Data and Testing Definitions and Intro

**wbassett** · **02-24-07, 04:09 AM**

Index

Intro- This Post
RGB values
CIE L*ab
Neutral Reference Point
More on the 'V' Curve
Q&A

DIY, it means different things to different people. To most I think it initially means Do it Yourself to save money. Some enjoy DIY screens as a hobby, while others think it’s cheap and inferior. It is less expensive that is for sure, but it has come a long ways since the days of bed sheets. Most I think are looking for a good screen while they decide on what commercial screen they really want... many are often surprised at how well some of the DIY methods perform and often rethink spending all that money on a commercial screen.

Data and testing are not something people are used to hearing or seeing when DIY screens are the topic. Sure people do screen shots and comparisons, but prior to a short nine months ago no real data was ever really done on DIY screens. Everything pretty much used to be a ‘visual’ check from a color swatch. Some have worked very well, but in the end nobody really knew anything about the color itself and it was a lot of guessing and opinions, even with mixes and advanced methods.

For those familiar with commercial screens, some of the data and testing that will be presented will be very familiar looking, other data may seem a bit foreign. That is because it is more in depth than what is usually seen at the consumer level. Much of the same type of testing does go into commercial screens; we just don’t see it at the end user level. DIY has a stigma and because of that there are more and higher hurdles to get past to show it isn’t sub par.

My goal is to try to help legitimize DIY screens and get more people to at least take a look at it as a viable option. A lot of people seem to think a DIY screen is only paint, and that is not the case. There are some methods that are not very different than buying a fixed frame screen. A commercial fixed frame screen comes with the screen material and the border. You have to assemble it and hang it. Some of the DIY substrate methods are very similar, the differences being you have to cut it to the desired size, make your border, then assemble it and hang it. Performance wise, some are on par with mid to high end commercial screens.

I will try to keep things to a minimum in the threads and not overwhelm things with too much data. I understand people really want to just find something that will work for them, but it is nice to see supporting data to back things up.

**wbassett** · **02-24-07, 04:32 AM**

All the methods I research and test are checked for their color composition. RGB is an excellent way to get a visual of the shade and hue of a color, and it also shows the color components. Too high (push) or too low (deficiency) in any of the components and that screen color will start to have an adverse affect on the image. RGB by itself is only part of the overall picture though. CIE color space plots show where the color falls, how much it leans towards one color shift, or how close it is to our neutral reference target of D65. Ideally the closer to D65 neutral the better, but I will try to show and explain why some colors are still very good choices.

CIE L*ab D65 is tied to sRGB so the two do translate very well for discussions.

First, I felt that before anyone could really do anything with DIY we should look at commercial screens. Knowing and understanding what they are doing allows us to build a foundation. Everything needs a strong foundation to build on. There are still a lot of aspects of commercial screens that are unknown, but a lot of ground has been made.

The color swatch gives a good visual idea of the colors, but there is much more than what our eye sees.
I took the RGB values and plotted them on a graph. I like the graph in addition to the swatch chart because I can see the order from light to dark, plus when graphed you can see the color curve better. We always look for a neutral color because that is the best palate, but it is interesting to see a lot of commercial screens are slightly green deficient.

Also once plotted it is easy to see how flat the color line is. Ideally a neutral color would be completely flat, yet none of these swatches are. Most look like a ‘V’, hence I coined the term ‘V Curve’ which will be mentioned a lot throughout the DIY forum. This is where that originated. So why are commercial screens Green deficient? There have been some very good discussions on this. Primarily the reason is green pushes the hardest of the three component colors. Not everyone goes through a thorough calibration with a disc like Avia or Digital Video Essentials. Projector manufacturers shoot for D65, but out of the box most are off some. A screen that is slightly Green deficient errs on the safe side and most people can get a satisfactory and very pleasing image right out of the box without knowing how to do a full calibration.

There are other reasons for a slight Green deficiency and they are:

Too much green makes skin tones look bad; too little is not very noticeable.
Titanium dioxide/lampblack/raw umber are inexpensive, nontoxic pigments.
Effective green pigments are expensive and toxic (cadmium, etc.)
A truly neutral screen is ugly when the lights are on.
True neutral does provide some advantage - meaningful to us, but probably not to them. Commercial companies make screens that work well for the largest number of users while providing good performance at the least expense.
A true neutral color would require extremely close manufacturing tolerances.

