RectLight Intensity Distribution and IES profile

Does the light intensity distribution of a RectLight conform to Lambert’s cosine law?

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When I add the IES profile for the rectlight , the file records the absolute light intensity at each angle. However, the engine multiplies the cosine value when calculating the light intensity at each angle, which causes me to measure the light intensity distribution incorrectly.

Is this an algorithmic problem or am I misunderstanding.