> kaip perskaiciuoti ?

http://en.wikipedia.org/wiki/Lux#Relationship_between_illuminance_and_irradiance :
"There is no single conversion factor between lux and watt/metre2; there is a
different conversion factor for every wavelength, and it is not possible to make
a conversion unless one knows the spectral composition of the light.
    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."

   http://www.physlink.com/education/askexperts/ae409.cfm: yra formulė ir grubi
akies jautrumo kreivė -- atsidarius Paint'e, įmanoma rasti koeficientą 635 nm
bangos ilgiui. Sunku pasakyti, kiek procentų paklaidą gausi. Jei rastum tą
kreivę didesne raiška, būtų tiksliau.

  Taigi geriau jau pasiskolinti normalų liuksmetrą ir susikalibruoti.

-- 
  saimhe