When you say a beamsplitter has __% transmission and __% reflection, is that across the entire spectrum?

Unfortunately, no. All coatings used are optimized for certain portions of the spectrum. Our beamsplitters use partially reflecting coatings designed for broadband applications in the visible spectrum and are optimized for a 45° angle of incidence. The visible spectrum in reference to coating performance typically covers the range of 450nm to 650nm, while our spectral curves typically give extended values from 400nm to 700 or 800nm. Percentage values stated are typically held within ±10% over the visible spectrum, while within ±5% at the optimized wavelength, typically 550 or 632.8nm. To determine the performance of a particular beamsplitter for an application, the reflection and transmission values should be closely inspected for the particular wavelength(s) of interest. For example, a plate beamsplitter with a 50/50 ratio will actually have a value of 35% average reflection at 800nm. Our dichroic cube beamsplitters have almost equal average amounts of reflection and transmission over the visible range. Variations in reflection or transmission values over the spectrum are typically due to the inherent characteristics of the type of coating used: metallic, dielectric, or hybrid.