can we measure luminance in cd/ m² with a lux meter? I measured illuminance of 35 lux. and since cd = lux*distance² I took some distance of 30 cm and got 3.15 cd. I know that my led-grid surface is 100cm² then I deduce that the luminance is 315cd/m² is my idea correct? if yes which surface should I take into consideration the led-grid surface (as I have done) or the surface of lux meter sensor
In the right situation luminance can be converted to illuminance. But I don't think you have the right conditions... the light source has to be diffused/integrated (such as with an integration sphere) and the only source of light.
In that case the formula for cd/m² is Luminance = (illuminance x reflectance)/π
You might be better off using a camera and converting the exposure value to the luminance value. https://www.ee.ryerson.ca/~phiscock/astronomy/light-pollution/luminance-notes-2.pdf