The beam splitter is used in reverse

Using a beam splitter in reverse generally preserves its transmission and reflection ratios, allowing it to combine beams or split light similarly to its forward operation.

Basic Principle

A beam splitter is designed to divide an incident light beam into transmitted and reflected components according to a specific ratio, such as 50:50 or 10:90. When used in reverse, the device can also combine two separate beams into a single output without significantly altering the original splitting ratio . This is commonly applied in interferometers, Mach–Zehnder setups, and three-CCD projectors where multiple beams are merged .

Transmission and Reflection Behavior

For isotropic, non-polarizing beamsplitters (both cube and plate types), the percentage of light transmitted and reflected remains essentially the same when the beam enters from the opposite side . Experiments with cube beamsplitters show that forward and backward operation produces nearly identical output intensities, with only minor variations due to experimental conditions or slight asymmetries in coatings . Plate beamsplitters with one antireflection-coated face and one partially reflective face also maintain their splitting ratio in reverse .

Practical Considerations

• Cube Beamsplitters: Typically more robust, with the reflective coating protected between two prisms. They are less sensitive to mechanical damage and maintain consistent performance in reverse .
• Plate Beamsplitters: Can handle higher radiation levels without damage, and their reverse operation is also effective, though slight beam displacement may occur due to the substrate thickness .
• Polarization Effects: Polarizing beamsplitters may behave differently in reverse because the reflection and transmission depend on the polarization state. Non-polarizing types are more predictable when reversed .
• Beam Combining: Using a beamsplitter backward allows two beams to be superimposed, which is useful in interferometry or color projection systems .

Summary

In most practical cases, reversing a beam splitter does not significantly change its optical behavior for non-polarizing, isotropic devices. It can be used to combine beams or split light with nearly the same efficiency as in the forward direction. Care should be taken with polarizing beamsplitters or specialized coatings, as these may introduce asymmetries or polarization-dependent effects.