Tuesday, November 15, 2011

Birefringence, Photoelasticity, Anisotropic Materials, Iridescence and the Rainbow Effect - Part 3

In the final blog post of this trilogy we will discuss Iridescence and how it can cause a rainbow effect on abrasion resistant coated Polycarbonate sheet. We will also discuss how the rainbow effect can be minimized.
Iridescence is the rainbow or oil slick type pattern that often appears on the surface of a Polycarbonate sheet particularly under artificial lighting conditions.

Coated Polycarbonate sheet has a thin film of coating on the surface of the sheet in order to protect the sheet against abrasion damage. It is this thin film of coating material that causes the problem, in much the same way that a thin film of oil on the surface of a pool of water exhibits the rainbow patterns. The effect is due to the process known as interference.

As discussed in previous blog posts, whenever light travels from a material with one refractive index to another material with a different refractive index, some of the light is reflected. In the case of the coated Polycarbonate sheet, when light moves from the air into the coating some of the light is reflected. Then, when the light moves from the coating into the actual Polycarbonate, some more of the light is reflected. When the light that is reflected from the first surface comes into contact with the light that is reflected from the second surface the light waves recombine.

Depending on how thick the coating layer is, the light waves may be in sync when they recombine or may be out of sync when they recombine. If they are in sync the two waves will added together and will have constructive interference. If they are out of sync the two waves will start to cancel each other out and will have destructive interference.

Since visible light has wavelengths of 380nm(violet) to 750nm (red) and a typical hard coat has a thickness of 4 to 7 microns [4000 to 7000 nm], the coating thickness is an order of magnitude thicker than the wavelengths of visible light. A small percentage variation in the coating thickness can therefore change whether the constructive interference or destructive interference occurs. If there is variation of coating thickness over a small area of sheet, even if the variation is only tens or hundreds of nanometers, then there will be areas of constructive interference and areas of destructive interference. This variation in the interference patterns is part of the cause of the iridescence or rainbow effect.

The question then becomes, how do we eliminate the variation in coating thickness? Abrasion resistant coatings are often added to sheet by a process known as flow coating. The sheet is hung vertically and coating solution is allowed to run down the surface of the sheet from top to bottom under gravity. The solvents are then allowed to evaporate. If the sheet is allowed to move before most of the solvents have evaporated, the coating surface can become uneven. However, we need to remember that the coating surface is not the only surface that we need to be concerned about - there is also the sheet surface that is reflecting light. The sheet is extruded between large chrome rolls which are powered by motors. If there is any variation in the motor speed of these motors or the motors pulling the sheet, there can be variation in the thickness of the sheet. While the variation in the thickness will be small, it only requires very small variation to cause iridescence.

The reality is that neither the sheet producers or the abrasion resistant coaters have the ability to control their processes to the level of 10-100nm thickness. If we look at sheet producers, many of them state that their thickness specification is plus or minus 10%. On a 0.118" thick sheet that corresponds to 300,000 nanometers. While this is an overall thickness tolerance and not a measure of local variation of thickness, it does give some idea of the magnitude of the problem. Using this information we can determine that the sheet producers and coaters cannot prevent the problem. In many cases sheet producers often blame the coaters for the problem and coaters often blame the sheet producers. This then leaves us with the question of how do we solve the problem?

To answer the question we will briefly move to another topic - different types of lights. Traditional incandescent light bulbs have a relatively smooth light spectrum across the visible region and are similar to sunlight in this respect. When sunlight is split into its component wavelengths (such as in a rainbow) there is a smooth transition from violet through the various colors to red. There are no wavelengths missing. An incandescent light bulb behaves in the same way (as do some full spectrum LED bulbs).
Fluorescent bulbs, mercury bulbs, sodium bulbs and non full spectrum LEDs are different. When the light is split into its component parts, there are peaks at some wavelengths and gaps at other wavelengths. For example, a low sodium bulb emits an almost monochromatic light source at 589.3nm and a standard fluorescent bulb has 22 peaks with the main four being Mercury at 437nm, Terbium at 543nm, Mercury at 547nm and Europium at 611nm. These wavelengths of a fluorescent bulb combine to yield a light that looks like natural light but has discrete wavelengths rather than the continual spectrum of natural light.

Having a light source composed of discrete wavelengths rather than a continuous spectrum is a major problem for iridescence; when the light is reflected from the two surfaces the discrete wavelengths make the problem much larger as there are no intermediate colors to cancel out the iridescent effect. In short, the light source can make the problem of iridescence much greater.

