This method will work with the Point and Spot lamp types - but the Spot will also be limited to a circular spot (in the direction of the lamp) so the Point lamp gives more control since you can simply provide whatever image you desire.
Replace the Image with an image of a plain square - or any other shape - for your desired result. The X and Y components are adjusted to remove the 'bending' caused by the spherical Normal coordinates (otherwise you get distortion of the image at the corners) before being passed through a second Mapping node (to adjust the aspect ratio of the image) before being passed to the Image node for the texture - simply apply your own image in the Image node. Here the Geometry Normal is passed through a Mapping node to rotate it into the desired direction before being split into its X,Y,Z components. The Geometry node Normal can be used to control the profile of the light emitted by a lamp and this can be used with an image texture to give full control of the lamp output.
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