Here are some examples of color push and how it affects the overall image. The first one is the correct color image that the projector should be putting out if calibrated to a neutral white reference screen.

Next is if the screen pushes Red.

Now we will see a Green push.

And last a Blue push.

It can be seen that Red and Blue are the most forgiving and some people actually like a warmer image (Red) or a cooler one (Blue), but these settings should be done at the projector and not the screen.

Green goes strong very quickly and really makes an impact on the overall image. Keep in mind, when there is a color shift, it isn't just that color that is affected. A Blue push will cause Reds to look purplish, yellows to look greenish, and so on.

What I have found is the 'V' curve isn't bad but it must have certain parameters met in order to function as a good screen. The reason I suspect some commercial screens have the slight blue push above just creating a cooler temperature screen is to punch up the whites. This makes sense because they moved the color away from neutral which would have some color shifting and could cause whites to dull. The blue is like adding bluing to laundry. By adding blue it gives a perceived extra brightness back in the whites so people don't notice that they were off. Since people don't normally go around comparing two and three test panels to their screens, they never really notice the difference or that their image can be dramatically better. That is unless the screen color pushes too far to one color or another, and then it is very noticable.

**wbassett** · **02-24-07, 04:59 AM**

As nice as RGB numbers are for getting a quick ‘visual’, the CIE Color Space graph shows exactly where a color plots in reference to our D65 neutral target. Between the two we get a complete picture of the color information. From there we now can determine colors that are best suited for a screen and compare them to commercial screens.

Here is a CIE graph of the above color swatch and RGB graph

(NOTE: The above readings were based on C and are in the process of being redone and referenced to D65)
Ideally we would want to hit D65 since that is the reference point manufacturers shoot for (but usually slightly miss with default settings). Since there are many consumers that do not run through a full calibration and go with out of the box settings with 'eyeball' tweaks, the 'V' curve screen is a safe zone for manufacturers. It can be improved upon though and that will be discussed in a seperate thread. The 'V' target is based on a D65 neutral reference point and then pulling the Green component deficient by 6 points. I went with 6 because the average deficency I have seen with commercial screens is around 5-7 points low. That plot is where it would fall if Green were deficient and the Red and Blue components were equal in value. This is not a neutral screen in the sense it doesn't fall within our D65 neutral reference point, but it is an extremely well balanced screen color. It is just about mid way between D65 and C, slightly to the right, and any projector can easily compensate and calibrate to a screen such as this. Colors that are close will have a very slight shift, but the closer to neutral D65 (or C) or the 'V' target, the easier it will be to calibrate and get the best image.

The CIE plot is divided up into different color quadrants. The illuminant reference points are neutral references for that specific illuminant. We deal with D65 as our target. There are several illuminants that are widely used by the color industry and these include A, C, D65, and TL84. Illuminants A and C were defined by the CIE in 1931 to represent tungsten light and natural daylight respectively. Illuminant C was found to be a poor representation of daylight in that it contains insufficient energy at the lower wavelengths and it has generally been replaced by a class of illuminants known as the D illuminants.

Next is a breakdown of the actual color zones in the CIE chart. The above is how it is normally seen, below shows the color areas more defined.

I want to take a brief moment to explain the above charts. Unlike the RGB values that provide us with a visual indication of what the color looks like, the CIE plot is based on the color's xy values. These values alone will not show what the color itself is, just where it falls in relation to our neutral reference. It is possible for two colors to have the same xy values and plot in the same location, but be completely different shades.

Neutral reference points are a moving target. They are different depending on the Illuminant value. D65 has replaced the C Illuminant as an international standard, and it is also what the Home Theater industry uses as their neutral/white point reference. That is what I target and reference as well.

**wbassett** · **02-24-07, 05:00 AM**

Place holder-- information to be added.