The best way to reduce the effect of iridescence is to change the lighting source to a full spectrum light source such as incandescent bulbs or full spectrum LEDs. If the only option is to use fluorescent bulbs, it is better to use a bulb with more emission peaks to more closely resemble full spectrum light.

Another option to completely resolve the problem is to use what is known as an index matched abrasion resistant coating. The Polycarbonate sheet has a refractive index of 1.585 and most coatings have a refractive index of 1.49. If an abrasion resistant coating with a refractive index of 1.585 is used, the light will treat the coated Polycarbonate sheet as a single layer material and the effect of iridescence will be completely eliminated. While this process sounds great (and HighLine Polycarbonate can offer index matched abrasion resistant coated products) there is a significant downside - index matched coatings are very expensive. In most applications it is better to install full spectrum bulbs to reduce the problem.

Finally, to illustrate the effect of lighting on the visual appearance of iridescence we will recount a case study about the problem. A rail car manufacturer was experiencing oil slick like patterns on the Polycarbonate windows of their railcars. The manufacturer of the windows was inspecting the windows prior to sending them to the rail car manufacturer to try and identify the problem. They were unable to detect the issue as their factory was lit with incandescent lights. When the windows were installed in the railcars, the oil slick appearance was easily visible because the internal lights on the rail car were fluorescent bulbs.
The most practical solution would have been to change the bulb type on the railcar, but unfortunately the window manufacturer did not understand the problem. They told the rail car manufacturer that the problem was due to birefringence, which, as anyone who has read these three blog posts knows, was not the cause of the problem. By understanding the cause of the problem it is easier to recommend a solution to the customer.

Thursday, November 10, 2011

Birefringence, Photoelasticity, Anisotropic Materials, Iridescence and the Rainbow Effect - Part 2

In the last post we discussed how stresses in Polycarbonate can cause the material to become Anisotropic and exhibit Birefringent properties. Light waves parallel to the stress direction will travel through the sheet at a different speed than the light waves perpendicular to the stress direction.

It is possible to visualize the stresses in the sheet due to the birefringent properties of the sheet. A technique known as Photoelasticity is often used. In this method light is first passed through a polarizing filter, in order to block all components of the light not vibrating in the direction of the plane. The light coming through the filter is then known as polarized light. The light is then allowed to pass through the Polycarbonate part being examined. The birefringent properties caused by the stresses cause the polarized light to be split into two perpendicular components each moving at different speeds which are governed by the amount of stress in each direction. The components of the light waves recombine on leaving the Polycarbonate. When this light is then viewed through a second polarizing filter it is possible to see the effect of the retardation of the light in the form of "rainbow" like patterns. There is a lot of theory that can be explored on the method of Photoelasticity and this theory can easily be researched by carrying out a web search. In this blog we do not plan to go into advanced theory of how the light waves recombine, but rather discuss how the method of Photoelasticity can be practically used.

In the picture at the top of this blog post is a photograph taken of a piece of Polycarbonate with a hole drilled through it. The photograph was taken with a simple phone camera and two polarizing filters bought from a camera shop for $25 each. One filter was put behind the Polycarbonate part and one filter was put in front of the part. Although this cheap set up does not compare with advanced equipment for visualizing and measuring Photoelasticity, it does provide a simple practical tool for visualizing stresses in Polycarbonate parts.

In the Photo it can be seen that there are high levels of stresses on each side of the hole. We suspect that these stresses were caused by poor drilling technique using the wrong drill bit for Polycarbonate and operated at the wrong speed. It is also possible that the drill was started while in contact with the sheet. The technique of Photoelasticity allows us to visualize these stresses and therefore allows us to adjust fabricating methods to minimize stresses. This information is particularly important as we know that areas of increased stress are prone to cracking and damage, especially when exposed to certain solvents.

We invite readers who are involved in fabricating Polycarbonate parts to try this test method themselves to see the stress areas on the parts. All you need to do is buy two Polarizing filters from a camera shop.

In this section of the trilogy of blog posts on the subject of the rainbow effect, we have seen how stresses in Polycarbonate sheet can lead to birefringence and that these stresses can be visualized through polarizing filters as a rainbow type pattern.
However, it should be understood that rainbow effect seen on some hard coated Polycarbonate sheet without the use of polarizing filters is not due to the birefringence of the material. These rainbow type patterns on hard coated sheet are often very easy to see with just the eye and can cause the visual appearance of the sheet to seem very poor. In the last post on this topic, we will discuss what causes the rainbow effect on coated sheet and how its effect can be minimized.

