Optical Coatings
Published on Apr 7, 2009 at 4:17 pm.
8 Comments.
Filed under astronomy, physics.
When you buy binoculars, eyepieces, or telescopes, you often run into terms such as coated optics, multi-coated optics, or fully coated optics. To the uninformed, these all sound the same. It seems like just a different choice of words to describe the same thing. Unfortunately, sometimes the sales people behind the counter selling the instruments don’t know the difference, either. Worse, sometimes the people marketing the product don’t know the difference, and they use the wrong phrase to describe it. But, the different choice of words are supposed to signify different things.
First, though, let me start off explaining what optical coatings are all about. To do that, I need to give a little physics lesson. I am a college professor, after all!
Light travels through space with a constant speed. It is always measured as 300,000 km/s, no matter how the observer is moving relative to light. The speed of light is a fixed value. However, when light passes through a medium, things are a little different. Light travels at a slower speed when passing through something than it does when it moves through a vacuum. The ratio of the speed of light in a vacuum to the speed of light in a medium is called the index of refraction, typically signified by an n in equations. The frequency of the light does not change as it passes from one medium to another, but the speed does, and thus so does the wavelength of the light.
Whenever light moves from a medium of one index of refraction to another one with a different index of refraction, then there will be a reflection. Even if both media are transparent to light, there will be some reflection from the boundary between the two. This is typical wave phenomena, and I won’t go into all of the physics behind it — you can look that up if you are interested. How much reflection occurs depends upon the properties of the media, but most of the time far less is reflected than transmitted. For a typical glass/air boundary 6 to 8 percent of the light is reflected. Sometimes this can be useful. A rather inexpensive way to protect drivers from being distracted by the headlights of a jerk driving behind you with their high beams on is to have a mirror that tilts the mirror a little, leaving simply a piece of glass to reflect light. That piece of glass reflects only a small percentage of the jerk’s high beams, making checking the mirror tolerable. But, with lenses, you normally don’t want to lose light through reflection. In fact, for astronomy, where the objects being viewed are dim to start with, losing light could make the dimmer objects difficult or impossible to see. Nonetheless, you always get a reflection from the air/glass interface. So, the trick is to try to minimize that reflection.
That is where optical coatings come in. Imagine a thin coating of some sort put on the surface of the lens, as in the above drawing. As the light passes from air to the coating, there is some reflection. Depending upon the coating, that reflection could be more or less than the reflection from air to glass. Let’s assume that most of the light keeps going (after all, if this is a lens, we want most of the light to pass through it!). When the light passes from the coating to the glass, there is yet more reflection. By the way, the same thing happens on the other side of the lens: as light leaves the lens to go back into the air, there is more reflection, though I didn’t show that in the diagram above.
Now, you might wonder what good the coating does to the lens if you are getting reflection from both sides of the coating. Isn’t that even more reflection than you’d get with just the glass? Well, maybe, and maybe not.
Let’s go back to talking about the nature of light. Light has wave properties: wavelength, frequency, etc. Waves have many interesting properties. Among those are that waves can add. But, when waves add, you get different results depending upon the relative phase of the waves when they add. So, what is this phase stuff? Imagine two waves that are completely in sync with one another. The peaks of one line up with the peaks of the other one. The troughs line up, too. This is shown below.
When these waves add, the peaks add to make bigger peaks. The troughs add to make bigger peaks in the other direction (they really are negative peaks, not troughs!). So, the end result is that you get a stronger wave. In physics, we call this constructive interference.
But, consider what would happen if the second wave were shifted a little to one side or the other. The degree of shift can be measured, and the measure of that shift is called the phase angle, or the phase shift, or sometimes just the phase. If the two waves are out of phase (in other words, they don’t line up just right), then you won’t get the constructive interference that is shown above. You might still add up the wave to be a little stronger than before, but it wouldn’t be the perfect case. But, if the two waves are completely out of phase (the peaks of one line up with the troughs of the other), then when you add them they tend to cancel each other out. That is called destructive interference, and it is shown in the following diagram.
But, how can you shift a wave? The simplest way is to simply make it travel farther. Now, that is where the lens coating comes in. Remember, some light is reflected off of the top of the coating (the air/coating interface) and some is reflected off of the bottom of the coating (the coating/lens interface). So, light that passes through the coating, reflects off of the bottom of the coating, and travels back across the coating to the top has gone a distance of twice the thickness of the coating farther than the light reflecting off of the top of the coating. That means that there will be a phase shift. If that shift is half of a wavelength, three halves of a wavelength, five halves of a wavelength, or any other shift that makes the distance such that a peak is shifted to a trough in the other wave, then destructive interference will result. If the shift is a full wavelength, two wavelengths, etc, then the peaks will be shifted to line up with other peaks in the first wave, so constructive interference will result. Such coatings change the optical properties of the lens, so they are optical coatings. Light carries energy. So, if some light is reflected, then there must be less light to pass through the lens. But, if no light is reflected due to destructive interference, the light being transmitted has to contain the energy that would have been reflected, and thus is more intense. That is what we want for lenses used for astronomy.
