Stop
A stop is an opaque plate with an opening: an iris, a slit, a double slit, a comb of slits, a knife edge, or the reverse, a beam block. It cuts the rays and the diffracted field alike, so diffraction from a slit or an edge comes out of the field.
The model
Shapes
The stop is a plate of its outer size, set across the beam. Its opening is one of:
| Shape | Opening |
|---|---|
| Iris | a round hole of the clear aperture’s diameter |
| Slit | a slit of the given width, and optionally a height out of the plane |
| Double slit | two slits, a separation apart, centre to centre |
| Slit comb | \(N\) slits a pitch apart |
| Knife edge | everything on one side of the edge |
| Beam block | an opaque disc, open round it |
An offset moves the opening, the slits, the edge or the block along the stop.
Rays
A ray through an opening carries on untouched. A ray striking a blade stops there. The panel gives the power the stop removes, as a share of what its sources emit.
The field
The diffracted field is multiplied by the stop’s transmission, 1 in the opening and 0 on the blades:
\[E_\text{out}(x, y) = t(x, y)\, E_\text{in}(x, y).\]The field after it then diffracts as it propagates, so a slit’s far field is the familiar \(\mathrm{sinc}^2\), a double slit’s has its fringes, and a knife edge shows its Fresnel ripples. An iris on a round beam keeps the field round. A slit, a knife edge or anything off-centre needs the 2D field.
Where an edge falls inside a grid sample, the sample takes the mean transmission over its footprint rather than the value at its centre. A hard edge is then anti-aliased, and the grid does not ring.
A Gaussian beam’s reference ray is not stopped by a blade: the beam’s power is cut as the field is, and a beam that lands half on the stop is split there, the part that passes carrying on (see Free-space optics: branches).
Settings
| Field | Default | Meaning |
|---|---|---|
aperture | 5 | mm: the opening (an iris’s diameter, a slit’s width). |
outer | 25.4 | mm: the outer size (a beam block’s diameter). |
shape | "round" | one of the stop shapes below. |
sep | 0.5 | mm: a double slit’s centre-to-centre separation. |
pitch | 0.2 | mm: a slit comb’s pitch. |
count | 5 | a comb’s number of slits, 2 to 400. |
offset | 0 | mm: the opening’s centre along the stop. |
height | 0 | mm: a slit’s height out of the plane (0 unlimited). |
side | 1 | 1 or −1: which side a knife edge covers. |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- The plate is infinitely thin and perfectly black: no edge diffraction from its thickness, no reflection from its blades.
- A slit’s height out of the plane applies to the field; rays stay in the plane, so they see only its width.