The beam splitter ratio is calculated as the fraction of incident light power that is reflected versus transmitted, often expressed as R/T or a percentage split.

Understanding Beam Splitter Ratios

A beam splitter divides an incoming light beam into two parts: a reflected beam and a transmitted beam. The splitting ratio is defined as:

where $P_{\text{reflected}}$ is the power of the reflected beam and $P_{\text{transmitted}}$ is the power of the transmitted beam . For example, a 50/50 beam splitter reflects 50% of the light and transmits 50%, while a 70/30 splitter reflects 70% and transmits 30%.

Calculating the Ratio

1. Measure or know the incident power $P_{\text{in}}$.
2. Determine the reflected power $P_{\text{R}}$ and transmitted power $P_{\text{T}}$.
3. Compute the ratio:

or as percentages:

For non-polarizing beam splitters, the ratio is designed to maintain the same polarization in both beams, while polarizing beam splitters are specified by the extinction ratio, which is the ratio of transmitted P-polarized light to S-polarized light .

Using Fresnel Equations

For a dielectric interface, the reflectance $R$ and transmittance $T$ can be calculated using the Fresnel equations:

where $n_1$ and $n_2$ are the refractive indices of the media, and ${\theta }_i$ and ${\theta }_t$ are the incident and transmitted angles . This is particularly useful for uncoated glass plates or custom coatings.

Practical Tools

Online calculators, such as the Beam Splitter Calculator, allow you to input the desired R/T ratio, incident power, and coating parameters to compute the reflected and transmitted powers, as well as lateral beam displacement for plate splitters . These tools simplify the calculation for both standard and custom beam splitters.

Summary

• Identify the type of beam splitter (cube, plate, polarizing, or non-polarizing).
• Measure or specify the incident light power.
• Use the ratio $R/T=P_{\text{reflected}}/P_{\text{transmitted}}$ or percentages.
• For precise optical design, apply Fresnel equations or use a beam splitter calculator.
• Consider polarization effects for polarizing beam splitters. By following these steps, you can accurately calculate the beam splitter ratio for your optical setup.