So, how big of a factor are these anti-reflective coatings? Remember that 6 to8 percent of the light would be reflected from the lens/air surface. Let’s take a number in between, and say 7% is reflected. That means that 93% is transmitted. So, for a single lens, you get 93% from each surface, that means that 0.93 x 0.93 = 0.86, or 86%, of the light makes it through. If you wear glasses, that would mean that things look 14% dimmer with the glasses on. Most of the time, that is not such a big deal. But, consider binoculars, with a lens at each end. Worse, you also have prisms in between. Each lens has a front and back surface, and each of the two prisms has two surfaces. That gives eight surfaces. Worse, the eyepieces are typically compound lenses, with more than one lens. So, let’s add at least two more surfaces for a total of ten. When you multiply out 0.93 ten times, you get 0.48, or 48%. That means that less than half of the light entering the binoculars would make it out the other side! Now, remember that the objective lens on the binoculars is still much larger than the pupil of your eye, so even though you lose a bit over half of the light, objects viewed through the binoculars would still look brighter through the binoculars than with the eye alone. If you are watching a football game or something where there is plenty of light, that won’t matter. But, for astronomy, your binoculars will act as if they had an objective diameter of only 71% of what they really are in terms of seeing the dimmest objects possible. So, your 10×50 binoculars would be acting like 10×35 binoculars. That is with no anti-reflective optical coatings. If all of the air/glass interfaces are coated, though, then this doesn’t happen.
This is where you get into the issue of the different terms that I started this post with. As you’d suspect, fully coated optics would be when all of the air/glass interfaces have optical coatings. However, optical coatings can be expensive if done right. So, some manufactures don’t coat all surfaces. Often the lenses are coated, but not the prisms. In that case, the binoculars are advertised as having simply coated optics rather than fully coated optics. Strictly speaking, all they need to have is a single glass/air interface coated in order to claim coated optics, though. Thus, some of the lesser quality binoculars are truly far lower quality than their cousins.
But, simply having coated optics, even every surface coated, doesn’t give perfect results. Each color of light is a different wavelength, so that means that you need to have a different thickness of coating in order to produce destructive interference upon reflection. A single coating can only work for one wavelength of light. So, if you are going to minimize reflection for only one wavelength of light, you need to be careful about what wavelength might have the greatest problem with reflection. Optics that have such a single coating tend to look slightly bluish when you hold them up and look at them at an angle.
A single coating works with only one wavelength of light. In order to have anti-reflective properties for multiple wavelengths of light you need multiple coatings. But layering coatings is even more expensive than putting on the coatings in the first place. Still, for optimal results, that is what you have to do. Optics coated in this manner often have a greenish or purplish tint to them when they are viewed at an angle. When there is more than one optical coating, you can bet that the manufacturer wants you to know about it, so binoculars having more than one coat on any of the lenses are often proudly labeled multi-coated or multiple coated optics. In general, though, the multiple coatings are on the lenses and not the prisms inside the binoculars (which may still have single coatings, though). If every surface is given multiple optical coatings, then the term usually used is fully multi-coated optics.
So, you have decided that you want coated optics. Now, it becomes important to get the right kind of coated optics. While most of the optical coatings are designed to reduce reflection, thus improving the images, there are other reasons for optical coatings. Some coatings are designed to alter the transmission of light in ways other than the anti-reflective coatings. For example, there are many types of binoculars and gun scopes that are designed to block or increase the reflection of certain colors of light so that objects look different through the binoculars or scope. This is nominally supposed to make it easier to spot discontinuities in background foliage or sand, helping to expose camouflaged objects, or to accentuate certain colors common to game animals (making them easier to see when hiding). These types of coatings are never good for astronomy, because they work by limiting the amount of light passing through the lenses. Unfortunately, some of these coatings are just plain cool to look at, such as the red lenses of the so-called ruby coated lenses (it normally isn’t really a ruby coating). So, some of these types of coatings are getting to be a bit of a fad because they rather look special. Normally, they don’t really do much to help the view through the binoculars, and often they can hurt the view. Most authorities tell you to stay away from them. But, for astronomy, they are definitely something to avoid. Stick with the plain anti-reflective coatings for best performance. Best of all, buy binoculars that are fully multi-coated. And, remember that you often get what you pay for.
-Astroprof
Sili on April 8, 2009 at 11:12 am: 1
Thank you for the fresher on optics. Makes me want to reread something on it - just not the book I used for my course …
mike on April 22, 2009 at 3:41 pm: 2
Nice
dale on April 27, 2009 at 1:08 am: 3
wow , i am a photographer and have a bunch of lenses . some cost well over 2000 dollars. i went and checked them out . yep! they are showing the purple.
Audrius on January 21, 2010 at 4:53 pm: 4
great article, this will help me to distinguish good optics from lesser quality just by checking the color on lenses
Ahmet Faruk YAZICI on May 16, 2010 at 2:07 am: 5
Thank you for explaning it with simple words.
Sameer Kadam on June 24, 2010 at 11:12 am: 6
I am an astronomy enthusiast from Khagol Mandal (Mumbai,India)IT’s really a wonderful post. lots of basics concepts are now cleared about optics & it’s coating ! I like the simplicity of the words & the way complex terms get unwind with your writing.I am loving it.
jose on October 16, 2011 at 9:51 am: 7
thanks it helped alot
Greg on December 13, 2011 at 7:20 pm: 8
Hi there, I am doing a report on optical coatings and would like to analyze the ruby coatings. Do you know what they are actually made out of? Thank you