Scientific Definition of "Optically Clear"

S

Setec Astronomy

Well-known member
Since some members like to use the term "optically clear" when describing certain LSP's, I was wondering if anyone could shed any light on the scientific or engineering principles associated with this. I have searched the internet and haven't really found any definitions or discussions of the optical clarity of coatings or films, and certainly not any comparisons, etc.



I did look into thin-film interference (the phenomenon that causes the rainbow effects in soap bubbles), but all the formulas regarding that involved measurable film thicknesses and their relationship to the wavelength of various frequencies of light...and as far as I know, no one has been able to state that there is a discernable thickness to an LSP.



How about some of you other engineering types, can you point me to some textbook-type reference which would indicate what kind of frequency shift we'd be talking about to change the color of the reflected light, whether that would be soley related to the thickness of the film or also due to the "clearness", which direction in the spectrum it would be shifted, etc.?



Since we are theoretically talking about a film layer (LSP) on top of another film layer (clearcoat) in most cases, perhaps there is some diffraction or other spectrum shift going on, any ideas?



EDIT: I forgot my other question...above I was talking about effects on appearance of your vehicle of an optically clear vs. non-optically clear LSP, whether one or the other causes a perceived color change to your vehicle, but the obvious question from a product development viewpoint is how is "optical clarity" measured, there must be some objective measurement for this parameter, but I have been unable to find any apparatus, equipment, or test methods, or even a definition for "optical clarity". Can anyone point me in the right direction? Perhaps someone in the paint or coatings biz?
 
as mentioned elswhere it's my opinion that some products change the appearance noticably. How exactly that occurs I couldn't explain from a scientific standpoint. I'm not sure if any manufacturers have means of testing why those changes occur and if they know if they'd share that knowledge with the public.
 
I'm sorry, but we live in a world with physical rules. Optics is based on science and engineering, whatever this phenomenon is, it is not some knowledge limited to LSP manufacturers. If it is a parameter that is capable of being controlled or measured, people have controlled, measured, and specified how to do so. Everything manufactured item or chemical you come in contact with (other than works of art or Spanish Fly) is engineered, specified, measured, and controlled. That's how the gasoline you buy in Pittsburgh works the same as the gasoline I buy in NJ, because there is a specification, a test method, etc. It's how when you say you wear a 42 size coat people know what that means. It's how your food has a label on it that says what's in it. If "optical clarity" is a real parameter, effect, phenomenon, its measurement and control has been delineated by some standards organization (ASTM, ASME, etc.), researched and documented by universities or other R&D organizations, or discussed in some trade magazine (Waxes Monthly).



EDIT: Here's a gloss meter Materials Test & Measurement :: Glossmeter - Description which allows you to measure gloss as related to a standard. This seems to have nothing to do with optical clarity, as it's not mentioned. But somone might specifiy that an item must have a gloss measurement of >70 when measured with a Taber Micro-Gloss 20°. So if the parameter you were desirous of was optical clarity, you would need a way to define and measure it, to make sure that your process/product was repeatable/consistent, to quantify any changes you were making during development, etc.
 
i have only heard the term used in zaino marketing and on this forum...but to me it is used incorrectly. IMHO, "optically clear" would mean that it is, well...CLEAR. no effect would be made on the underlying layers' appearance since it is clear. since no optical effects take place, then only durability would be added and appearance would depend 100% on prep.



BUT...that is not the case...most LSPs make a surface "shiny" or "bright" or "deeper". so, it is not "clear" then...but changing the optical appearance somehow.



this also reminds me of how i question various layering habits discussed here. i've read that someone will use something like collinite as the first layer for protection...stating that it doesn't look good enough...so then natty's or similar is put on top to make it look better (while keeping the durability of the collinite's layer). if collinite has already made the appearance "bad"...how can a top layer correct that since the initial optics are already disturbed?



just throwing out ideas to talk about. :)
 
I suppose "optically clear" is a marketing term, not a scientific one.



Diffuse and specular reflectivity are more useful terms if you want a scientific discussion. I expect most people would regard a product with low diffuse reflectivity as being "optically clear".
 
Yes, I think the term is a marketing one. A product which would be totally and truly "optically clear," would, quite frankly, leave the paint looking exactly as it did before.



What people really look for in LSP is optical distortion - but a certain kind. Whether its a "darkening" of the paint, or making the reflections "deep" or "wet," it is a certain type of distortion. I'm not sure if I'm making any sense, but this is what I think.
 
