Why not use built in support for lighting in Unity? Spot light would do exactly the effect you need. Unity 2D "mode" is a bit tricky conserning the lighting:
You'll need to add a material to your sprites for this which uses the
Sprites Diffuse shader. As standard, sprites don't support lighting as
we expect users to use them unlit and use lighting baked into the
sprite design itself.
Simply make a material, assign it to the sprite, and choose the
Sprites Diffuse shader from the material.
and also, from the linked thread:
Resurrecting this thread for anyone still looking for solutions for
the binary lighting thing, just figured it out myself. Two important
things to check:
1) Is the light's render mode set to "Important"? This should fix the
issues with scaled stuff.
2) Are you using scaling or rotation to flip
your sprites? Sprite renderers work like textured planes, they only
receive light on one side. If you rotate the characters to mirror
their direction then the renderer is suddenly facing away from the
light, but if you just scale the object by -1 you can flip the
character without flipping the renderer's normal.