Leybold Optics GmbH

Low-E Coating

Low-E is short for low emissivity coating. This type of coating is typically used on one inner surface of a double or triple glazing unit.

In residual and in commercial buildings a window has three general functions:
1) It has to transmit light, to illuminate the interior of the building and to give a view of the world                 surrounding us. 
2) It should stop or reduce heat transfer from the inside to the outside and vice versa
3) The view trough the window should be more or less neutral in colour

Point number one is obvious: This is the reason to place a window inside a building.
Energy is expensive and less energy consumption is our common challenge to keep our nature in shape.
The comforts of human beings depend strongly on the ambient light and their individual views. For that reason it is essentially that we still see the familiar blue sky looking through a window. It would irritate us to see a purple grass field. A white curtain turned to yellow by the view inside trough a colour changing window would be disaster.

A low-E coating is designed and developed to match these functions. In addition to that the choice of the coating depends on its reliability and of course its production cost.

Looking at the spectrum of electromagnetic waves it is clear that a low-e coating should be pass-filter:
• High transmittance in the visible part of the spectrum (400nm to 780nm)
• and low transmittance in the infrared part of the spectrum (800nm to 50µm)

The following graph shows this:

Blue Line:           Transmission
Red Line:            Reflection coated Glass Side
Magenta Line:     Reflection uncoated Glass Side
All sputtered Low-E coatings are based on a thin, closed but transparent silver layer. This thin layer of 5nm to 20nm thickness acts as an infrared mirror. Silver is the metal with the best electrical and thermal conductivity. Alternatively gold or copper could be used, but both of them are not a thin as a comparable silver layer. For that reason it is nearly impossible to compensate the absorbance colours of these layers by other dielectric layers. The high transmission of a low-e film is the next problem with other IR-mirrors than silver.
Silver is a weak and soft material that tends to oxidize and agglomerate at higher temperatures. The pure silver layer needs to be protected and the optical appearance has to be tuned to a neutral colour. Therefore other layers are needed for the full layer stack. At least a dielectric layer between the silver and the glass substrate is necessary as well as a second one on top of the silver. The first commercial systems of this type were introduced by ASAHI in the late 80ties: Glass-ZnO-Ag-ZnO and alternatively Glass-ITO-Ag-ITO (so called IMI-Layers).

Before two pieces of glass come together in a double glazing unit both substrates are needed to be washed. The coating has to survive the washing procedure and it has to last fore decades inside the double glazing (in an Ar- or Kr-atmosphere). The Low-E layer has to fulfil a certain mechanical and chemical stability. That causes protective and adhesive layers inside the layer stack to strengthen the silver layer. Especially oxides were used for dielectric layers, sputtered in an aggressive oxygen atmosphere, which normally oxidises the silver layer. Thin metallic or suboxidic layers were introduced to capture the oxygen and protect the silver. NiCr, Cr, Ti, Nb and the sub oxides are commonly used for this purpose.
 

Machines
 

Apollon

Apollon

Horizontal in-line sputter coating plant for architectural glass more...
 

Detailed Information
Low-E

Thick silver layers reflect the infrared light pretty good, but they also absorb a high amount of the visible spectrum. The goal was to minimize this visible absorption by keeping the conductivity constant.
more... 		 
   

 


Low-E is short for low emissivity coating. This type of coating is typically used on one inner surface of a double or triple glazing unit.

 

In residual and in commercial buildings a window has three general functions:

 

 

 

 

Point number one is obvious: This is the reason to place a window inside a building.

Energy is expensive and less energy consumption is our common challenge to keep our nature in shape.

The comforts of human beings depend strongly on the ambient light and their individual views. For that reason it is essentially that we still see the familiar blue sky looking through a window. It would irritate us to see a purple grass field. A white curtain turned to yellow by the view inside trough a colour changing window would be disaster.

 

A low-E coating is designed and developed to match these functions. In addition to that the choice of the coating depends on its reliability and of course its production cost.

© 2012 LEYBOLD OPTICS