Anti Glare Coatings explained

Anti-glare coatings are different to anti-reflective coatings.  Anti-glare coatings are generally produced using an abrasion resistant hard coat with small particles in the coating to give a matte surface.  This matte surface stops light being reflected from the sheet surface back to the viewer so that the user's view is not obscured by glare from lighting or the sun.

One down side to the matte surface is that the light transmission of the sheet is lowered and the view through the sheet is hazy.  The more of the matte agent that is put into the sheet the more the glare is reduce, but also the sheet becomes more hazy and the view more obstructed.

To illustrate the effect of an anti-glare coating we have taken three pictures of an anti-glare sheet with a 40% gloss level.  The 40% gloss is quite a high level of matte agent - we commonly supply product with gloss levels of 60% and as high as 80%.  The 80% gloss level is much more transparent but does not reduce the glare as much as the 40% gloss level material.

We are often asked how much does the reduction in gloss level obscure the view through the sheet?  The answer depends on what you are trying to view.  If you are trying to view something that is a long way away through the sheet, the object is still able to be seen but the view is very blurred.  To show this effect, we positioned a typed page only 15" behind the anti-glare sheet.  The page is visible but the details are not.

We then moved the page to 5" behind the sheet.  Again the page is visible and you can even start to make out the detail of some of the larger font.  48 Point font is clearly legible, even 28 Point font is just visible, while smaller font can be seen but not read.

We then moved the typed page to immediately behind the sheet and the page was even touching the sheet.  Nearly all of the font, even the smallest can be clearly read.  

When choosing an anti-glare gloss level it is important to test it in your application.  The questions that need to be answered are how much do you need to reduce glare and how much haze can you accept.  The answers to these questions depend on what environment you are you using the sheet in and what do you need to see through the sheet.    

 

 Photo 1 - Typed page 15" behind the anti-glare sheet

 Photo 2 - Typed page 5" behind the anti-glare sheet

Photo 3 - Typed page immediately behind the anti-glare sheet (touching)

The Quality of Polycarbonate and Light Transmission

As we explained in a previous blog post, we would typically expect the light transmission of 0.118" one side hard-coated Polycarbonate to be in the range of 90% [with 5.1% reflectance on the uncoated side, 4% reflectance on the coated side and a little internal loss of light transmission due to the internal structure of the Polycarbonate itself]. As the thickness of the Polycarbonate increases, we would expect the internal loss of light transmission to also increase a little.

We were recently asked by a customer to apply an anti-reflective coating to the uncoated side of 0.236" one side hard-coated Polycarbonate. The Polycarbonate was provided by the customer and had been produced by another manufacturer. By applying the anti-reflective coating we were expecting to reduce the reflection on the uncoated side from 5.1% to around 1.0%. We were therefore expecting to increase the overall light transmission from 89-90% to around 94%.

After we had coated the material with the anti-reflective we discovered, to our surprise, that we were only getting a light transmission of 89%. The application of the anti-reflective coating appeared to have failed. We examined our coating process and found no obvious problems. We then decided to test the light transmission of the material before we applied the anti-reflective coating. To our surprise we found that the light transmission was only 84-85% instead of the 90% that we expected. The problem was with the quality of the competitors Polycarbonate and not the anti-reflective coating.

We then measured the reflection on both surfaces and calculated the internal loss of light transmission across the entire visible spectrum. We then repeated this process with our own 0.236" Polycarbonate. We then plotted our the internal loss of light transmission for both materials over the visible spectrum. This plot can be seen in the diagram at the top of the page (for a better view, click on the picture).

The results were shocking.

Over the range of 450-500 nm, our material had an internal loss of light transmission of 2% and the competitors had a loss of 5%

Over the range of 525-575 nm, our material had an internal loss of light transmission of 4% and the competitors had a loss of 7%

Over the range of 650- 750 nm our material had an internal loss of light transmission of 1% and the competitors had a loss of 7%

The end result was that the customer would have been better off buying our HighLine Polycarbonate without an anti-reflective rather than applying an expensive anti-reflective to the competitors material. In the end the customer decided to use our Polycarbonate with an anti-reflective and achieved a light transmission of over 94%.

