The approximate conversion is 1 Lux = 0.0079 W/m^2 for sunlight that has the peak at 500 nm.

https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg

The conversion depends on the wavelength but 470 nm is very close to 500 nm.

https://www.rapidtables.com/calc/light/how-watt-to-lux.html#meter

Dear Vanesa Fernández-Moreira , as pointed by Valentin Bogatu , there is not a direct conversion from lux to J/cm2, but there is some sort of equivalence to W/m2 or W/cm2.

These units are not really equivalent, because the lux is the SI derived unit of illuminance and W/m2 is the SI unit of irradiance.

The lux (illuminance) is a photometric unit, and it means it has into account the different sensitivity of the human eye to each wavelength of the visible spectrum (we don´t see equal red than blue light).

The W/m2 or W/cm2 (irradiance) is a radiometric unit, and it means that we measure the physical power of light with all the wavelengths weighted equally over a surface area (power density at a surface).

There is no simple conversion because one unit depends on the wavelength or colour of the light and the other is independent. The conversion provided by Valentin is for sunlight (white light with the Sun spectral distribution and our optical perception of each visible colour), so if your source is monochromatic at 470nm the conversion wouldn´t be accurate.

Your experiment looks like an irradiation experiment, and you would be interested on the dose of blue (470nm) light your sample will get with the conditions described (10 min at 7 cm from the source). So it would be beter to buy a radiometer than a lux-meter. Radiometers (like lux-meters) have a photosensor (a photodiode) that measures the power density by area. They are designed for a certain range of power densities and also they can be broadband or monochromatic (usually by means of a filter). This sensor can be placed on the instrument or it could be connected by a wire (or wireless) to the instrument. This instrument contains the electronics for reading the measures. It is also possible to connect the sensor directly to a laptop or pc, together with a software to read the measurements.

Some sensors can also be able to measure the total cummulative dose (J/m2).

Another thing to take into account when measuring the these parameters is the angle of incidence of the light. Imagine your light source is a LED, it emits light within a light cone, so that some light comes from the cone main axis and some light comes from some other direction (with an angle different of 90º). Sensors often has a diffuser to correct this effect. The sensors must be callibrated for the wavelength of interest if they are monochromatic or for the whole spectral range they can measure if polychromatic.

Simple versions just let you measure power density by unit area, and some more advanced versions also allow you to measure the total dose.

There are affordable and portable devices at prices in the same range of lux-meters. Some more advanced systems are proportionally more costly.

Please, have a look to the following short video with some clear descriptions and specifications for a radiometer that allows to measure both W/m2 and J/m2. It is a video from a manufacturer, there are other, just googling.

https://www.youtube.com/watch?v=7DrGNrO58eE

The specifications of sensor and reading unit can be find here:

https://www.ophiropt.com/laser--measurement/laser-power-energy-meters/products/LED-Measurement/Irradiance-and-Dosage-sensors/PD300RM-UV

https://www.ophiropt.com/laser--measurement/laser-power-energy-meters/products/laser-power-meters/starbright

Hope this helps. Good luck with your experiments.

Salu2

Dear Vanesa Fernández-Moreira

These links might help, have a look:

1. https://www.researchgate.net/post/How-can-we-calculate-UV-light-intensity

2. https://www.researchgate.net/post/How-can-I-calculate-the-UV-dose-in-J-m2

Kind Regards

Qamar Ul Islam

Is there any ratio between nm to Lux ?

Dear Vanesa Fernández-Moreira, as pointed by Valentin Bogatu and Manuel Gómez. Besides that, 1 W = 1 J/s.