absolute beginners I recommend downloading the excellent (and free) Light Measurement Handbook from Alex Ryer.

I am afraid that in your case they are not convertable. There are, however, some empirical conversions between solar radiation and PAR, but even those are not very accurate. For example it is estimated that in Europe PAR = 0.48 x Solar radiation and in North America PAR = 0.45 x Solar Radiation.

It is generally accepted that: 1800 micromols m-2 s-1 ≡ 427 W m-2 but this equation was estimated for sunlight. In your case, working with a xenon lamp, I dont think this general equation can be applied since the percentage of PAR (Photosynthetically active radiation) from the total is different than that percentage for sunlight.

Lux is defined as the power equivalent to the human visual sensitivity (the "photopic" sensitivity of color vision) peaking at 555 nm. You would need a spectroradiometer to resolve the actual power distribution in (W/m2)/nm, or a chemical actinometer to measure the effective part of the spectrum specifically, if the actinometer suits for this spectral region (a Xe lamp usually emits light from the UV into the infrared).

To support the previous comments, lux meters measure only the total irradiance in the visible part of the spectrum whereas a 'light meter' may be similar or be able to measure accross the visible and some of the UV as well depending on its construction/ optics. Strictly speaking it is only possible to use a lux meter to measure the power of a similar source to that against which it was calibrated (though this is often ignored).

It is not possible to convert lux to W/m2 unless you know the calibration curve of the meter - as stated above it is however often possible to gain an approximation. Even so the true power from a Xenon lamp must include the UV and IR components which a lux meter will not give you.

A final thought - if you contact the makeres of the Xenon lamp they may well be able to tell you what the power output is both in total and as a spectral power distribution

A follow up question for you: What is the purpose of the light?

If for plant growth, the quantum sensor (PAR meter) will provide what you are most likely looking to find. In general, if you have a quantum meter, or 'PAR meter', it will give a response that tells you the number of photons reaching the sensor for the range of 400 - 700 nm. Of course, it partly depends on the type of meter you have. For example, I have an Apogee quantum sensor that is calibrated for both sun and electric lamps. Depending on how the meter is calibrated (or in my case the setting), the readout may be high or low by as much as 10%. If you are looking to get a measurement for the purpose of human vision or lighting a room, the PAR meter may give you a different (perhaps slightly elevated) reading because it will be looking more at the total incident radiation in that waveband hitting the surface. Even though PAR is almost the same as human vision, I believe a 'lux meter' will give you a readout that is tuned specifically to the average human eye and will not represent the total radiation hitting the sensor. Again, this is where it is important to define the purpose of using the light.

For converting to W/m2, it also depends on your light source. For example a cool-white fluorescent will emits different amounts of energy at different wave lengths compared to a warm-white fluorescent. Some energy will also be emitted as unusable wavelengths and heat. The same is will be true of your xenon lamp. Consider referring to the following paper for a table to help you convert specific to your situation (although xenon is not featured):

Thimijan, Richard W., and Royal D. Heins. "Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion." HortScience 18.6 (1983): 818-822.

Hope this helps.

There are two terms used to measure lighting intensity that should be distinguished based on their definition.

First, Photometric :which measured in lumens per square meter (lm/m2), or lux (lx),

Second , radiometric, which refers to measurement of radiant energy.

In the other word ,lumens are units of luminous flux while Watts are units of radiant flux and they are not exchangeable quantities .

Yep, you should be very careful. It is all about spectral efficiency of the combination of source and detector. If you go the "photometric/lux" way your quantities are modulated by the spectral response of the eye (only thatof the eye, and in fact, only of the daytime eye). So you need be aware you are speaking of an integrated quantity modulated by the detectors response. Some manufacturers even refer "pseudo-photometric" data integrating the response of their detector, even if made of silicon, and using photometric units.

But to me the "PAR region" is an unknown as "detector". So UV and IR bands may be within... or not. Get the Xenon lamp spectral emission data, get the detector (or region) you need to study and simply integrate the emission multiplied by the detector response. You'll get candelas, strictly, if using the daytime eye as detector.