The lesson to be learned from this recent experience is that not all Polycarbonate sheet is equal. The Polycarbonate sheet from this competitor, who is a major international supplier of Polycarbonate sheet, clearly had a much lower light transmission across the visible spectrum than the Polycarbonate sheet from HighLine Polycarbonate. This lower transmission is caused by inferior resin, use of regrind and the commodity production methods used by some of the large producers. In the vast majority of applications, particularly commodity applications, this loss of light transmission is not important. However, in some quality and high-tech applications, a 6% light transmission loss in the 650-750nm range can be critical. Any application requiring an anti-reflective coating should seriously consider the quality of the base Polycarbonate and should be extremely cautious about buying an off the shelf product from a distributor. Polycarbonate sheet for high quality applications should always be bought directly from the manufacturer so that you can have the material produced specifically for the required application.

All of HighLine Polycarbonate's material is designed for high quality optical applications. If you are using another supplier's material it would be wise to ask for them to provide the light transmission curve for the actual lot number of the sheet you will be receiving. We were certainly surprised by the poor quality of some of the material that is being sold as high quality product.

Anti-reflective coating options for Polycarbonate

There are several options available for improving anti-reflective performance of Polycarbonate sheet. The correct choice depends on a number of factors including the level of anti-reflection required, the size of the part, the number of parts required and the cost sensitivity of the application. In this blog entry we will discuss how to make the correct choice for the application.

Anti-reflective coatings are typically applied to Polycarbonate that has an abrasion resistant coating applied to the surface. The abrasion resistant coating provides a better surface for the anti-reflective coating to adhere to than the uncoated Polycarbonate. The finished product is therefore more durable. The abrasion resistant coating itself also improves the anti-reflective properties of the Polycarbonate sheet, as discussed in a previous blog post.

There are essentially two broad types of anti-reflective coatings, liquid anti-reflective coatings and vapor deposition anti-reflective coatings. Liquid anti-reflective coatings are applied to the sheet in a solution and are then cured using either ultraviolet light or heat. Vapor deposition coatings are applied using a sputtering process.

Level of anti-reflection achieved.

The following table shows the amount of reflection from each surface of the sheet with each of the anti-reflective options. These figures are over the visible light range of 420-680 nm.

Uncoated Polycarbonate sheet 5.1%

Abrasion resistant coated Polycarbonate sheet 3.9%

Liquid anti reflective on Polycarbonate sheet 2.0%

Vapor deposition anti-reflective on Polycarbonate sheet 0.75%

If a very low level of reflection is required a vapor deposition anti-reflective is normally used. However, it is often possible to use a liquid anti-reflective or even just an abrasion resistant coated sheet for applications not needing such a low level of reflection.

Cost of anti-reflective solutions.

A liquid anti-reflective coated sheet typically sells for about five times the price of a standard abrasion resistant coated Polycarbonate sheet.

A vapor deposition coated anti-reflective sheet would sell for about five times the price of a liquid anti-reflective sheet.

These broad pricing guidelines obviously depend on a number of factors including part size and the number of parts required, but they do give some indication of what you can expect to pay for increasing levels of anti-reflective performance. Often only very high technology applications can justify the cost of a vapor deposition anti-reflective coating.

Part size and minimum order quantity.

One of the problems of vapor deposition technology is the limitation on the size of the part. Parts of up to 14” x 18” can be produced on a standard sputtering machine in reasonably small quantities. However, once you get above this size you need to use a very large sputtering machine that requires large set up costs and thus large production runs. Parts up to 24” x 36” are easily possible but may require production of at least 1000 parts at a time; this makes it very difficult to obtain a couple of parts for a prototype development if parts over 14” x 18” are required. Once you require parts of over 24” x 36” you need very specialized equipment and the cost is extremely high.

For liquid anti-reflective coatings it is possible to easily coat sheets of 48” x 96” or larger and the minimum production size is much smaller. The easier production makes liquid anti-reflective materials much easier to obtain for prototype development. For large parts we typically recommend that liquid anti-reflective coatings are evaluated first, before trying the expensive vapor deposition anti-reflective coatings.

Why the Light Transmission of Coated Polycarbonate sheet is higher than Uncoated sheet

This topic is a follow up of previous blog on the 28^th October 2009 – “Transmission – Anti Reflectives and Anti Glare”.  The previous blog gives an introduction Refractive Index and Reflection. 

