Lambert's law, the law according to which the luminance L of a light-scattering (diffuse) surface is the same in all directions. It was formulated in 1760 by the German scientist J. Lambert. Simple relations between photometric quantities follow from the definition of Lambert's law: between the luminous emittance M and the luminance L: M=πL; and between the luminous intensity of a flat scattering surface along the perpendicular to it (I0) and at an angle θ(Iθ): Iθ=I0*cosθ. The last expression means that the luminous intensity of such a surface is greatest along the perpendicular to it and, falling off as θ increases, becomes zero in directions tangential to the surface.
In reality only a few real bodies scatter light without substantial departures from Lambert's law even in the visible region of the spectrum. They include surfaces covered with magnesium oxide or barium sulphate, and gypsum; among turbid media, opal glass and some types of cloud; among self-luminous emitters, the black body and powdered phosphors. Lambert's law nevertheless finds wide use not only in theoretical work, as a model of ideal light scattering, but also for approximate photometric and lighting-engineering calculations.
In reality only a few real bodies scatter light without substantial departures from Lambert's law even in the visible region of the spectrum. They include surfaces covered with magnesium oxide or barium sulphate, and gypsum; among turbid media, opal glass and some types of cloud; among self-luminous emitters, the black body and powdered phosphors. Lambert's law nevertheless finds wide use not only in theoretical work, as a model of ideal light scattering, but also for approximate photometric and lighting-engineering calculations.