**wbassett** · **02-24-07, 05:18 AM**

This is also posted in the Neutral Gray thread as well. A lot of topics overlap each other and are very much intertwined. Some people may not be interested in a particular thread, so they may never see certain information. Normally I would just post a link, but this is key to screen color balance in my opinion and this is definitely on topic in here.

A quick note: There has been some recent talk by some that RGB is meaningless and shouldn't be used. I disagree. It is an excellent way of presenting a 'snapshot' of a color. Not only does it give us a visual representation of what a color looks like, it also gives us a quick indication of that color's balance. RGB alone doesn't tell the whole story, and for those of use doing testing and research we tend to dig deeper and go into some areas that quite honestly most people just don't care about. So in that sense RGB is perfectly fine in my opinion as a presentation tool and quick color balance indicator, but it certainly is not what everything is solely based on.

There was also some debate on conversions. Some of the charts and numerical values may change slightly and I will update them if that is required. Still the "V" theory is a sound principle. I have discussed this with a few college professors that teach light and color theory and they agree with the concept.

So what is the 'V' curve, and why do I feel it is important? First let's take a look at the typical Home Theater. A Home Theater that is comprised of a front projector has many components and variables that ultimately determine the overall performance. Some items and elements we can control, like the projector's color balance, brightness, contrast, gamma settings and so on... and then there are other items that are harder to control or in some cases impossible, such as sunlight, room layout, etc. We really shouldn't be using the screen as an 'adjustment', only if there is a reason to do that. The color balance of a screen may be one of the most critical items in an entire home theater setup. If the screen skews the colors of the projected image, it is not a very good screen. Ideally we want a screen that reflects the projected image the most accurately and efficiently- accuracy for the image color, and efficiency for the overall brightness.

White screens are the easiest and most forgiving, but even they need to have a good color balance or it can shift the projected image's color.

There always seems to be an ongoing debate about color balance, especially with gray. Either about its ambient properties, or that it takes more than gray to make an ambient light screen, or it kills whites and makes images look muddy… to it is the best type of screen a person could have. So who’s right and who’s wrong? Everybody and nobody.

A lot depends on a person’s personal tastes and what they like. Gray undeniably helps with black levels and perceived contrast for projectors with less than stellar contrast ratios. It is also better with lighting in a room and can deal with ambient light better than a white screen can. I still do not feel this makes it a true ambient light screen, but as stated by others I agree it is better than a white screen and more appropriately should be called 'ambient light friendly'. What is important and something every DIY method should strive for is color balance.

I find it interesting that neutral gray is viewed by some as the Holy Grail of gray screens. This has been debated for a long time now. Some have gone back and forth in this debate, stating neutral is the way to go and then later questioning why the importance of a neutral. I have questioned this myself since manufactured screens are not completely neutral. If you look at the color curve once the spectrophotometer values are converted to RGB, most have a distinctive ‘V’ curve. So the question is why would they do this? Here are some explanations I got:

They did it on purpose- so the screen will look good even if the projector is off a little. A red or blue push doesn’t look too bad, but a green push kills skin tone- and this is how many people judge image quality.
They did it accidentally- when gray came into popularity, they just used the most common standard, which is illuminant C (6774K). Many commercial grays are almost exactly on for illuminant C, but there are some that are not…
Probably the biggest reason, most projectors have a green push out of the box. Since many people just run through a quick calibration, and most of the time just by eye, it makes a screen the easiest to calibrate to for the widest range of projector's and users.

Here is what those pushes can look like as compared to the correct image.

Correct Color

Red Push

Green Push

Blue Push

Red is the most acceptable and forgiving to a point. Some people actually prefer a cooler image, so they may not mind a blue push, but green is definitely a bad thing to have any type of push in. Seeing that some Director's and Director's of Photography film with a blue tint (check out the Terminator movies sometime, especially T2, it has a distinct blue tone to it) a screen that already pushes blue would certainly take the movie image over the top and beyond what the Director intended it to look like. In some cases to a point that is unwatchable to some people.

Hopefully the reason why screen color balance is so crucial is starting to make sense.

So I asked the question if commercial screen companies aren’t concerned about their gray’s being totally neutral, should we? That’s like the opening statement about who’s right and who’s wrong… yes we should be concerned, yet no we shouldn’t… but both of those answers have a reason in my opinion.

