Chromatic Focal Shift

The chromatic focal shift is the position of the paraxial focus as a function of wavelength, measured from the focus at the primary wavelength. For each wavelength, a paraxial marginal ray is traced through the system, and its focus is found where it crosses the axis in image space.

from optiland.analysis import ChromaticFocalShift
from optiland.samples.simple import CementedAchromat

lens = CementedAchromat()

lens.draw()

(<Figure size 1000x400 with 1 Axes>, <Axes: xlabel='Z [mm]', ylabel='Y [mm]'>)

By default the curve spans the optic’s defined wavelengths, here the F, d and C lines. Pass wavelength_range (in µm) to plot a different range.

cfs = ChromaticFocalShift(lens)
cfs.view()

(<Figure size 800x550 with 1 Axes>,
 <Axes: title={'center': 'Chromatic Focal Shift\nMaximum focal shift: 21.7711 µm'}, xlabel='Wavelength (µm)', ylabel='Focal Shift (µm)'>)

How to read the plot:

• A positive shift means the focus at that wavelength lies farther along the optical axis than the focus at the primary wavelength.

• The markers are the optic’s defined wavelengths; the curve is zero at the primary wavelength.

• The vertical distance between two points of the curve is the axial separation of their foci. Wavelengths at the same height on the curve focus at the same place.

• The maximum focal shift, shown in the title, is the full range of the curve: the spread of the foci over the plotted wavelengths.

This achromat has a minimum near 0.52 µm: wavelengths on either side of it, at equal heights, are brought to a common focus. The F and C foci are not equal in this design; the C focus is about 19 µm beyond the F focus.

print(f"Maximum focal shift: {cfs.max_focal_shift() * 1e3:.2f} µm")

Maximum focal shift: 21.77 µm

cfs = ChromaticFocalShift(lens, wavelength_range=(0.45, 0.75))
cfs.view()

(<Figure size 800x550 with 1 Axes>,
 <Axes: title={'center': 'Chromatic Focal Shift\nMaximum focal shift: 44.5301 µm'}, xlabel='Wavelength (µm)', ylabel='Focal Shift (µm)'>)