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)

Variable Message Signs (VMS) and Polycarbonate

Over recent months we have had a large number of customer contact us regarding Variable Message Signs (VMS), also known as Dynamic Message Signs (DMS), and the use of Polycarbonate for these signs. These signs are often used as traffic signs to warn drivers or give special information.

The signs often consist of a bank of either yellow or red LEDs behind a protective Polycarbonate front shield. The Polycarbonate is used to protect the sign against impact damage and environmental conditions.

Most of the questions that we get asked relate to a technical standard such as the European Standard EN.12966 for VMS. The main concern relates to the test, which simulates reflection of sunlight when the sun is at a low angle in the sky (5 or 10 degrees). In this situation, the sun is reflected off the Polycarbonate shield to the driver and partially obscures the light coming from the LEDs, making the sign difficult to read.

The sign can be made easier to read by either reducing the reflection of the sunlight or increasing the amount of LED light transmitted through the sheet – either by increasing the LED brightness or increasing the light transmission of the Polycarbonate sheet.

The test apparatus used for EN.12966 is shown in the picture accompanying this blog post [Please click on the picture to enlarge]. The principal of reducing reflection and increasing transmission is the same as that discussed in our previous blog posts with the exception that we are not concerned with the entire visible spectrum. We are specifically concerned with how the Polycarbonate interacts with the Yellow LEDS (wavelength 635 nm) and the Red LEDs (wavelength 590-595 nm) for the vast majority of VMS.

The problem that most VMS manufacturers have experienced is that they frequently buy general purpose Polycarbonate sheet, that has not been optimized for VMS, from distributors or manufacturers that are not aware of the options available. Much of this material has been produced with the idea of minimizing the production cost; as a result there is often large amounts of second grade (regrind) material in the product. As discussed in our previous blog posts, this regrind has the effect of lowering the transmission across the visible spectrum and in particular in the yellow region of the spectrum used by the yellow LEDs of VMS.

The first method improving the visibility of VMS signs in low sunlight is therefore to use an optical grade of Polycarbonate that has been design for VMS use, such as grades offered by HighLine Polycarbonate. The next method is to reduce the reflection and increase the transmission by the use of specially designed coatings. The added advantage of these coatings is that they improve the UV and weather resistant performance of the Polycarbonate, preventing the material from yellowing over time, which would also reduce the transmission in the yellow part of the spectrum. The coatings also add scratch resistance to the sheet, which is important in a road traffic environment.

The following table shows the effect of using a high quality VMS Polycarbonate and using an anti-reflective hard coat. The sheet used is 3mm / 0.118” thick.

Yellow LED Transmission

Uncoated GP Polycarbonate (*) 83.8%

Uncoated VMS Polycarbonate 89.0%

VMS Polycarbonate with anti-reflective hard coat 91.0%

VMS Polycarbonate with anti-reflective hard coat outside and optical coating inside 93.6%

Red LED Transmission

Uncoated GP Polycarbonate (*) 86.0%

Uncoated VMS Polycarbonate 89.7%

VMS Polycarbonate with anti-reflective hard coat 92.0%

VMS Polycarbonate with anti-reflective hard coat outside and optical coating inside 95.5%

[* the GP Polycarbonate was purchased from a distributor and was produced by a major manufacturer as their standard product].

For Yellow LEDs it is therefore possible to increase the transmission by 8.6% [91.0/83.8 = 8.6% increase] by using a properly designed Polycarbonate with an anti-reflective hard coat, for Red LEDs the increase is 7.0% [92.0/86.0 = 7.0% increase].

For both color LEDs the anti-reflective hard coat is also able to reduce the reflection by 25%.

The combination of the increase in transmission and the reduction in reflection significantly increases the readability of the signs in sunlight.

A further option to improve the performance is to use an advanced optical anti-reflective on the inside surface. The use of the advanced optical coatings is not recommended for the outside surface, as they are not suited to use in a dusty and dirty roadside environment. By using these materials on the inside surface the transmission for yellow LEDs rises to 93.6% and the transmission for red LEDs rises to 95.5%.

These figures give an increase in transmission of 11.6% for yellow LEDs and 11.0% for Red LEDs. They also reduce the reflection by 56%. One question that has not yet been completely answered is whether the additional cost of an optical grade anti-reflective is justified by the performance advantage over an anti-reflective hard coat.

The other option for VMS is to use an anti-glare hard coat. At the moment we are investigating the performance of these materials in this application. Anti-glare materials are different from anti-reflective materials in that they scatter the light to reduce reflection; so while you can reduce reflection you also significantly lower the transmission and the clarity of the sign. It remains to be determined whether the loss in transmission is acceptable. At the moment we are very reluctant to recommend anti-glare coatings for VMS applications even though we are able to provide anti-glare coatings.

To summarize, for VMS signs it is important to use a Polycarbonate sheet that has been designed for VMS applications rather than use general purpose Polycarbonate sheet. With an anti-reflective hard coat the transmission can be increased 7.0% for red LEDs and 8.5% for yellow LEDs and the reflection can also be reduced 25%.

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.