By the way be careful with words in photometry! Intensity may only be applied to point sources, as it should be defined as power per solid angle emitted... Photometry/radiometry are tricky disciplines: everythng is very well defined but you need exactly to now what you speak about!!

Dear I faced same problem during my research work. It is true that lux represents only brightness not light intensity. So you to proceed with solari meter or not possible then mail me at [email protected]

Dear, for measuring the light intensity of my electrodeless discharge lamps (Hg, Cd, S, et.) I am using the USB2000 (Ocean Optics) apparatus together with the calibrating source LS-1-CAL (Ocean Optics) and SpectraSuite software.

Download the free PDF version of Alex Ryer's Light Measurement Handbook, URL : www.intl-light.com/customer/handbook/handbook

I found link to convert LUX to W/m2 @ specific wavelength.

http://www.cactus2000.de/de/unit/masslux.shtml

Lumens measures the intensity of light to the human eye, but not necessarily the spectrum of the light. Photosyntethically Active Radiation (PAR) meters measure light intensity within the spectrum that plants use most to photosynthesize (excluding UV light and near IR) i.e a solar spectral window from 400 to 700 nm. nanometers, which doesnt cover near UV of 700 red spectra(near IR). A Xenon Arc Lamp has a Spectral characteristics of Emittance Light.The Spectral Emittance is wavelength dependent.The Lamp Emittance Spectrum has sections of higher intensities and lower intensities.Light intensities have a physical meaning and values only for the Emittance Spectral Bandwith of the Lamp beeing dependent on lamp size,manufacturer and lamp`s age.For a narrow spectral bandwidth using a broad interference filter(e.g.500-550 nm) a PAR meter can be used as indicative ,for comparison experiments. If accurate values are necessary as for example quantum yields it`s better to use a photochemical actinometer which gives accurate light intensities from quantum yields measurement.In all other cases it`s enough to use the values given by the Xenon Lamp manufacturer!These values when given are quite accurate in watts per meter square,see for examples in attachements here.

All data necesarry for Xenon lamps as for in this PDF attachements.It`s best to use manufacturer`s values in watts per square meter.

Look on internet for the "Light measurement handbook" written by Alex Ryer

The Lux Meters are measuring in Photometric Units in Lux(es),1Lux =1 Lumen per square meter.That`s a system based on a definition of the human eye sensitivity to visible spectrum.Light Meters measure Light Intensity in energy units,in power per square area unit,e.g.:watts per square meters,therefore it is an absolute energy based system.There is no simple correlation between the photometric units and the energy units.While the so called the luminous function, pretends to correlate between the two,the Lumen and Lux units are human eye wavelength dependent,(of visible light energy)and the energy system is per wavelength(any of,including UV ,VIS, IR,etc.).In photometry and radiometry intensity has a different meaning,while it is the luminous or radiant power per unit solid angle (this is mainly intended for antenna flux radiance measurements).These differences can cause all kind of confusions and bewilderments.

Therefore for scientific Light Intensity measurements the Energy Units System is recommended;the Lux Meters and the Lumen photometric units system is recommended for Lighting Measurements as for Lighting in rooms,houses etc.,it has an "environmental engineering" meaning nowadays.

A bit old topic, but...

I strictly agree with Vladimir - only spectral measurements in wide wavelength range will let you transform between photometry and radiometry with any curve you need (I mean Vlambda, PAR or any other). Of course there are some specific problems with that.

I also strictly agree with Santiago Royo - ligh technology (in sense photometry) is very tricky with names and definitions.

Ankur - lux does not represent brightness - it represents only the luminous flux on the surface - brightness as we see it is represented by luminance and you have to use completely different meter to measure it. To connect brightness with the illumninance you need to know the reflection characteristcs (e.g. spectral BRDF) of the material that you look at.

Finally, although Yaacov Harel gave a nice general resume, I can't totally agree that photometric quantities have only the enviromental engineering meaning nowadays - it seems to be not very deep conlcusion and of course depends on the point of view. Who cares watts on the road (automotive lighting) or in general in lighting (interior, exterior etc.). Of course from the point of energy efficiency - all of us. But you can't forget that the main goal of lighting is to let us see better and we can't see watts - we can see lumens/(m2*sr). Therefore in this part of science watts are not recomended. Photometry is a specific area of science relating everything to the human eye, so detector has to obey strict rules and there are still so many scientific problems to solve.

