[TakesALickin]

refraction: the change of direction of a ray of light, sound, heat, or the like, in passing obliquely from one medium into another in which its wave velocity is different.

I have a couple of watches that have domed crystals that exhibit a refractive quality that I don't see in my other watches. I've been wondering for quite some time what causes it.

When I got my Alpha PO homage, I noticed the tendency of the crystal to refract light when turned at an angle to my eye (see pic below). Initially, I chalked it up to the crystal being mineral glass and, I presumed, low grade. I have a number of Seikos with domed Hardlex crystals that don't seem to refract light quite so severely.

A few months later, I bought an Ocean7 LM-1. I was a little shocked at first to discover that the LM-1, with a sapphire crystal, also refracts light - so much so that it reminds me of a window on a submersible diving bell.

It was at this point that I began to suspect that the refraction might not just be a result of the crystal being cheap, but that it might have to do with thickness, chemical composition, or other properties of which I was not aware. I'm wondering if anyone here knows - what causes some crystals to refract light when turned at angle, whereas others do not?

 

[Pei Der Han]

The simplest thing I can think of is that these crystals all become lenses at some point. As soon as you deviate away from the flat(both sides) type of crystal, there will always be a bending of light passing through them.
Perhaps the ones that don't show it as much are the 'double' domed type where the inner surface is also domed in order to mitigate this phenomenon. I'm guessing the one you've shown is a thick crystal to handle the rated pressure and may also be flat on it's inner surface.

I think this is the type of question that an optician might be better at explaining...Mike, where are you?

 

[TakesALickin]

Pete, I can't comment on the PO, but I had to open the LM-1 to remove some lint I noticed on the underside of the crystal (the curse of owning a 10X loupe... ;D ). When I had it open, I noticed that the underside of the crystal is indeed flat. You may be onto something there.

 

[time]

The coating (IF used) makes a huge difference.
But as with any reflective material, at some point, light will become distorted when held at certain angles.

 

[TakesALickin]

So, you're saying anti-reflective coatings will aid in reducing refraction? Would it make a difference if both sides of the crystal were coated, as opposed to only one side? Is there a qualitative difference between certain coatings?

 

[time]

So, you're saying anti-reflective coatings will aid in reducing refraction? Would it make a difference if both sides of the crystal were coated, as opposed to only one side? Is there a qualitative difference between certain coatings?

Yes, it does make a difference if both sides use AR coating.
I do not have enough data as I haven't actually tested different types of double coatings yet against each other, but I assume there is a qualitative difference between different types.

 

[NelsonE]

The coating has nothing to do with it. It depends on the thickness of the xtl and most importantly the shape of the xtl. Whether its domed top and bottom or just one side and the degree of curve.

 

[time]

The coating has nothing to do with it. It depends on the thickness of the xtl and most importantly the shape of the xtl. Whether its domed top and bottom or just one side and the degree of curve.

While your points are valid, I beg to disagree on the coating.
The thickness and shape play a major role, but the coating can reduce the refraction significantly.
Light still has to pass through the glass and pass through 'whatever' material is applied on it and diverge accordingly.


Here is a quick example with 2 different materials, not the same as coating, but the principle applies.

 

[NelsonE]

Coatings used on a sapphire xtl are either bottom only or top only or both and they are designed to stop reflections. If you have a domed xtl they do a pretty good job. With a flat xtl less so. The appearance you are talking about is innate to the shape of the xtl. The coating is a cosmetic thing and may decrease glare of it but the quality itself will not change. I have many watches with differing shaped xtls and the ones with the qualities you describle aren't really affected that much by any coating. Its just a characteristic. For example, I have a "yes" watch which has the very refractive qualities you describe. It has coatings on both sides but the distortion is still very apparent. However if you go out in the sun with it you don't get much reflection off it.