Converting Lux to W/m^2 for Solar Radiation Measurement

[SilverSparrow]

TL;DR

Need help to convert Lux into W/m^2 (is it even possible?).

If not, how else can I measure intensity of light which would give me a result in W/m^2?

For a physics project, I'm planning to investigate the relationship between the number of slits in a diffraction grating and the intensity of the central maxima. The light meter which I'm planning to use to measure the intensity gives me a result in LUX.

I know the wavelength of the red laser light which I'm using is approximately 550 nm. I'm wondering if it's at all possible to use a conversion to change the lux value in W/m^2?

If not, is there any other way I could possibly measure the intensity of the central maxima that would give me a result in w/m^2?

Thank you so much.

 

[Vanadium 50]

What makes you think there is such a conversion? If I had a zillion watts per m2 of IR or UV on a surface, I'd still have zero lumens (and therefore lux).

 

[hutchphd]

What makes you think there is such a conversion? If I had a zillion watts per m2 of IR or UV on a surface, I'd still have zero lumens (and therefore lux).

The OP says the light in question is 550nm so a conversion is reasonable.

 

[berkeman]

The OP says the light in question is 550nm so a conversion is reasonable.

https://en.wikipedia.org/wiki/Luminosity_function

 

[Vanadium 50]

The OP says the light in question is 550nm

Ah...missed that.

 

[hutchphd]

Summary:: Need help to convert Lux into W/m^2 (is it even possible?).I know the wavelength of the red laser light which I'm using is approximately 550 nm. I'm wondering if it's at all possible to use a conversion to change the lux value in W/m^2?

One needs to include the value of the photometric curve at (eye sensitivity) at 650nm. I point out that 550nm light is not red! I assume you mean 650 ??

 

[jtbell]

I know the wavelength of the red laser light which I'm using is approximately 550 nm. I'm wondering if it's at all possible to use a conversion to change the lux value in W/m^2?

If the wavelength stays the same, the conversion is a constant factor, therefore either lux or W/m² should be suitable for comparing different intensity measurements. Why do you specifically need W/m²?

 

[anorlunda]

I thought I could help the OP using information from the Wiki article linked by @berkeman . It says

Aha, that number 683.002 looks like a luminosity to power conversion factor, so I googled that. LOL, The Internet sometimes takes us in strange directions. Here is the results I got for 683.002.

• Texas Transportation Code § 683.002 | FindLaw
• American Standard 1660.683.002 10-Inch Modern Rain Easy Clean Showerhead,
• Docket Nos. ER03-683-002 and ER03-683-003 2 Notice of Filings and Responsive
• Mirror, Chest. 683-002 Dresser
• Sec. 683.002. ABANDONED MOTOR VEHICLE
• But it finally found the right one.

Lux - Wikipedia

https://en.wikipedia.org/wiki/Lux
For monochromatic light of this wavelength, the amount of illuminance for a given amount of irradiance is maximum: 683.002 lux per 1 W/m 2; the irradiance needed to make 1 lux at this wavelength is about 1.464 mW/m 2. Other wavelengths of visible light produce fewer lux per watt-per-meter-squared.

 

[davenn]

Summary:: Need help to convert Lux into W/m^2 (is it even possible?).

If not, how else can I measure intensity of light which would give me a result in W/m^2?

I know the wavelength of the red laser light which I'm using is approximately 550 nm.

I seriously doubt it, 550nm is in the green part of the spectrum. Shorter wavelength than red
Did you really mean to say "my green laser" ?

Red lasers are usually around the 633nm

 

[mamut]

The peak of the luminosity function is at 555 nm (green); the eye’s visual system is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the irradiance needed to make one lux is minimum, at 1.464 mW/m2.

That is, one obtains 683.002 lux per W/m2 (or lumens per watt) at this wavelength. Other wavelengths of visible light produce fewer lumens per watt.

Solar Radiation is a shorter wave length and different color spectrum, so it has a different factor of Lux to W/m2:

Bright sunlight is approximately 136000 lux = 1075 W/m2 = 126.7

The factor changes slightly depending on time of day, conditions such as cloud cover, moisture in the air etc. and most sources agree on the 126.7 as a reasonable factor to use.

The conversion in the green visible light spectrum spectrum at 555nm is 1W/m2 = 683 lux, and is used of human eyes.

Some of our weather stations allow to calibrate the gain to whatever light spectrum you wish to base solar radiation on.