Role of the Pinhole in a Confocal Microscope

Why is a pinhole required?

The pinhole is the defining feature of a Confocal Raman Microscope, providing major advantages in spatial resolution and imaging contrast over a conventional optical microscope.

Why is a pinhole required?

When laser light is focused into a sample by a Raman microscope, a three-dimensional excitation volume is formed within the sample. This volume is known as the excitation point spread function (PSF) and a representation of its shape is shown in Figure 1a. As the light converges towards the focal point, the diameter of the PSF decreases, reaching a diffraction limited minimum diameter at the focal plane followed by an increase in diameter as the light diverges. The exact shape of the PSF is heavily dependent on the optical properties of the sample such as opacity at the excitation wavelength, refractive index, and how it scatters the incident excitation. Raman scattering will occur throughout the entire excitation volume and simply measuring the Raman spectrum from this excitation would, therefore, result in spectral contributions from the entire volume with limited axial (Z-axis) discrimination.