One question that we are often asked is why the Light Transmission of our abrasion resistant coated polycarbonate sheet (90%) is higher than the Light Transmission of our uncoated polycarbonate sheet (88%)?  

This question is asked because there is a belief that the coating should reduce the “optical properties” of the sheet.  Some people even believe that we must be using a “purer” base sheet for our coated product. 

The answer to the question is related to the reflection of light.  As we discussed in our previous blog post, light is reflected from uncoated sheet on the front surface and the back surface. 

Uncoated sheet.

At the front surface, the light passes from the air (with a refractive index of 1.00) to the Polycarbonate (with a refractive index of 1.585).  Using the Fresnell Equations, the reflection can be calculated as 5.1%.  [See the previous post for details of the Fresnell equations].

At  the back surface, the light passes from the Polycarbonate (with a refractive index of 1.585) to the air (with a refractive index of 1.00).  The reflection from this surface is also 5.1%.

The total reflection is 10.2% giving a light transmission of 89.8%.  Typically we report a light transmission of 88% to be conservative. 

Coated sheet.

In the case of one side coated sheet we introduce another layer – the coating.  The coating material typically has a refractive index of 1.49.  With this information we can calculate the transmission of the coated sheet.

At the front surface, the light passes from the air (with a refractive index of 1.00) to the coating (with a refractive index of 1.49).  The reflection from this surface can be calculated as 3.9%

The light then passes from the coating (refractive index of 1.49) to the Polycarbonate (refractive index of 1.585).  The reflection from this surface can be calculated as 0.1%.

At the back surface, the light passes from the Polycarbonate (with a refractive index of 1.585) to the air (with a refractive index of 1.00).  The reflection from this surface is again 5.1%.

The total reflection is 9.1% giving a light transmission of 90.9%.  Typically we report a light transmission of 90% to be conservative.

It can be seen that adding a coating actually increases the Light Transmission of the Polycarbonate sheet.  The application of a layer with a Refractive Index between that of air and Polycarbonate is actually the theoretical basis of advanced reflective coatings.

Transmission - Anti Reflectives and Anti Glare

The term transmission is often used when specifying Polycarbonate or other clear plastics.  The terms anti-reflective and anti-glare are also used, often without a clear understanding of the meaning.

Polycarbonate sheet made from a high quality resin has a refractive index of 1.585  

This number means that light travels in Polycarbonate at 1 /1.585 or about 2/3 of the speed of light in a vacuum.

When light passes from one substance to another substance with a different refractive index two effects occur.  Firstly the light changes direction slightly and secondly some of the light is reflected.

The amount of light that is reflected can be calculated using the Fresnell Equations:

R = [ (h₀ - h₁) / (h₀ + h₁) ]²

Where  R is the amount of light reflected and h₀ and h₁ are refractive indices of the two materials.

If the refractive index of air (1.001) and polycarbonate (1.585) are used, the reflection on the surface is calculated to be 5.1%

However it should be remembered that there are two surfaces giving a total reflection of 10.2%; this is the reason why high quality Polycarbonate sheet has around 89% transmission as the remaining 10.2% of the light is reflected.

In display applications it is important to both increase transmission and reduce reflection. Increasing transmission allows a brighter display for a given backlight.  Reducing reflection makes the display easier to see for the user, particularly in bright sunlight.

There are two solutions to reduce the reflection from the surface back to the user.  The first is to use an anti-glare coating.  This reduces the light that is reflected back to the user by scattering the light, much like a matte surface.  Unfortunately this method also reduces the light passing through the sheet and the transmission can often be reduced to 80% or lower.

The second and better method is to use an anti-reflective coating.  With an anti-reflective coating the reflection can be reduced to 0.75% on each surface giving a total of 1.5% reflection. With an anti-reflective coating, the total transmission of a Polycarbonate sheet can be raised to 98.5%.  Anti-reflective coatings allow the goals of increased transmission and reduced reflection to be achieved.

HighLine Polycarbonate LLC produces Polycarbonate sheets with a range of anti-reflective coatings.  These can be combined with transparent conductive ITO layers for the display industry.