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Answer Posted / ravikishore
Lambert's cosine law says that the total radiant power
observed from a "Lambertian" surface is directly
proportional to the cosine of the angle θ between the
observer's line of sight and the surface normal. The law is
also known as the cosine emission law or Lambert's emission
law. It is named after Johann Heinrich Lambert, from his
Photometria, published in 1760.
An important consequence of Lambert's cosine law is that
when an area element on the surface is viewed from any
angle, it has the same radiance. This means, for example,
that to the human eye it has the same apparent brightness
(or luminance). It has the same radiance because although
the emitted power from an area element is reduced by the
cosine of the emission angle, the observed size (solid
angle) of the area element is also reduced by that same
amount, so that while the area element appears smaller, its
radiance is the same. For example, in the visible spectrum,
the Sun is almost a Lambertian radiator, and as a result
the brightness of the Sun is almost the same everywhere on
an image of the solar disk. Also, a black body is a perfect
Lambertian radiator.
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