Collimated light

Collimated light is light whose propagation directions occupy a narrow angular range. In geometrical optics, ideal collimation is represented by parallel light rays. A collimator approximates this by placing a small source at the focal position of a lens. A reticle projected by an autocollimator provides a practical example: the outgoing pattern travels as an approximately collimated beam before reflection from a test mirror.^[ref]

A finite beam still undergoes diffraction. For an ideal Gaussian beam in free space, let $w_0$ be its beam waist radius, measured where the intensity has fallen to $1/e^2$ of its axial value, and let $\lambda$ be its wavelength. Its far-field beam divergence half-angle and Rayleigh range areThe radius at distance $z$ from the beam waist is $w(z)=w_0\sqrt{1+(z/z_R{)}^2}$. Thus small beam divergence can make a beam nearly constant in width over a useful distance without making it perfectly parallel everywhere.^[ref]

For an original example, choose $\lambda =500\mathrm{nm}$ and $w_0=1\mathrm{mm}$. Then $\theta \simeq 0.159\mathrm{mrad}$ and $z_R\simeq 6.28\mathrm{m}$. At $z=z_R$ the radius is $\sqrt{2}\mathrm{mm}$, so even this small-divergence beam eventually widens. Collimation describes propagation directions; optical coherence describes phase correlations and is a separate property.