Just Ask Jeff: What is Face up Brightness and Tilt Brightness?
What is Face up Brightness and Tilt Brightness?
It is pretty simple really. Face up brightness is the light return of a stone/design when it is viewed from the face (top) of the stone. Viewed perpendicular to the table, looking straight down into the crown. Tilt brightness is viewing the stone from an angle off of perpendicular. Like 10 degrees of perpendicular, i.e. with the stone tilted 10 degrees.
Gemcad will calculate the tilt angle of a design, perpendicular (face up) to plus/minus 90 degrees (I did not realize this until someone pointed it out to me, I thought it would only work to 15 degrees. It was to that angle on an older version of Gemcad, I did not realize it had changed) on the “x” and “y” axis’s. Depending on what you tell the program when running the Gemray feature.
There has been some talk of making designs “bright with high tilt brightness”. This is a good idea in theory, but it really will not work in real life with most commercial materials that do not have a high refractive index. There are some advantages and valid reasons for being concerned about tilt brightness if you are cutting material with higher refractive indexes (or even low RI materials, if you can get them to have good tilt brightness).
Note: Most designs tilt brightness will improve naturally as the refractive index of the material goes up. It is just a function of the higher refractive indexes being used.
I am going to use a standard round brilliant for an example. It is the easiest to show the problems I am going to cover and because it is a round, the design can be tilted in any direction without any differences because of L/W. It is the same all the way around. The high symmetry of a round brilliant also gives the best all around performance in general (as compared to an oval or rectangle with a longer L/W), in both face up brightness and tilt.
This is a very standard round brilliant with a 41 degree pavilion and 42 degree breaks on the crown. The refractive index (RI) is 1.54 for Quartz, critical angle (CA) of Quartz is 40.49 degrees.
Note: I prefer to cut Quartz’s (especially lighter colored material) in a deeper design. I generally use 42 degree crowns and 41 degree pavilions… Of course if I was cutting very dark Amethyst, I might choose the lower crown.
The ray trace on the top is the round brilliant (1.54 Quartz) face up. The ray trace to the bottom is the same stone with it tilted 10 degree on the “x” axis. I chose 10 degrees because as you can see there is a large hole, and anything higher in tilt will just make the hole bigger. Notice the large window (hole in the reflection) that falls out of the stones reflection, when it is tilted 10 degrees in any direction.
The ray traces above are of the same round brilliant, but with the crown lowered to 34 degrees (what I call a hubcap design because of the low crown). The degree of tilt is the same 10 degrees. As you can see, other than being a little brighter (out around the girdle) because of the lowered crown angles, the window is identical. Lower crowns will not help the tilt brightness at all. It could be argued that the lower crown actually lowers the look of the overall stone, to me anyway, but some people like the lower crown, each to their own on that.
The above ray traces are with a 42 degree pavilion and a 42 degree crown. Notice there is no change in the window or tilt brightness. The overall brightness of the stone is lower because of the extra degree on the pavilion.
The above ray traces are of the round brilliant with a 42 degree pavilion and a 34 degree crown. Notice the lower crown helps brighten the stone slightly, making up for the extra degree on the pavilion. However the tilt brightness and the window is still the same. I have run these tests changing the angles on both the pavilion and crown all the way up and down. These trends do not really change much, and the performance of them is just not acceptable. I would suggest that you try doing it yourself, it is an interesting learning experience. Part of the reason I chose a round brilliant is because it is easy for a new Gemcad user to create in the program, and run these tests.
As you can see, as the angle on the pavilion goes up above 41 degrees ( it cannot go below 41 degrees because of the critical angle of Quartz) there is absolutely no gain in tilt brightness.
Also, you can see that lowering the crown angle helps the brightness slightly, but has no effect on the tilt brightness of the stone. The crown angles can be raised, and it will improve the tilt brightness a little bit. But 42 degrees is about as high as you can go and still maintain a decent face up light return. In other words, it is about the best compromise available…
Now if you have been reading my article’s you have heard me say that I cut Sapphires at Quartz angles and prefer them that way. It improves the yield, the color (because there is more depth on the stone) and also the tilt brightness of the stone. Here is some proof.
Above is the same round brilliant 41 degrees pavilion and 42 degree crown (RI 1.76). As you can see the face up brightness on the top is great and the tilt brightness is also very good (bottom), almost no loss when it is at 10 degrees tilt. Although you can just see it beginning to loose some out around the bottom edge at the girdle.
So in conclusion, on lower refractive index materials 1.54 to 1.62 (Quartz, Beryl, Tourmaline, Topaz) it will not matter what the design is, there is just really no tilt brightness available because of the low refractive indexes. The tilt brightness is really not much of a factor here, no matter what you may have heard.
Also I would like to point out that even if you have a good tilt brightness say of 10 degrees (which is pretty good), that is hardly anything when compared to the actual viewing conditions the stone will actually be seen under. If the stone is set in a ring or pendant that swings, 10 degrees is a very small axis change. Just watch any woman’s hands as she talks with a ring on her finger and uses her hands. So I am not really sure how important tilt brightness really is, although I do design for it, if the refractive index of the material I am working in will allow some. Tilt brightness should be considered as you get into the higher refractive indexes. Be Aware that the L/W of a design can dramatically impact and effect the tilt brightness of any design and that the designer usually needs to make some compromises, depending on the design itself.