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%.

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.

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.