Dichroic beamsplitters, like excitation and emission filters, are generally selected and evaluated based on their transmission and reflection bands, transmission value, edge (cutoff) steepness and blocking. However, another important consideration, which is often overlooked, is the dichroic’s flatness, which can significantly impact overall imaging performance depending on the application in which it’s used.
High-performance optical filters are produced by depositing multilayered thin-film coatings on the surface(s) of plane, parallel glass. The highest quality filters employ hard coatings, which, in combination with non-perfect substrates, can result in filters exhibiting a slight bending or curvature of the filter surface. When flatness is critical, this bending can, however, be compensated for by applying an additional coating which counteracts the bending caused by the primary coating. The key to choosing the right filter is to know when this “compensation” coating is required, and how precisely the uncoated substrate surface and compensation coating must be made.
Let’s consider three typical applications where dichroic flatness should be considered.
• Imaging only the transmitted light
• Reflecting a small diameter illumination beam such as a laser
• Imaging the reflected light with a large diameter beam as in wide-field microscopy
Case 1: Imaging only the transmitted light
In traditional fluorescence imaging where only the transmitted light is being imaged, the only appreciable effect of bending in the filter is a slight divergence of the beam axis with image quality being unaffected. In these situations, high flatness is not required.
Now let’s take a look at the two cases where images are generated or obtained from the light reflected off the beamsplitter. In contrast to transmitted-light imaging, when imaging the light reflected off the beamsplitter a non-flat dichroic beamsplitter will produce two problematic effects (Figure 1). First, the location of the focal plane shifts along the optical axis; and second, the size and or shape of the focus spot changes causing blurring of the image. Generally, a small shift of the focal plane can be corrected by a minor adjustment in the camera/tube lens or camera position.
Case 2: Defocus error associated with reflecting small (laser) illumination beams
One case where a simple adjustment will not work is when the same lens is used to focus the reflected light off a non-flat filter onto a sample and then refocus the light returning from the sample in the opposite direction. A good example of this is a laser-based microscope in which a dichroic reflects a laser beam that passes through the same microscope objective that collects fluorescence (or Raman) light from the sample. Here, if the filter is flat enough to provide a focal shift less than one “Rayleigh Range” (a standard measure of depth of focus) for a given beam diameter the system will appear to be in focus.
Case 3: Spot size blurring associated with larger-diameter (imaging) reflected beams
Unlike shifts in focal plane, mechanical adjustments cannot compensate for a change in the size or shape of the focused spot associated with a non-flat dichroic. A common issue with a reflected, large diameter imaging beam is that light reflected off a curved dichroic (at a 45 degree angle) exhibits appreciable astigmatism. As illustrated in Figure 2 below the smallest spot size (the “Circle of Least Confusion”) can be significantly larger than the diffraction-limited spot size.
Therefore, the quality of an image formed using reflected light off a non-flat dichroic might be significantly degraded compared to the ideal, diffraction-limited image produced by a flat dichroic. For example, in Figure 4 below the radius of curvature should be greater than about 50 meters to ensure that the curvature induced aberrations are less than blurring caused by diffraction.
Table 1 below provides a guide to selecting the appropriate level of flatness for different application.
For a more in depth discussion of this topic please see our Technical Note.