If we look at what I call the ‘V’ curve, we see there is a green deficiency, but usually the red and blue components are relatively balanced. There are instances where a slight blue push, or a slight red one, (or a slight deficiency) can be beneficial, but mainly it would be more of a user preference. A slight red deficiency could help some with incandescent lighting since it leans in that direction, while a slight blue push can make whites appear ‘whiter’ (another color conditioning we all have had since we were born). The trick and key is knowing when the push is too much.

A few years ago DIY was dealing with these questions and ideas, but for the most part there was no data. The spectrophotometer tests that have been done recently, as well as the converting of that data to RGB values has done a lot in the way of understanding screen balance as well as now knowing the color composition of DIY methods. RGB is widely debated by some as being useless when discussing screens. I disagree. It’s hard for the average person to understand CIE data and looking at those numbers isn’t easy for someone not used to what they mean to determine if a color is pushing to hard one way or another. RGB makes it easy to quickly see the color curve. If the ‘V’ is in line with typical commercial screens, then yes that color would most likely make a nice screen- as long as the color itself isn’t some wild off the wall color. If it is a white or gray then I would have no problem using it as a screen.

So I just completely contradicted myself when it comes to neutral colors vs a well balanced color? Not really. Even though it was just stated that many screens tend to have a slight green deficiency, that doesn’t mean a neutral color is bad… just that the commercial companies strive for the best image across the widest range of projectors AND consumers tastes. As stated earlier, green can push hard even with the slightest increase in RGB value. Most people only do basic calibrations. This way the commercial screen looks good out of the box to most users and projectors.

Seeing color is a sensation, like hearing, taste, or smell. Sensations are not felt the same way by every person. Food tastes differently to each person. In the same way, there is no absolute color that is inherently seen the same way by every person. Nor is every person’s vision the same. This is where neutrals come into play.

Neutral gray will eliminate/reduce color contamination from reflected light. Even though slight variances in screen composition and colors will work fine, a neutral palate is the best at reflecting the colors back the most accurately.

Also different light sources affect the colors that you see. For instance, a color viewed under fluorescent light will look radically different when viewed under incandescent light. Fluorescent light adds green to colors while incandescent light adds red. (This is why a slight red deficiency in screen color can be helpful with incandescent lighting)

A front projection Home Theater system consists of several devices that all deal with color at some level, and in different ways. Projectors deal with colors being created by light, and the screen deals with colors being reflected by pigments. Because all of these components in the system handle color in different ways, color reproduction between them is not so obvious. They use different color models, have different color gamuts and different Gammas. Moreover their colors are influenced by calibration settings and environment. Again neutral colored screens reflect the light from the projector with the least amount of color skewing. The other option is a screen that has a good color balance.

I am not abandoning any previous research and testing I have done. I think both can coexist… as long as the ‘V’ curve isn’t radically pushed by the red or blue components it will work just fine. Neutral grays however will allow us to go darker with fewer color problems than if the gray is not neutral and meant to be an aesthetically pleasing color as a wall paint. Also a neutral color reflects light the most efficiently, therefore the image tends to be brighter even with darker shades of gray.

Many people have been searching for neutral grays for a long time now. It’s not that they didn’t know what they were looking for; it has more to do with technology and more readily available data. Now we can actually see spectro data and the color breakdown where a couple of years ago that type of data didn’t exist in the DIY realm, at least not from what I have seen. So both a neutral screen or a well balanced screen or 'V' curve screen will definitely work, and like I said earlier, everyone’s tastes are different. I know when I finally found a ‘compromise’ with one gray paint I used, it still wasn’t optimal. Even though I probably could have gotten used to the color shift, anyone else walking in for the first time would most likely see it was off since they were not ‘adjusted’ to the screen themselves. To me that is a bad screen. People should not be told ‘Well you’ll get used to it…’ there should be no reason to... and that is where color balance comes into play and is so important.

**wbassett** · **02-24-07, 05:18 AM**

If anyone has any questions I will try to answer them. If I do not know the answer, I have some contacts and resources that are color scientists and they should be able to answer them. I am also enrolled in the color and light theory course at Rensselaer Polytechnic Institute, so hopefully I will be better able to explain things as well as further my research on screens and their construction.

