In PhotoBioMedicine the units used are dose in J/m2 or irradiance in W/m2. But in the electrical it seems to be Illuminance and lux/m2 or lumen. I am wanting to measure the received dose from a Panel of 640nm and 850nm LED's. It will need to be measured at a precise distance, I understand, as it is divergent light.
Is there a simple light measuring tool to give me a reading in W/m2?
Is there a calculation to convert lumen/lux to W/m2 (at the specific wavelength)?
I am not looking for a calculation for lumen produced for a given watt of energy input.
Thanks for your help as I am getting out of my usual area.
there is no such conversion since Lux and Lumen are based on the spectral response of the eye. Which means that Lux and Lumen does not exists in UV or infrared. However you could define W in all the wavelength.
Another difference is that Lumen is based on solid angle. So transforming Lux which both depend of the Solid angle and the surface (Lux is Lumen/m2) in W/m2 should consider a constant shape of the luminous flux.
In this case and for Green-yellow peak, you could consider that 683 Lux correspond to 1W/m2 , as example going to red where the eye is less sensitive, 1W/m2 could correspond to much less lux, around 50 lux
Look on google for the light measurement guide written by Alex Ryer.
Thanks - yes been Googling a lot. Fortunately also found irradiance meters/pyranometers that look to do the job I need. Alex book is awesome but did not really have the info I needed.
Sensitivity of the human eye (giangrandi.org) please find in the link the exact transfer between Lumen and Watt at specific wavelength. So if you use a Luxmeter on the surface, you could via this table get the Watt equivalent
Dear Jayson,
Complicated but important question! All quantities related to the optical power using the unit [W = Watt], like the irradiance [W/m2] are radiative quantities and thus proportional to the number of emitted photons emitted by a surface or perceived by a surface. The area dose [J/m2] is the irradiance multiplied by the irradiation time. The integral power of a light/radiation source can be determined by an Ulbricht sphere and is expressed in W.
The illuminance is a photometric quantity and thus dependent on the spectral sensitivity function of the receiver, here the human eye. To this end, one need the eyes sensitivity curve, which is dependent on the illumination level. However, the illuminance [lux = lm/m2] is the visible light which illuminates a surface. For the calculation, you need the emission spectrum of the light source and the radiative flux. An example: A single LED with 1 W optical power and an emission spectrum with a luminous efficacy of 300 lm/W has a luminous flux of 300 lm, which is an integral quantity! If this LED illuminates an area of 1 m2 one obtains 300 lm/m2, which is 300 lux. (Remark: The angle dependent luminous intensity is expressed in Candela [Cd].
In other words, you cannot convert radiative quantities such as W/m2 or J/m2 without the knowledge of the emission spectrum into photometric quantities. More can be found in my lectures, which I add to this answer.
All the best for your research!
Thomas
- Badges
- Science topic