What is the ratio of the beam splitter

The ratio of a beam splitter, also called the splitting ratio, defines how incident light is divided between the transmitted and reflected beams.Definition of Splitting RatioA beam splitter ratio specifies the proportion of light that is reflected versus transmitted. For non-polarizing beam splitters, this is usually expressed as a percentage, such as 50:50, 70:30, or 90:10, indicating the fraction of light in the reflected arm compared to the transmitted arm (Thorlabs) . A 50:50 ratio, often called a half mirror, means half of the incident light is reflected and half is transmitted . For polarizing beam splitters, the ratio is described by the extinction ratio, which measures the separation of polarization states, typically the ratio of P-polarized light to S-polarized light in the transmitted or reflected beam .Types of Beam SplittersCube Beam Splitters: Made from two triangular prisms cemented together, often with a coated hypotenuse surface. The coating thickness is adjusted to achieve the desired splitting ratio, commonly 50:50 for interferometry applications .Plate Beam Splitters: Thin glass plates coated on one surface to reflect a portion of the light while transmitting the rest. They are usually placed at a 45° angle of incidence and can be designed for specific ratios like 30:70 or 50:50 .Adjustable Splitting RatiosSome beam splitters allow continuous adjustment of the splitting ratio. This can be achieved using:Rotating disks with gradient coatings, where the local reflectance changes with angular position.Combination of a rotatable half-wave plate and a polarizing beam splitter, which adjusts the polarization of the input beam to control the power distribution between output ports according to Malus' law .ApplicationsThe splitting ratio is critical in applications such as interferometers, laser systems, optical monitoring, and imaging setups, where precise control of light intensity in each path is required . Choosing the correct ratio ensures optimal performance for measurement accuracy, signal strength, or imaging contrast. In summary, the beam splitter ratio determines how light is divided between transmitted and reflected beams, varies by type (non-polarizing vs polarizing), and can be fixed or adjustable depending on the optical setup and application.