The rest of the threads will not be like this. They will have references to data but I will try to keep it to only what is needed to show a particular method's strengths or weaknesses.

**wbassett** · **08-09-07, 11:45 AM**

Lumen ratings are what the projector puts out. One thing to note is this can be misleading in several ways.

First, many times the Lumen rating is the maximum Lumens the projector can produce. Although this is not lying, it can and usually is vastly different than what the Lumen output is when the projector is calibrated and video optimized. Typically video optimized Lumens runs around 1/3 of the max rating.

There is a formula to calculate foot Lamberts at the screen and it is-
ft lamberts = (projector ansi lumens * screen gain) / screen surface area in square feet

Remember though to use video optimized lumens and not the raw max lumen rating.

To make things even more complicated, some manufacturers are now listing projector Lumen ratings with the video optimized rating, while others are still listing the max Lumen rating. This can be very misleading because two projectors, both rated at 1000 Lumens could have drastically different light output levels. The projector that has video optimized Lumens specified will actually be brighter. This really confuses people when they are looking at a 1000 Lumen projector and a 1200 or 1500 Lumen rated projector. They tend to see the numbers and instantly want the higher specs.

CR is also a victim of the numbers game and here is an outstanding explanation of contrast ratio and how the numbers are inflated.

So, what does all of this mean and what are we really looking for?

12 foot-Lamberts is considered the minimum screen brightness for a completely light controlled room. Many people may disagree with that, and some have far less than that and find it acceptable, but that is the recommended min. The SMPTE and THX standard is 16 foot-Lamberts, and movie theaters usually produce between 12 to 22 fL of light at the screen. Television produces 35 fL or more depending if it's standard definition TV or HDTV.

So if you are less than 12 fL at the screen, that means you either should think about

A brighter projector
A smaller screen
Higher gain

For those that already purchased a projector, option 1 on the list is out of the question. That leaves a smaller screen or higher gain.

Gain is tricky. Some people guess at gain based on comparisons to other materials. This could be dead on accurate, but it certainly isn't reliable. Even commercial companies are sometimes guilty of using testing methods that do not adhere to industry standards and as such they may list 'custom' gains that are unrealistic. One company lists a gain of 1.8 for their CRT White, however when it was formally tested and reviewed, it was not as bright as a StudioTek 130 which has a gain of 1.3. The estimated gain was placed at 1.0 when the company stated it was 1.8.

Gain can bring the fL up to the level needed for a good sharp and vivid image, but many times people get caught up in the numbers. Higher gain is not always better. When gain goes up, the viewing cone goes down. Viewing Cone is the angle at which the screen brightness drops off dramatically from the on axis image. This cannot be avoided, as gain goes up viewing cone goes down, period.

Gain is a measurement of the reflectivity of any screen or projection surface. The gain number represents a ratio of the light that is reflected from the screen as compared to the light reflected from a magnesium oxide reference source. So a screen rated with a gain of 1.0 will reflect the same amount of light as a magnesium oxide reference, and a screen rated at a gain of 1.5 will reflect 50% more than the reference source. This does not mean light is being produced. There can never be more light than what the projector itself creates, and from the instant it leaves the bulb, the energy starts to decrease.

How this works is a 1.0 gain screen is referred to as a unity gain screen. This type of screen has an even amount of light dispersion across the entire surface of the screen and will look just as bright off axis as on axis. Once the gain increases, the light has to come from somewhere and in essence it comes from the off angle reflected light. Since more light is being focused and returned to the on axis viewing position, the off axis angles become dimmer. For the most part this is not a big issue since people normally do not sit at that far of an angle to a screen, but in certain venues such as a church, auditorium, or a bar, there usually are extreme viewing angles present. The angle at which the gain reading drops to 50% of the peak value is known as the Half Gain Viewing Angle. A person viewing the screen from this angle will see an image half as bright as the person seated at the center position.

A high gain screen does not typically reflect red, green, and blue equally. This can produce color shifts in the image that are very noticeable when the screen is viewed from different angles.

