There are two main approaches to generating a sheet of light. A cylindrical lens or similar element can be used to focus light in one axis (the sheet thickness) while leaving it spread in the other axis (the sheet width), thus illuminating the entire field of view all at once. This is commonly referred to as a “static” sheet. The other method is to use a galvo mirror to scan a beam of light across the field during each exposure of the imaging camera, sometimes called a “digital” or “scanned” sheet. Each approach has advantages and disadvantages. Static sheets spread the excitation dose out in time which can reduce photodamage and are less complex to generate. However, illumination intensity usually isn’t constant over the field owing to source’s origin as a Gaussian beam. Digital sheets are uniform over an easily-changed width, have a “stop motion” effect which is beneficial for moving samples, and can be combined with a camera’s rolling shutter for 1D confocality. Due to sCMOS camera readout, static sheets can usually attain higher frame rates. But the rolling shutter can allow multiple light sheets to be created simultaneously in multi-view schemes.
The ideal light sheet would be extremely thin, with intensity completely confined to the focal plane of the detection objective, and long enough to cover the entire field of view of the imaging optics. But physics intervenes. Focused light diverges, so the thinner the sheet the shorter the thin region. The distance over which the sheet is relatively thin is called the confocal length and generally scales quadratically with sheet thickness. In practice the confocal length is generally matched to the size of the sample or field of view.
Most often sheet has an approximately Gaussian profile because the laser source originates as a Gaussian beam. For the case of a static sheet this Gaussian profile is in the focused direction, mathematically the same as sweeping a Gaussian beam. The thickness and confocal length of a Gaussian beam/sheet can be derived analytically as:
waist thickness = k1 * λ / NAill
confocal length = k2 * λ / (NAill)2
where k1 and k2 are dimensionless constants and depend on the chosen definitions of thickness and confocal length, λ is the vacuum wavelength, and NAill is the numerical aperture of the illumination beam. For common definitions k1=k2=0.64. The illumination NA can be no larger than the NA of the illumination optics but most often is only a small fraction of it so that the confocal length is sufficiently large as previously explained.