How to control diffusivity light of rectangular light

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I need a rectangular light with no diffusivity.
Like this

344247-requiredlight.png

But default rectangular has some diffusivity like this.

344248-defaultlight.png
344248-defaultlight.png1152×648 3.05 KB

So could you tell me how to control diffusivity light of rectangular light or make a rectangular light with no diffusivity.