Ideally for Home Theater use, the gain is normally between 1.0 to 1.8. That isn't a set rule though and some may prefer a brighter image, but the brighter the image is, the more eye fatigue that occurs. There will be a point that even a two hour movie can induce a headache from strained eyes caused by a screen being too bright.

**wbassett** · **08-09-07, 01:15 PM**

Here is an outstanding video that explains the basics of CIE and color balance, neutrals as well as a demonstration of an actual spectrophotometer.

This is for the Whibal White Balance card, but all of the color theory information is still the same for us.

Now this isn't an in-depth class or anything like that, but it will shed some light on all of this color talk... yeah I know, bad pun!

Anyway, if you've read through this thread this far then you at least have an interest in all of this. Some people may already know all of the information presented here inside and out, while others are reading this thread and saying 'I still don't quite understand it all'. Hopefully this excellent video will clear things up for many.

Oh, he even shows why paper isn't always good as a reference and in our case, as a screen. What may look white usually isn't, especially with paper. That's exactly what was explained in this and the Neutral Gray thread about how what we think is gray usually isn't... again, enjoy the video, I thought it was very good at explaining things in an easy to understand way.

**wbassett** · **08-17-07, 09:35 PM**

It was requested/suggested to add these discussions to the main sticky. I'll only transpose the informational posts and the discussions can still take place in the other thread.

There is always an interest in gain when it comes to screens. I have recently seen many discussions and questions about gain. Some statements are valid, some are conjecture, but there does seem to be a lot of confusion about screen gain and the various ways it is achieved. More importantly there seems to be some questions about the pros and cons of gain and how it is achieved.

First a brief bit of history about the ‘Silver Screen’.
At the turn of the century, 1909 to be specific, motion pictures were becoming the rage. One problem that plagued the fledgling theater industry was that projectors and screen materials at that time were extremely limited and crude by today’s standards. This resulted in images being very dim and quite hard to see, but the public was still fascinated with moving pictures.

Adele DeBerri owned a theater in Chicago during this era. She was a unique individual, remember this was an era when women typically did not own and operate a business. Not only was she a pioneer in that respect, but she was an innovator as well. Adele had the idea to paint the screen image area with a silver paint that was highly reflective and therefore would reflect more light back at the viewing audience. That’s how the ‘Silver Screen’ was born. What many may not be aware of is Adele went on to developed a silver painted canvas projection screen that quickly became the standard for the industry. Da-Lite Screen Company, Inc. is the successor to the business founded in Chicago in 1909 by Adele DeBerri.

The reason for high gain screens date back to the earlier example of turn of the century projectors that needed something to focus the light and make the image brighter. Today projectors are much more sophisticated and ten fold better than the old ‘moving picture’ projectors back at the turn of the century. The projectors currently available are so bright that if a person was to look directly into the lens it could cause permanent eye damage. So if our newer projectors are significantly brighter than even projectors made ten years ago let alone turn of the century technology, and gain is to produce a brighter image- why do people still seek a high gain screen? Situation and setting is often the main reason. Excessively large screens also come to mind.

Why does “higher gain” make a screen appear better? Or does it? And how is “gain” achieved?

First, gain is the ratio of brightness of your projection screen material to a white standard such as barium sulfate or magnesium carbonate. These materials are used by the industry to set a flat white “Lambertian” light distribution where every point in the audience would see the same image brightness. A gain of 1 represents a screen as bright as magnesium carbonate. A gain of 1.1 means it is 10% brighter and a gain of 2 means a screen is twice as bright.

If a surface exhibits Lambertian reflectance, light projected on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view. More technically, the surface luminance is the same regardless of angle of view.

Like the saying, “Nothing in life is free” and a screen cannot create light. Gain is not the creation of light, it is the focusing and redirecting the diffused light back at the viewer. The price of this increased brightness is field of view, or viewing cone. The center of the screen and on axis viewing in the ‘sweet spot’ looks great, but anyone sitting off to the side will see a darker, sometimes even unwatchable image. So to increase the gain, or image brightness on axis, the light has to come from somewhere. Since a screen cannot create light, the off axis light is refocused back at the viewer.

