Lambert's Law

Lambert's law [′lam·bərts ‚lȯ]

(optics)

The law that the illumination of a surface by a light ray varies as the cosine of the angle of incidence between the normal to the surface and the incident ray.

The law that the luminous intensity in a given direction radiated or reflected by a perfectly diffusing plane surface varies as the cosine of the angle between that direction and the normal to the surface.

(analytical chemistry)

Bouguer-Lambert law

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Lambert’s Law 

A law according to which the luminance L of a light-scattering (diffusing) surface is the same in all directions; formulated in 1760 by J. Lambert. Today this law is considered to be a model of idealized diffusion of light, convenient for theoretical studies.

It follows from Lambert’s law that there is a constant relation between the luminosity M and luminance: M =π L. It also follows that the luminous intensity radiated by the plane scattering area Δ5 in any direction depends on the angle α between the direction and a line perpendicular to ΔS: I_α = I ₀ cos α.

This expression signifies that the luminous intensity of a plane surface is a maximum (I₀) along the perpendicular to that surface and, decreasing with increasing a, becomes equal to zero in a direction tangential to the surface.

In reality, only a few real bodies diffuse light without major deviations from Lambert’s law, even within the visible spectrum. Among such bodies are the dull surfaces of gypsum, magnesium oxide, and barium sulfate; some types of clouds and of milky glass among turbid media; and ideal black bodies and powdery luminophors among luminescent emitters. Nevertheless, Lambert’s law finds application not only in theoretical works but also for approximate photometric and illumination calculations.

REFERENCES

Gurevich, M. M. Vvedenie v fotometriiu. Leningrad, 1968.
Sapozhnikov, P. A. Teoreticheskaia fotometriia. Leningrad, 1967.

D. N. LAZAREV