Dichroic filters are multi-layer thin-film coatings deposited on a glass substrate using high vacuum deposition techniques. Each film layer is approximately one one-thousandth of a millimeter thick. The film coating typically consists of between 20 and 50 separate layers. The substrate is a special type of glass that has low thermal expansion properties.

Dichroic film structures typically consist of one of the following design types: short wave pass, long wave pass, bandpass, or notch filter. These design types comprise the basis of color determination and color separation. Performance is determined by the transmittance and/or reflectance of a band of wavelengths. Transmission in dichroic filters typically average 90% or better. Gel filters of comparable hue typically average much lower transmission, usually in the range of 50% to 60% or less. Dichroic filters are also environmentally durable and their performance will not deteriorate over time.

The films are deposited on large diameter substrates that are later fabricated into smaller sizes either by scoring and breakout or with a waterjet cutter. The waterjet cutter allows complex shapes to be programmed and cut.

• Dichroic filters reflect unwanted wavelengths back to the light source. Gel filters absorb the unwanted wavelengths and dissipate them in the form of heat, ultimately leading to performance degradation (melting, bleaching out, scorching, etc.).
• Dichroic filter transmission levels are substantially higher than an equivalent gel filter providing considerable savings in energy and money.
• Dichroic filters are highly color selective producing brighter, cleaner appearing, saturated color.
• Dichroic filters will not melt, wrinkle, shrink or fade like gel filters.
• Dichroic filters retain their characteristic spectral transmission properties indefinitely for long life and value.
• Dichroic filters are low maintenance and seldom if ever require replacement.
• Dichroic filters offer superior performance and long term cost savings.

Tolerance is important because the tighter the manufacturing tolerance is the closer the color matching will be. So when you order a particular color and a supplier works to a loose tolerance the end result may be very a noticeable color mismatch.
Most commercial dichroic filter houses provide material that is within ± 10-15 n.m. of specification. More specialist suppliers, such as Lightwave Research, provide a much tighter tolerance of ± 5 n.m. Let's say the supplier just happens to run a batch that is within 5 n.m. of the target color. You may be satisfied because the color match will be quite close. On the other hand if the run has produced material that is at one of the outer edges of tolerance, say +15 n.m., the color match would be very noticeably "off" and you may be quite unhappy with the end result.
The problem becomes particularly acute when a user orders filters at different times, e.g. not taken from the same manufacturing batch, as the color may vary quite noticeably from batch to batch if the manufacturing tolerance is fairly wide. So, at ± 15 n.m. tolerance, filters may potentially be supplied at + 15 n.m. at one time and - 15 n.m. at another time. A variation that would be very noticeable when new and old filters were used next to each other.
Rejection of light energy at undesired wavelengths is also a very important filter property. This parameter needs to be controlled very closely. For example, leakage of only a few percent of red in a blue filter can wash the color out.

Substrate
The medium that a thin film is coated on. Typically this would be glass but metal, plastics, and other materials are also often used.

Bandpass filter
Transmits light in usually small band of wavelengths and rejected light above and below the selected band. The passband is usually referred to as the area of transmission and the areas above and below are known as rejection bands.

Notch
This expression refers to the degree that filters notch out (eliminate) various frequencies of light. A notch filter is the opposite of a bandpass filter. Example: A magenta filter that transmits red and blue will have the green color bands notched out.

Contamination
Is a small amount of light spuriously transmitted from within a rejection band. It is also called a "leak" since it leaks light when it should be blocking it. This leak may or may not affect the perceived color of the filter.

Adhesion
Refers to the measure of force necessary to detach an area of film from the substrate or from a neighboring film in a multilayer coating. Adhesive quality is very difficult to measure accurately and tests are usually of a "go/no go" nature. A piece of tape is stuck to the film and peeled off. The film fails if a significant portion of the coating comes away when the tape is peeled off.

Thermal expansion coefficient
The measure of how the length and volume of a type of glass changes in response to an increase or decrease in temperature.

Quartz (SiO²)
A substrate material which has a low index of refraction. This is the most common material used in the composition of most optical glasses.

Tempax
A trademarked material produced by Schott industries, Tempax is boroscilicate glass.
Makeup is: >80% Si0² and >8% Boric Acid (B²O³). It has a low thermal expansion coefficient and has been replaced by Borofloat (tm) glass which is of higher quality than Tempax.

Pyrex
A brand name adopted by the Corning glass company for their boroscilicate glass products. Pyrex has a low coefficient of expansion but also tends of have internal inhomogeneity that makes it better suited for first surface mirrors than for transmissive components.

Soda Lime Glass
Typically used for window glass and bottles, etc., soda lime glass's makeup is about 70% SiO², 15% alkali (Na²O) and 15% alkaline earths (CaO + MgO). It has a relatively high thermal expansion coefficient.

Slope:
This expression refers to the degree that filters progressively cut off (eliminate) or cut on (allow through) various frequencies of light. To put it another way, slope is the measure of how quickly a coating changes from a rejection band to a passband or vice-versa.

Lambda 5 = Wavelength where absolute transmittance is 5%.

Lambda 80 = Wavelength where transmittance = 80% of average transmittance in the pass band.

Mixing paint, where the more colors you mix the blacker (darker) the color becomes, is familiar to most people and is called subtractive mixing.
Placing a number of different color filters in one lighting fixture also results in subtractive color mixing of the light beam. As more filters are added the color progressively becomes darker, edging its way toward black as any filter placed in a beam of white light produces a particular color by subtracting all the unwanted colors from the white light so achieving a light beam of a particular color.
Some of today's automated fixtures mix cyan, yellow, and magenta in various quantities to produce many different colors and shades so making very good use of the subtractive color mixing principle. Mixing light from several different colored light sources is called additive mixing because the addition of each color brightens (adds to) the light more for every fixture and color used. In additive mixing every color added brings more light. The final product of adding all these colors in always closer to whites. It is, however very difficult to predetermine exactly what color will arise as the result of shining a number of different colored light beams at one object or surface.

Front surface mirrors (also called first surface mirrors)
The mirror coating is applied to the front surface of the mirror so light is reflected directly from this coating.

Rear surface mirrors (also called second surface mirrors)
The mirror coating is applied to the rear surface of the mirror so light is passed through the substrate, reflected from the coating and then passed back through the substrate again.
Generally speaking a front surface mirror has a higher reflective efficiency then a rear surface mirror. because in a rear surface mirror some of the light is absorbed by the two way journey through the substrate as well as being reflected backwards and forwards between the substrate's inside front layer and the mirror coating. Another disadvantage of rear surfaced mirrors is that some light is reflected from the surface of the substrate forming a parasitic second reflection which causes a somewhat diffused looking return beam. A rear surface mirror is more durable since it is protected by the substrate but it needs a substrate of higher optical quality since the light has to pass through the substrate twice.

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