Too much gain has other negative effects too. Those viewing the image directly in front of the projector screen may have to endure a movie that is uncomfortably bright. And then there is the effect called “hot spotting” where the image literally shows a bright circle in the center of the image.

Gain in front projector screen materials is made by adding “mirror-like” materials that will reflect light back at the projector instead of diffusing the reflected light in a Lambertian distribution.

In the beginning it was silver, then came glass beaded products. In 1954, the first pearlescent materials were introduced. Pearlescents were innovative because they were clear and sparkly instead of silver. (That’s right, pearlescents aren’t something new or revolutionary)

Pearlescents work by the process of interference. The first interference pigments were obtained and produced by Nature. Mother of pearl is an excellent example. Some seashells also have the interference effect. Interference is the separation of white light into its component colors much like what a prism does when it refracts light. By making a series of thin layers that are clear and refract light, an interference pattern is set up. Sparkle and shimmer are two ways to describe pearlescence.

Man made pearlescents are made from the mineral mica. The mica is processed into small particles and then coated with a very thin layer of titanium dioxide. The layer is so thin that it actually allows light to pass through instead of acting like a normal pigment when it is used in paint.

Here is the problem: each layer reflects a small percentage at the front face of the particle, most of the light is transmitted (passed through the mica flake) and refracted. When light hits the back surface of the particle, a small percentage is again reflected and most continues on through. If the particles get stacked one upon another, the number of light reflections gets to be quite large. The resulting sparkle can look bright and somewhat impressive. If the concentration of flakes isn’t dense enough, then the screen can look ‘sparkly’ when hit with high powered projector light. The down side is color shift and light separation.

There is a newer solution and way around this prism effect and to increase gain without introducing color shifting other than 1954 technology and methods, and that is by the use of non-interference pigments. Instead of being based on mica, the key particle is aluminum oxide. It is thick enough so that it is opaque and will not allow light to pass through it as mica flakes (Pearlescent) do. It is then coated with the same thin layer of titanium dioxide that the interference pigment was but the optical results are much different. The reflections are reduced and the degree of color separation is minimized. Also the prism effect is eliminated, which in turn eliminates or greatly reduces any color shifting.

Surface sheen is another way to quickly increase the gain, but it also raises the specular gain which almost always results in hot spotting. The higher the projector’s Lumen output, the more noticeable the hot spotting becomes.

Specular gain is gain that is not created from the base color. In other words, a bright white screen will have a higher natural gain than a darker gray screen. If a shiny surface coating is applied over the same two screens, the gain on the white screen would be higher than the gray screen since the base itself is adding to the overall gain. The base gray also adds to the overall gain, but at a lower ratio, therefore it will have a higher specular gain.

This can be confusing, and people may say “But how can a screen with lower gain hot-spot when a higher gain screen with the same surface doesn’t?” As explained this is due to the surface sheen of the coating on the gray contributing more to the gain than the base.

Here are some graphs from when this was discussed about the laminate screens that hopefully will make more sense:

The pie charts as a whole represent the total gain. With the same surface coating for each, what changes is the ratio's of the specularity to the surface color and the basic gain of the color. If the coating remains the same, and the color gain decreases, then the ratio of specularity goes up even though the coating hasn't changed or added extra sheen.

So even though the surface coating is the same for both materials, it makes up a high percentage of the overall specular gain for Fashion Grey than it does with Designer White. With more of the gain being from the coating, the specularity has increased, and that can increase the possibility of hot spotting.

Hopefully this shed some 'light' on a few things and this thread is intended to discuss some of the confusing aspects about screens, color, and light.

**mechman** · **11-09-07, 04:24 PM**

Spectral Curves

What's a Data thread without some data?!?!?

I know a lot of you are aware that I've purchased a spectrophotometer. I've already taken oodles of readings and I think this will be where we put our findings. I'd like to take this time to thank all the members who've sent me samples to measure! One of the things we're trying to track is store variance. Especially with the True Value stores where they still have the mechanical machines mixing their paint. Thanks and keep them coming!!

We'll get the ball rolling with spectral curves!

