Beam Splitters & Partial Transmitters
Beam Splitters separate incoming light into two beams. In reverse, they combine. Partial transmitters allow a portion of incoming light to pass & reject the rest. Can be metallic, dielectric or a mix & rejected light absorbed, reflected or both.
Beam Splitter Coatings
Beam Splitter (BS) is a term used to describe various coatings which divide a beam of light into separate beams. Dichroic filters are often called beam splitters. In this section, we will be describing beam splitters that divide light at each wavelength of interest into two separate beams. These beam splitters are typically designed for an incident angle around 45 degrees from normal. Partially transmitting metals also make very useful beam splitter coatings. Two common metals used for this purpose are Inconel and chrome. Metal beam splitters are often very broad and can cover a much wider spectrum of wavelengths than their dielectric counterparts. Dielectric coatings as described here have the advantage of being non-absorbing and so allow for greater throughput of energy. These dichroic filters for example, have a dielectric coating that can be used for a 50/50 beam splitter. In contrast, Inconel is limited to 30% transmission and 30% reflection due to the absorption inherent in the metal film. Standard dielectric beam splitter coatings include 30/70, 50/50, and 70/30.
Neutral Density Filter Coatings
There are some applications for which the light used is too intense. In those cases, a filter is often needed to apply a reduction in the intensity of the light without influencing the spectral profile. These wavelength neutral filters are often specified in terms of their optical density. Optical density can be calculated by taking the negative of the log (base 10) of the transmittance. For example, an optical density of 0.5 would be equivalent to approximately 32% transmittance. An optical density of 1.0 would be equivalent to a transmittance of 10%. Standard neutral density filter designs are available for the UV, Visible, and IR bands and for a variety of optical density values. A similar optically significant case involves surfaces for which reflectance and transmittance are required to both be minimized over a narrow or broadband spectrum. This type of coating is commonly called a Dark Mirror due to the black non-reflective appearance. Dark mirror coatings are often used to reduce stray light reflections at surfaces intended to be opaque. A dark mirror coating can be applied to a variety of glass and metal substrate materials.
Semi-Transparent Mirror Coatings
An emerging trend in hotel, restaurant, and retail displays is the use of aesthetically pleasing high performance Semi-Transparent Mirrors to create a hidden smart display mirror combination. The display unit is located behind the mirror, hidden from view until powered on, projecting a clear and sharp image on command. The semi-transparent mirror coating is available on low iron soda lime glass for low absorption and on grey glass when a more opaque look is needed; as when hiding TVs, security, and surveillance equipment.