Sunday, October 23, 2011

Birefringence, Photoelasticity, Anisotropic Materials, Iridescence and the Rainbow Effect - Part 1

One question that we are often asked about Polycarbonate is what causes the rainbow like patterns on coated sheet and how can they be eliminated.
The answer is not simple and we will need to answer the question over two or three posts. There is also a lot of confusion in the industry about what causes the effect. Often people try to explain the effect using the wrong terms.

Birefringence and anisotropic materials
The first term that we will discuss is Birefringence or the double refraction of light when it passes through an Anisotropic material. At this stage, don't worry too much about these terms, we will explain them as we go. Birefringence is often the term that is incorrectly used to explain the rainbow patterns seen on the surface of some coated Polycarbonate sheet. As we will explain, Birefringence can allow us to see stresses in the sheet using polarizing filters - they allow us to see the stresses which will appear as rainbow like effects. However, birefringence is not the cause of the rainbow like effect which can be seen with the eye on the surface of hard coated Polycarbonate sheet.

To explain birefringence and anisotropic materials we will start with a discussion about the structure of Polycarbonate. Polycarbonate is a long molecule containing Carbon, Hydrogen and Oxygen atoms. A simple web search can give details of the chemical formula. When Polycarbonate is heated and allowed to cool without being subject to any stresses, these molecules will be arranged randomly.
During the production of extruded sheet, the Polycarbonate is melted and then extruded through a wide die into a sheet format. The sheet is then pulled out of the die by some pull rollers through some chrome polishing rolls to create a smooth surface on the sheet. The pull rolls create some stress in the sheet in the direction of extrusion, but not in the direction perpendicular to the extrusion. The sheet is cooled and allowed to "set" while still being pulled by these rolls. This difference in stress in the sheet between the extrusion direction and the direction perpendicular to extrusion is commonly referred to as shrinkage. We have discussed shrinkage in more detail in previous blog posts; shrinkage is able to be controlled below 1%, although often it is possible to find sheet with high levels of shrinkage of 10% or more.

The stresses in the Polycarbonate can be eliminate by annealing the sheet - heating it above its glass transition temperature and then allowing it to cool. Also stresses can often be added to the sheet by some fabrication methods.

The more shrinkage that the Polycarbonate sheet has, the more stress it has in the extrusion direction and the more the Polycarbonate molecules are aligned in the extrusion direction. This alignment of the Polycarbonate molecule chains causes the Refractive Index of the Polycarbonate in the direction of the extrusion to be different than the Refractive Index in the direction perpendicular to the extrusion. As explained in previous blog posts, the refractive index is a measure of how fast light travels in a material. The difference of refractive index in the two directions causes extruded Polycarbonate to become what is known as an Anisotropic Material - where the speed of light traveling through the material is dependent upon the direction of the material.
If a Polycarbonate sheet is produced without any stress or 0% shrinkage, it would not be Anisotropic.

The difference in the Refractive Index between the two directions can be calculated using the Stress Optics Law:

(RI1 - RI2) = C x (Stress1 - Stress2)
RI1 = Refractive Index in extrusion direction
RI2 = Refractive Index in direction perpendicular to extrusion
C = Stress Optic Constant
Stress1 = Stress in extrusion direction
Stress2 = Stress in direction perpendicular to extrusion.

If the Refractive Index in one direction is different than the Refractive Index in the other direction, the components of the waves of light moving through the Polycarbonate in one direction will travel at a different speed than the light in another direction. The more Polycarbonate that the waves travel through, the more the one wave will lag behind the other. This effect is known as Retardation of the wave.
The retardation of the wave can be calculated using the following formula:

Retardation = C x thickness of Polycarbonate x (Stress1 - Stress2)

The amount of retardation of the wave is therefore proportional to both the thickness of the sheet and the differences in the stresses in the two directions. The retardation will be much lower on thin sheet with low shrinkage.

When the components of the light in the two directions emerge from the sheet they will recombine. However, how they recombine will be a function of the phase difference caused by the retardation of the light. There could be constructive or destructive recombining of the waves at different wavelengths.

In the next post on this subject we will look at how these waves combine. We will also look at how we can use a polarizer to look at the stresses in the sheet using an experimental method known as Photoelasticity.

Thursday, July 14, 2011

Bonding Polycarbonate Sheet

One question that we are often asked is how can two Polycarbonate sheets be bonded together?