Here are Winter Mist and Winter Mountain:

If there was ever a reason not to paint a neutral screen with pearlizing medium mixed with poly... Winter Mist and Mountain again with a pearlizing topcoat.

The above 2 spectral images are of a N8 and a N9 gray paint coated with a 1X pearlizing medium topcoat. I know that there's been a lot of recommendations to two coats of the 2X mix. NOT RECOMMENDED! Stay away from the pearl/pearlizing topcoats!

Here's a very interesting curve! This is benven's, hopefully soon to be announced, CGiv.

Wilsonart's Designer White

EasyFlex08 - this is the first and only EasyFlex that I will measure.

Behr's SilverScreen - closer to neutral than EF08

CMRA's S-I-L-V-E-R -

I've got three samples of Silver Fire. One from zductive, which turned out to be the closest to neutral but the mica was so prevalent on it that it seemed like glitter scattered on the surface. Also in this shot is Harpmaker's Silver Fire. Harp's is the black line.

And then there's my Silver Fire mix. I have been in contact with MMan trying to get some of this mix directly from him but so far I haven't gotten anything. If I do I'll edit it in here.

Sherwin Williams Gray Screen

Sherwin Williams Soothing White

Behr Ultra Pure White 1050

Kilz2

I'll have more later including some readings off commercial screens. We'll also have the RGB, L*ab values etc.

mech

wbassett-
So basically what we are doing here is pulling everything together. Some people can visualize things better one way than another.

With the RGB numbers, we can punch them in anything, MS Paint for example, and get a quick down and dirty look at a visual representation of the color. With the CIE chromacity chart we can see where a color falls in relation to our D65 neutral reference point. If it starts moving too far away we can see in which direction there will be a color shift in.

Some people have expressed a desire to see spectral graphs, and feel this is the one and only way to look at thing- so here they are!

Pretty much what we are looking for is as flat of a line as possible. If we see something that looks good, then we check the L*ab 'a' and 'b' values to see if they fall within our +/- .5 tolerance for a neutral or if it is a near neutral +/- 1. If we see a wave, or line with sharp rises or large bumps, that would be a color we would want to stay away from.

When you put all of this together, it's a very compelling set of specs that allows us to actually look at two screen methods and determine if one really is better than another. Prior to this it was a lot of speculation and opinions. And there certainly is nothing wrong with having an opinion, this does let a person step back and look at the data as well as the screenies and personal testimonials. When everything is taken into account, it is much easier to make a decision.

**mechman** · **11-09-07, 06:06 PM**

Store Variance Experiment

Posted and discussed here.

**mechman** · **11-09-07, 06:15 PM**

CIE Plots

I'm hoping that Bill or Jim will post their CIE plots and such here. It takes me a bit of time to get the data and put it into a spreadsheet and they are kind enough to do the plots!

**mechman** · **11-09-07, 06:53 PM**

Magnifications

One of the main purposes I had behind magnifying commercial screens and DIY screens is to compare quality to DIY offerings. Why? To see where DIY is at compared to commercial. I've always stated that the current crop of DIY products is only 90% of the way there and they'd never get the last 10% mainly due to better quality control at a screen manufacturing plant as opposed to someone's garage.

DL Glass Beaded Screen

HoloDisplay's HoloVega 200x

DL Silver Vision 200X

DL Silver Matte 200X

DL High Power 200X

DaLite Silver Matte at 10X

My Pigment Free Gray experiment.

Silver Fire

CMRA's S-I-L-V-E-R

Pearlizing medium over both True Value Winter Mountain and Winter Mist. This is the 1X formula applied once. The current recommendation is a 2X formula rolled on twice!

To stress again how bad of an idea this really is, here are the spectral curves for Winter Mountain pre-topcoat and post-topcoat.

If you look closely at each of the images you notice several things. For example the size of the glass beads from the High power and the Glass Beaded screens, the texture of Silver Matte, etc. What really stands out is the difference between the craft store paints and the manufactured products. The DIY screens which add mica to their mix have little specs of blue, red, yellow, green, etc. You do not see that in any of the manufactured screens or, for that matter, in my rendition of the PFG. The light hitting the screen is reflected back accurately not shifted color-wise.

Advertisement