Losses are caused by both absorption and reflection, with the latter being dominant in most cases. The purpose of an AR coating is to reduce the reflection losses so that nearly 100% of the light passes through the viewport.
The simplest AR coating is a single layer of material, the thickness of which is chosen to cancel out the reflections from the surface of the glass (or other substrate) via destructive interference. This material must have a refractive index lower than that of the substrate.
Applying the same coating on both sides of the optic ensures that reflections from the other surface are also cancelled in the same way (symmetrical). Glass which has been AR-coated on both sides typically reflects only 3-4% of light, as compared to 6-7% when uncoated. This will in turn result in 96-97% transmission as opposed to 93-94% uncoated.
Where a lower level of reflectance is required (for instance <1%), two or more layers are used, typically utilising a higher index material in addition to the coating material with lower refractive index. These materials are typically alternated to induce further destructive interactions which can more precisely cancel out reflectance at the desired wavelength(s).
For a single wavelength target, a 2-layer ‘VAR’ coating is used, so called because of the shape of its reflectance profile – where the bottom of the ‘V’ indicates very low reflectance at one wavelength. For two distinct wavelength targets, a ‘WAR’ is utilised – the shape of the ‘W’ representing the two separate troughs in the reflectance profile.
The example below shows a VAR coating optimised for 780nm. The ‘V’ shape is not so apparent here but narrows significantly when optimised for smaller wavelengths. The model (theoretical) curve is shown on the first graph, with the real data, as measured by Torr Scientific’s spectrophotometer, on the second one for comparison.
In the manufacturing process the coating is first designed using software. The materials and thickness are then programmed into the computer controlling an e-beam (electron beam) deposition process. After completion, the coating is tested in the spectrophotometer to check that the reflectance is within specification at the wavelength(s) specified by the customer.
If low reflectance is required over a range of wavelengths a broad-band anti-reflective coating (BBAR) will be required. Depending on the complexity, four or more layers may be needed to achieve the required performance. Typically, the wider the range over which the wavelength targets extend, the more difficult it is to maintain an even distribution of low reflectance.
As a general rule for BBAR coatings, the smallest wavelength in a range should usually be greater than or equal to half of the largest wavelength. For example, 500nm to 1000nm would be acceptable, but 350nm to 800nm would be considered too wide. However, all coating requests are considered individually and, where possible, a more complex coating design can be quoted to achieve the desired performance.
Torr Scientific processes all AR coatings to customer specifications, so please contact us with your requirements and one of engineers will produce a model reflectance curve to show what’s possible. Our AR coatings are typically applied for wavelengths in the range 250-1550 nm, but we are able to consider requests outside of this range on a case-by-case basis.
Questions to ask yourself before requesting a coating.
- Is it a laser application or for visual?
- Which wavelengths, or wavelength ranges are you working at?
- What level of transmission/reflectance is required?
AR >96% transmission (<4% reflection)
VAR/BBAR/WAR >99% transmission (<1% reflection)