Although I am an engineer, I have to admit that optics is not my forte. Its been a few years since my physics classes but here goes:



The term ‘optical clarity’ is difficult to define, as it is often highly subjective. However, we can measure light transmittance which does have standard test methods (i.e ASTM D-1003 - Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics). Basically, this test method is used to evaluate light transmission and scattering of transparent plastics for a defined specimen thickness.



There are multiple factors that all effect optical clarity or light transmittance. This may include surface reflections, internal scatter, refraction, angle of the incidental light, crystallinity and likely many more that I don’t claim to know or remember at this instance.



The perceived transparency (optical clarity) is dependent on the thickness of the sample. Very thin films such as LSP’s and the like can appear “optically clear� since a majority of the incidental light is transmitted. As thickness increases, so does the possibility of encountering constituents within the specimen that absorb light (pigments, contamination, etc.) Plastics (like polycarbonate) are often used in safety glass because they have a high light transmittance and are very strong. Previous attempts at safety glass (I can’t remember what was used exactly) tended to look yellow or slightly opaque. This was due in part by the extreme thicknesses required, on the order of inches, and also the fact that this particular polymer tends to absorb some light.



I hope this makes sense and sheds some light on the issue for you.
 
etml12 said:
However, we can measure light transmittance which does have standard test methods (i.e ASTM D-1003 - Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics).

The perceived transparency (optical clarity) is dependent on the thickness of the sample. Very thin films such as LSP’s and the like can appear “optically clear� since a majority of the incidental light is transmitted. As thickness increases, so does the possibility of encountering constituents within the specimen that absorb light (pigments, contamination, etc.)
Click to expand...



Ok, now we're getting somewhere! I presume the ASTM test method you describe would not be suitable for a coating (although presumably there must also a coating test method which would use an "optically clear" substrate, or one whose effects could be subtracted).



So would you say that any perceived spectrum shift would be entirely due to the thickness of the LSP? Since we are talking about transmittance/attenuation/phase shift of reflected light as it passes thru the LSP (and the clear coat, if applicable) and is reflected off the paint/color coat, what kind of instrument would be used to measure that? I was just looking at laser interferometers, but that doesn't seem to be the right thing. This does seem like a parameter that might have been measured for something like clear coating road signs or something else that has a protective layer on it, so there must be some existing methods. I'll have to run through the ASTM index.
 
Mike,



I think what’s causing your confusion here isn’t so much an optical effect as a psychological/linguistic one. (But I'm sure there's an optical effect buried in there somewhere.)



People choose their words based on their own experiences and the slice of culture/language/education/traditions that they’ve been exposed to. As a technoid you’re used to dealing with the scientific perspective on things. You have an understanding of the common language of science and how to describe things in those terms.



Most people haven’t been exposed to the scientific way of doing things. And since they can only use words in a way that’s consistent with their background it won’t necessarily coincide with the textbooks. That doesn’t mean their observations are invalid, only that verbal descriptions will need to be interpreted differently.



I think if you ask guys with formal optics training how they would interpret “optical clarity� they’d probably say something about high (however you choose to define low/high) percentage transmission of energy in the spectral range of interest combined with low (ditto) geometric distortion of image structure. I doubt the typical person in society would think along those lines. Or if they did they won’t use those words to convey the thought.



I find it usually takes a fair amount of interactive discussion to zero in on how the verbal descriptions provided by a non-academically trained observer relate to the physics of a situation. Sometimes it can’t be done at all without extensive experimentation and statistical correlation of verbal descriptions to physical measurements.



It’s not just people without technical training. I’m constantly amused, aghast, annoyed, etc by the interactions I witness between highly trained technical people from different backgrounds. They can be by far the worst.





PC.
 
I'm thinking that the "darkening" effect that some products have is really a low surface reflectance. That is, it looks darker because you are seeing the paint and not the lsp.
 
Interesting post/topic.



Maybe even a 'sticky post' if we can get a scientific explanation for several other commonly used terms on the board(s).



How about....



Clarity



Wet



Liquid Depth



Reflective



Gloss



Shine



3-D



[ ADD MORE HERE ]
 
The only deffinition of "optically clear" that refers to detailing products is when a product is CLEAR, this is IMO. TW ICE would be an example.



However.... again IMO, LSP layers are so minimal that I dont believe the hype that some LSPs actually tint the vehicles surface. For instance.. when I use #21 on silver or light colored vehicles.. I deffinately do not see a purple tinge to them afterwards. Same thing with #26... never seen the yellowing some talk about.



Personally.. there are a small handfull of people here that love to push that "optically clear" line when speaking of some products. I find it comical actually. Id like to see some kind of scientific proof that an "optically clear" product will produce a better surface appearance then say... something that is not optically clear.



