One of the biggest challenges when working with multi-color systems is dealing with chromatic aberrations. We will explore why reflective optics can be an asset when designing the excitation path of a fluorescence microscope.
Broadly speaking, aberrations are the deviation from what is expected from an optical system. For example, chromatic aberrations happen when different wavelengths are focused at different positions along the optical axis.
People used to commercial objectives, perhaps are familiar with terms like apochromatic, which means the system corrects both spherical and chromatic aberrations.
However, the effort that is placed in objectives is hard to replicate along the entire optical path. It would become economically unfeasible to work exclusively with apochromatic lenses all over a microscope.
Chromatic aberrations are the consequence of using elements with slightly different refractive indexes at different wavelengths. Glass, the material most lenses are made with, shows that behavior. Achromatic lenses use different types of elements to compensate the shift in specific wavelengths, but they add more surfaces that reflect light, and get bulkier.
Reflective optics have the advantage of not having a refractive index to worry about. A parabolic mirror focuses all the wavelengths in the same spot. Mirrors do not suffer from chromatic or spherical aberrations in multi-color applications. That is why they are mostly used to collimate point-sources, such as light coming out of an optical fiber, or to tightly focus collimated light.
Cruzaโs Neo1 was built using reflective optics. That is what allows it to be compact and to display very limited chromatic aberrations. The optical layout is better described in the paper “Compact and reflective light-sheet microscopy for long-term imaging of living embryos” by Moretti et al.
