Hi, I was wondering what’s the best way to achieve a high NA widefield illumination using a point source such as a fiber? From my understanding, if I place the point source in a conjugated image plane I don’t have widefield illumination. If I put it in a conjugated Fourier plane I have widefield illumination but a low NA as I don’t fill the back focal plane of the objective. Any way to overcome this? Thanks!!
Hi Ediz,
A point source is not the best source for widefield illumination for several reasons: the difficulty in achieving the combination of irradiated area and NA, as you mentioned; if the source is coherent, you may see speckle in your illumination pattern; and whatever you do to work around these drawbacks will lower your efficiency.
If you’re stuck with that source, you can try placing a diffuser in front of it and using the resulting scattering surface as your conjugate plane, or place your point source in an integrating sphere and use the output aperture of that as your conjugate plane.
Best,
Jasper
Hi Ediz,
Whatever you do with a point source, it does not have the etendue to give you what you seek. Large field, large NA is large etendue. If you want, etendue is the size of the ligh and a point source is by definition as small as it gets. Within the limits of the Lagrange invariant, you can transform between solid angle (NA) and spatial coverage but the product cannot ever increase. But it can get smaller if you crop it with an aperture.
What you seek if fundamentally difficult and is in principle governed by the light source. There are some work-arounds because the Largrange invariant is the result of Snell’s law so any combination of shperical surface (including flat) will not be of any help. Hence, the work-arounds use surfaces with various topoligies that break the spherical symmetry. One such class is diffusers, then there are various vibrating plates and so on. All have their optical limitations or consequences typically originating from coherence.
If you light source has a limited temporal coherence length, you can use that to increase your the etendue, but again with some (lets say less severe) optical limitations.
A light source that can give you large-NA illumination over a larger area is a discharge lamp, such as those used in DLP projectors while single-mode single-frequency lasers will be the opposite.
You can find more on theis optic in the Tech Talks at Blog — Senslogic - Optical System Design
Regards,
Jarek