At HighLine Polycarbonate we are mainly involved in producing Polycarbonate sheets with a wide range of high tech properties. We only engage in a limited amount of fabrication which includes routing of the sheets into finished part shapes.

We do not engage in fabrication that requires bonding of two sheets together. Some of our customers do engage in this type of fabrication and we will list some of the methods that we know about for joining two sheets of Polycarbonate together. We would be very interested to hear from our readers about other methods that they know about so that we can update the post with additional information.

We do not plan to cover physical methods of joining sheets together such as rivets, screws and tapes.

- The first method that we know about is using Methylene Chloride or a 60%/40% mixture of Methylene Chloride and Ethylene DiChloride. This solvent bonding technique is known to give a good bond strength and excellent optical clarity along with low capital investment. The mixture of Methylene Chloride and Ethylene DiChloride gives a slightly longer curing time than neat Methylene Chloride allowing more time to get the parts in the correct position; this is particularly important for larger parts. Suppliers of these chemicals can be found on Google. We recommend reading the Material Safety Data Sheet for information on safe handling and disposal before using any chemicals. We also recommend that you test any method on a small part before using on critical parts.
Before starting the solvent bonding process, both surfaces should be cleaned with warm water. If there are greasy areas, IsoPropanol (IPA) should be used to wipe the surfaces clean. Some fabricators recommend dissolving between 2% and 5% Polycarbonate saw dust in the Methylene Chloride or Methylene Chloride/Ethylene DiChloride solvents before use in order to give a stronger bond strength. We have yet to see any evidence that the saw dust improves the bond strength. In any case, if you choose to try this method, make sure that all of the saw dust is fully dissolved before use, because otherwise lumps of saw dust may prevent good surface contact between the two parts. Another recommendation that we have heard from fabricators is that in order to prevent whitening of the joint occurring, 10% Glacial Acetic Acid should be added to the solvents. Whitening does not always occur, so we would only recommend that you try this solution if you are having problems with whitening on your particular parts.
Having made up the solution, the solvent should be applied to one of the clean parts. The two parts should then be clamped together with several hundred psi pressure for about 5 minutes. The parts should then be allowed to cure in a well ventilated area at room temperature for between two and five days.

- The second method is to use an adhesive; this is a cheaper solution than solvent bonding but we believe that the bond strength and the optical clarity are not as good. Many customers have had excellent results with products such as "Weld-on". These products can easily be found using Google.

- Other methods such as vibration welding and ultrasonic welding have had varying degrees of success depending on the part shape and thickness. We would suggest that you contact manufacturers of the equipment to see if these options are suitable for your needs. These methods would require capital investment.

- The final option that we know about is to laminate the two parts together using an interlayer material such as transparent Polyurethane. This method is often used to manufacturer ballistics laminates where Polycarbonate layers are bonded to glass. This method requires a lot of specialist knowledge and equipment, such as an autoclave so it is unlikely to be viable for the majority of applications.

We look forward to hearing about more bonding methods from our readers.

Wednesday, May 25, 2011

FDA and NSF Standard 51 grades and UV absorbers

Polycarbonate sheet is widely understood to block UV wavelengths below 385-390 nm. What is not so well known is it is not the Polycarbonate that blocks these wavelengths, but rather the UV absorbers that are added to the Polycarbonate that block the UV light.
Polycarbonate sheet that has no UV absorbers will only block wavelengths below 290 nm. Unfortunately wavelengths below 385 nm will cause the Polycarbonate to weather and become brittle and yellow. Manufacturers therefore add UV absorbers to the Polycarbonate resin to give it some protection against the UV light. Some outdoor grades of Polycarbonate also have an additional cap layer or coating heavily loaded with additional UV absorbers to further protect the sheet against the affect of UV light.

There are some grades of Polycarbonate, that are often known as FDA or NSF Standard 51 compliant grades that have no UV absorbers. The reason that no UV absorbers are added is that these grades are designed to be used in the Food Processing environment and the UV absorbers are not approved by the FDA to be used in Food Processing areas. The manufacturers therefore produce grades without the UV absorbers. Because these FDA grades of Polycarbonate sheet have no UV absorbers, they should not be used outside as they will yellow very quickly.
One question that we are often asked is are the FDA approved grades safe to be used in food contact applications? The FDA grades of Polycarbonate sheet do not have UV absorbers in them because they are not approved for materials used in Food Processing environments. However, the Polycarbonate itself does still have Bisphenol A or BPA in it and there is currently a great deal of debate about whether BPA is safe in food contact applications such as baby feeding bottles. As a result of this debate, at HighLine Polycarbonate we do not sell any Polycarbonate sheet that will be used in applications where it comes into regular, direct contact with food. However, FDA grades of Polycarbonate sheet can be used as machine guards to protect operators on food packaging lines when the machine guards do not come into contact with food that will be eaten.

