In the heat and the rain – that’s where you typically expect to see rainbows. A nice spring day, a light shower, and a pot of gold hidden off in the distance. Not so much when it comes to your lenses. Anyone that’s dealt with lenses long enough has experienced this before – an oil-slick like effect, or rainbow, in the lenses. As this is one of the few places in life where you probably don’t want a rainbow, I wanted to address some concerns over it, and see what can be done about it. This also has a few other fancy names, such as “Newton’s Rings” and “Chromatic Aberration”.
Shame on the Light †
So the first question, of course, is ‘what causes this?’. It really comes down to the way that light behaves when it moves through materials. Birefringence is the three-dollar-word of the day that explains this. In our corner of the world, this really boils down to a very simple thing. That is, the difference between the refractive index of our lens material and the refractive index of the various coatings on the lens. That’s why you tend to see this more on polycarbonate and higher index lenses. Most standard lens coatings come in a refractive around 1.49-1.50, which is similar to basic plastic lenses. This explains why you very infrequently see this on plastic lenses. The closer the refractive index of the coating and the lens, the less likely you are going to be to ‘catch the rainbow’. Since it’s a phenomenon occurring between lens and hard coating, you don’t tend to see it on glass lenses, as they don’t need a hard coat, so there’s nothing to cause the effect to manifest. So it’s not so much an issue of anything being wrong with the lens, and is a product of the physics of light. That means it’s not down to the manufacturer, but rather is all light’s fault. So, shame on the sun.
Stand up and Shout †
It’s the first thing that the patient is going to do when they see this. Maybe. It’s important to make sure that you and your staff know what this phenomena is so that you can adequately explain it to the patient. You want to make sure that they know what it is, so that way they don’t think there’s “something wrong” with it. The good thing about birefringence is that it’s purely aesthetic – it doesn’t impact the functionality or optics of the lens at all. When wearing it, the patient isn’t going to be able to detect the color variations, so it’s not going to interfere with their vision. Indeed, often times people don’t really notice it or see it as out of place. This is because, really, it isn’t. It’s simply the way that the physics of light work in this situation.
Rainbow in the Dark †
How do we minimize this effect? Well, the easiest and most impractical way to fix it is to simply avoid all light. When there’s no lighting, the rainbow won’t get you down. One interesting and also useless piece of information is that if you have non-white light, you get patterns that are more monochromatic designs instead of the color spectrum produced by white light. However, at this point you probably want some information that’s a little more practical and/or useful.
Last in Line †
So by the time you’ve recieved the lenses, they are already done. You’re not going to be chanigng the coating process from your office. Basically, as an end-user, there’s really not a lot that you can do. Anti-reflective coating can help to lessen the effect, but it’s still going to be visible to the discerning patient and professional. It’s up to your lab and/or manufacturer, as it’s purely a result of the coatings that they use. Since it’s a result of the components used in the manufacturing process, that means different manufacturers may produce different results on their materials. For example, manufacturer A may use the same, lower-index coating for all of their lenses. Manufacturer B may use lower-index coatings for their plastic lenses, and higher-index coatings for their polycarbonate and high-index lenses. By using a coating of a higher refractive index on these materials, the rainbow effect will be lessened since the refractive index of the coating and material are closer to each other.
Caught in the Middle †
Why don’t we just use a higher index coating? A very good question that, of course, has a slightly complicated answer. To answer that we have to look at where the lenses are coated, who’s coating them, and the kind of result you get at the end.
In terms of where a lens is coated, it usually doesn’t make a difference, until you start to travel outside of the United States. Some manufacturers make their lenses in other countries and then import them into the U.S. This means that they can use some chemicals in their coating process that may be deemed too hazardous to do the same type of coating in the U.S. That’s not to say the end product itself is dangerous, rather the large volumes of concentrated liquid required for a coating machine can be too hazardous to produce in the U.S. For example, it’s not very easy to even obtain a hard coating that goes above a 1.60 refractive index in the United States, which makes it almost impossible to completely eliminate the rainbow effect on all materials.
Who does the coating can also make a difference. Is the coating being applied by the manufacturer, or by the lab? If it’s the manufacturer, it’s easier for them to cost-justify having higher index coating available. This is because higher index coating can typically cost 3-4 times as much as standard hard coating. With an increased cost, it’s also important to note that when doing a dip-type coating, the batch of hard coating has a life span of about six to eight weeks before needing to be replaced. That’s regardless of the number of lenses that have been processed. So the more lenses that are coated, the easier it is to absorb the higher cost. This is why you can sometimes get manufacturer-coated lenses that seem to have less chromatic aberration than those that are coated by an intermediate party.
Quality can also be a concern when you go with a higher-index hard coating. As you increase in index, you can often decrease in scratch resistance. This is often a hard thing to choose between, especially when it comes down to trying to select what’s right for the patient. If you have to choose between scratch resistance and cosmetics, which one do you pick? That is largely going to depend on your particular clients, but most people would rather not have those lenses come back for a remake, and people are much more likely to scratch their lenses than they are to have a problem with our rainbow.
— Ronnie James D.O. (RIP)