rad

for a circle of radius r is ‘1 rad’ the angle corresponding to length r on the circle,

say
$1\,\mathrm{rad} = \frac {360^\circ} {2 pi} = \frac {180^\circ} {\pi} \approx 57{,}29577951^\circ$

accordingly :
$1\,\mathrm{mrad} = \frac {1 rad} {1000}$ :

$1\,\mathrm{mrad} = \frac {360^\circ} {2000 \pi} = \frac {180^\circ} {1000 \pi} \approx 0{,}05729577951^\circ$

Randlichtabfall

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Ray

Light rays indicate the direction of energy propagation and are normal to the wavefront surfaces.

In a homogenous medium light rays are straight.

Rayleigh Criterion

The diameter of the smallest disk that a lens can produce as image of a point size object is called Airy-disk

According to the so called “Rayleigh Criterion” holds:

The smallest possible Airy-Disk that a (diffraction limited) lens can generate is

$D = 2 * 1.22 * Wavelength * F\#$

The best possible resolution on image side (at 20% contrast) is the radius of this smallest possible Airy-Disk

$R = 1.22 * Wavelength * F\#$

When you double the F#, you lose factor 2 of resolution in each x and y direction.
If the lens supported 5 Megapixel before, then it supports only 1.3 Megapixel after

When you double the wavelength (for example 850nm IR instead of 420nm blue), you lose factor 2 of resolution in each x and y direction. If the lens supported 5 Megapixel before, then it supports only 1.3 Megapixel after

See F-number

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rad

Randlichtabfall

Ray

Rayleigh Criterion