Dear,photometric quantities can have only technological use.What part of Science can use lumens?!Astronomy etc..anything else it`s Applied Engineering and/or Photography.The Lumen unit is based specifically on Lambda=555nm as a mean value for human eye vision spectrum(specifically green part).Therefore It is certainly not recommended for use in photochemistry or photophysics experiments as e.g .:in calibrating instruments for light intensity measurements and/or in experiments as for calibrating an arc lamp as a Light Source as in Question asked.Please note also Fokko Wieringa reference above for using right the systems units.

Of course lumens are not useful for PAR region! This is why I wrote the only solution is to use spectral instruments adding that conversion between watts and lumens is also possible (but of course only between 380 and 780 nm - it is obvious).

As I said everything depends on the point of view and it comes from the discipline you work with. In some of them V(lambda) is NOT needed, but in some of them it is a necessity. This is what the Alex Ryer handbook is also about (he cares lumens) - I know this material very well also from other, much wider sources, but it is only a basic knowledge of lighting. That is why I was joking that noone on the road (applied engineering) cares watts - we can't see them (once again - human beeing can see lm/(sr*m2) which is the unit of luminance), but in other applications noone cares lumens, especially if she/he works with UV or IR. Oh, you asked who else can use photometric units - medicine (eye), ergonomy etc - everytime someone has to relate to human visual system.

Greetings!

Greetings to you too!Except of what is written above,Kevin Yong didn`t comment further anyhthing else about the type of his experiments.Of course the use of photometric units system is worlwide recognized,especially in photography and lighting.So the use of the unit system depends on what one intends to do and on the accuracy necessary.I have explained in details the diferrence between the common use and the scientific use.It was not meant to argue with anyone`s particular preference.After all in a manual,like in the above mentioned handbook there are conversion charts for the units and systems.The main point I adressed was that Science prefers the use of Energy units because of Lambda dependence and accuracy,and that`s allways a true case. Scientifically speaking One can not average output of a Lamp according to Lambda=555nm but for his own practical reasons He might be willing to do so.

For example if One wants to use the Xenon Lamp Light for Photography,why not!

Specifically for PAR Instruments accuracy pl see attachement here.

Be very carfull with the spectral response of the device. if the device do not have the photopic spectral response you can make huge errors on the conversion from lumens to W/m2. Check this paper: https://academic.oup.com/mnras/article/467/3/2966/3061573

Dear All

I am using Visible LED of a very high intensity about 311300 lux. I want to convert this value to a lower value by converting lux to other unit. Could you please help me in that case.

Regards,

Dear all,

Lamp spectra measured with a spectroradiometer produce the physical spectral power distribution. In case one wants to measure the effectiveness of a lamp for a particular photochemical or photobiological process, one can apply a so-called action spectrum as a spectral weighting function upon the physical spectrum. If the phenomenon of interest is e.g. human vision then the human eye spectral response curve is applied as a weighting function and the result is expressed in LUX. By definition a good lux meter hence CANNOT see infrared or ultraviolet light, because a good lux meter only responds to visible light. A meter for PAR heavily takes into account the spectral regions that contribute to photosynthesis and disregards the rest. A dermatologist that wants to dose UV-therapy uses a radiometer with the standard erythema action spectrum, etc.

With a well calibrated spectroradiometer and a library of specific action spectra, one can derive all these specific responses like LUX, PAR, erythema, etc. Unfortunately a good spectroradiometer is expensive, which is why there are many field portable meters using a wide band detector in combination with spectral filters to shape the various desired response curves for various purposes. As long as one uses the right light probe for the right purpose, that is a very practical approach in daily routine use.

Within the EN 12455-series of standards pretty detailed information can be found on how to perform reliable optical exposure measurements for specific photobiological processes. And for absolute beginners I recommend downloading the excellent (and free) Light Measurement Handbook from Alex Ryer.