I like this thread. :waxing: :woot:
 
Yeah, I agree that a coat of lsp is too thin for it's light absorption properties to be significant.



It would be the reflective properties that matter most.
 
35TH LE said:
The only deffinition of "optically clear" that refers to detailing products is when a product is CLEAR, this is IMO. TW ICE would be an example.



However.... again IMO, LSP layers are so minimal that I dont believe the hype that some LSPs actually tint the vehicles surface. For instance.. when I use #21 on silver or light colored vehicles.. I deffinately do not see a purple tinge to them afterwards. Same thing with #26... never seen the yellowing some talk about.



Personally.. there are a small handfull of people here that love to push that "optically clear" line when speaking of some products. I find it comical actually. Id like to see some kind of scientific proof that an "optically clear" product will produce a better surface appearance then say... something that is not optically clear.



I like this thread. :waxing: :woot:
Click to expand...



Have you tested/used Zaino CS? (No, I'm not calling you out)



It's the first product that I've ever called 'optically clear' after application, but I'm not a scientist.



Wipe on and walk away.



And yes... the surface appears clear in terms of not having to wipe off a residue/film.



This topic was carried over from another thread, so I wanted to 'clear' it up a bit :cooleek:
 
35TH LE said:
However.... again IMO, LSP layers are so minimal that I dont believe the hype that some LSPs actually tint the vehicles surface. For instance.. when I use #21 on silver or light colored vehicles.. I deffinately do not see a purple tinge to them afterwards. Same thing with #26... never seen the yellowing some talk about.
Click to expand...



I originally interpreted those remarks to be a suggestion that the color of the LSP in the jar was changing the appearance of the vehicle. That may be the suggestion of some, but others are suggesting that the LSP changes the way the light is reflected off of the car, and imparting a perceived color change to the car, irrespective of the color of the LSP. Enough people have suggested this, and that certain LSP's do it and others don't, that it piqued my scientific curiosity as to what the effect is and how it would be measured/quantified.



After all, it is true that thin films on things do have measurable effects; consider anti-glare coatings on eyeglasses or low-E coatings on house windows (or the soap bubbles I previously mentioned).
 
I'm not trying to disprove anybody’s notions about this clarity issue as is could be quite complex but here is some food for thought. One of the properties I mentioned in my earlier post that affects light transmittance is refraction. In simple terms, the refractive index (RI) of a material is a measure of how much slower the light travels through it. This has a visual effect of “bending� the light. Think of a stick partially submerged in water and how the submerged portion is not collinear with the upper portion in air. So, RI’s occur any time there is an interface between two dissimilar products. If one was to layer multiple products it could be possible to cause a significant amount of refraction despite these layers only being thin films. Product lines like Zaino on the other hand may not exhibit this behavior if all the layered products are optically clear and have the same RI. In this case the light won’t be bent multiple times allowing more light transmittance.
 
mgm121499 said:
Interesting post/topic.



Maybe even a 'sticky post' if we can get a scientific explanation for several other commonly used terms on the board(s).



How about....



Clarity



Wet



Liquid Depth



Reflective



Gloss



Shine



3-D



[ ADD MORE HERE ]
Click to expand...



So are you saying that "optically clear" is just another term of marketing hyperbole like the ones you have listed? I was giving "optically clear" the benefit of the doubt as being a real term, whose significance could be objectively measured.
 
the other pc said:
Mike,



I think what’s causing your confusion here isn’t so much an optical effect as a psychological/linguistic one...People choose their words based on their own experiences and the slice of culture/language/education/traditions that they’ve been exposed to.
Click to expand...



Basically, (to borrow from Setec) "marketing hyperbole."



Reminds me of wine tasting and the "hint of tobacco" or other phraseology that'll describe a red wine. Now I'm a former smoker and a wine drinker and you couldn't get me to say that there is a tobacco flavor in it. Nor do wine makers put tobacco in their wine.
 
But Spilch, Autopians do note differences in appearance between LSP's, don't they? Such as some mute flake or cause it to pop. Or is it really a complex interaction (as others have noted) between polishing done, brightness and/or angle of viewing light?



On a sunny day, the sky looks blue...but at the end of the day, when the sun is going down, the sky looks other colors...has the air changed? No, we're at a different viewing angle, looking through more air, seeing a spectrum shift, etc.



So, is it our imagination that different LSP's can give different properties, that we are not realizing the full extent of other variables on how we perceive our vehicle? Or is it that different LSP's result in different film thicknesses, which refract light differently? Or is it actually that different LSP's have different refractive indexes (which should be measurable, as in my original postulate)?
 
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