One un-intended market for FDA approved grades of Polycarbonate sheet is to customers who bond Polycarbonate sheet to other materials using a UV cured adhesive. The adhesive requires light from a UV lamp to pass through the sheet in order to bond it to another material. The UV absorbers in Standard Polycarbonate sheet block the UV light from the lamp preventing the adhesive from curing. By using an FDA grade of Polycarbonate sheet, the adhesive is able to be cured effectively. After bonding, the sheet can be protected against UV light by adding a coating with UV absorbers.

Monday, May 9, 2011

Kinetic Energy of Ballistics rounds and transparent armor

We are often asked about the difference between bullet resistant windows installed in 24hrs stores or banks and the transparent armor used by the military.
The bullet resistant windows in convenience stores and banks are often made of cell cast acrylic sheet or a combination of acrylic and Polycarbonate. They are often about 1.25" to 1.375" thick and are designed to protect against threats that are likely to be encountered in that environment. Typical bullet resistant ratings of UL.752 Level 1 to Level 3 are encountered. But what does a UL.752 Level 1, Level 2 or Level 3 mean and how does it compare to the transparent armor of military applications?

A UL.752 Level 1 material is designed to stop 9mm FMCJ rounds weighing 8.0 grams traveling at a velocity of up to 394 meters/second.
A UL.752 Level 2 material is designed to stop 0.357 Magnum JSP rounds weighing 10.2 grams traveling at a velocity of up to 419 meters/second.
A UL.752 Level 3 material is designed to stop 0.44 Magnum rounds weighing 15.6 grams traveling at a velocity of up to 453 meters/second.

But what does this mean? One of the most important factors in determining whether a bullet resistant structure will stop a ballistics round is how much Kinetic Energy does the ballistics round have.
Using the equation for Kinetic Energy:
Kinetic Energy (Joules) = 1/2 x Mass (Kilograms) x Velocity (meters/second)^2

Calculating the Kinetic Energy for the UL.752 Level 1 ballistics round we find:

Kinetic Energy = 1/2 x 0.008 x 394 x 394 = 620 Joules

For the three UL.752 Levels we get:
Level 1 620 Joules
Level 2 895 Joules
Level 3 1600 Joules

We can see as the weight and the velocity of the round increase the Kinetic Energy of the round increases. The bullet resistant material needs to be able to resist a larger amount of Kinetic Energy.

We can now look at the military grades to compare the amount of Kinetic Energy they are designed to stop. Military grades of transparent armor are composed of multiple layers of glass and polycarbonate. The glass can be of various types. In some cases advanced materials such as Spinel and ALON are also used. Often the structures can be many inches thick.

For US military grades a standard known as ATPD.2352 is used. The different rounds that the materials must stop is listed but the velocities are classified. The fact that the velocities are classified makes it difficult to calculate the required Kinetic Energy that must be absorbed; it would be possible to take an educated guess at the velocities, but for the purposes of this blog post, we do not need to do this is we can use the NATO standard AEP55 STANAG 4549 Volume 1.

STANAG 4549 has 5 protection levels for Light Armored Vehicles. For the purposes of the discussion on transparent armor we will just look at Levels 1 and 4.

Level 1 material is designed to stop a 7.62 mm x 51 NATO ball round weighing 9.65 grams traveling at 833 meters/second.
Level 4 material is designed to stop a 14.5 mm x 114 API/B32 round weighing 64 grams traveling at 911 meters/second.

A Level 1 round has a Kinetic Energy of 3,348 Joules
A Level 4 round has a Kinetic Energy of 26,557 Joules

You can see that the energy that a UL.752 Level 1 material needs to stop is over 40 times less than a STANAG 4549 Level 4 material. The reason for this difference is that the type of ballistics rounds likely to be encountered at a convenience store are likely to be very different from those encountered by the military. Indeed the deterrence factor of bullet resistance glass in commercial applications should not be underestimated.

It should be noted that this discussion is very much a simplification and is only meant to compare the Kinetic Energy of the different rounds used for the different tests. There are a number of parameters that have not been discussed in this blog post such as the multi shot spacing and the shape of the round.

Sunday, February 13, 2011

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%.