= Collimated light
\b[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.<Measurements with autocollimators by TRIOPTICS>[Autocollimator operating principle; illuminated reticle at the focal plane]{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> are
$$
\theta=\frac{\lambda}{\pi w_0},\qquad z_R=\frac{\pi w_0^2}{\lambda}.
$$
The 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.<Propagation of a Gaussian Beam Through a Thin Lens by Francesco D'Angelo>[Equations (1)–(5); 1/e2 intensity radius, propagation law and far-field half-angle]{ref}
For an original example, choose $\lambda=500\,\mathrm{nm}$ and $w_0=1\,\mathrm{mm}$. Then $\theta\simeq0.159\,\mathrm{mrad}$ and $z_R\simeq6.28\,\mathrm{m}$. 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.
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