Elements of Light Return
- Brightness & Contrast

Some content on this page are contributed by Good Old Gold, with thanks.

What is Brightness?

GIA defines brightness as the appearance, or extent, of internal and external reflections of "white" light seen in a polished diamond when it is viewed face-up. AGS defines brightness as the amount of white light returned to the observer.

A diamond does not produce its own light. It needs to reflect an external source of light. The amount of light the diamond reflects, is a key determinant of its beauty.

When a ray of light strikes a diamond, part of it is reflected. This is called External Reflection. Part of it, however, enters the diamond and bends because the diamond is more optically dense compared to air. This is called Refraction.

Assuming that the diamond is cut well, most of the light entering the diamond will reflect off the internal surfaces and returns to the observer. Some of these rays of light exit as white light.

A diamond will appear bright if there is a lot of light being reflected off the diamond.

Low Ambient Hemisphere Lighting	Bright Ambient Hemisphere Lighting

To illustrate the point, we place a diamond in low light as well as bright ambient hemisphere lighting. Obviously, the diamond in the environment with more light will appear brighter since there is more white light reflecting off its surface.

Similarly, if the diamond is badly cut, it will reflect less light to the observer, and render the diamond less bright.

So, both the lighting condition and the cut of the diamond affects its brightness. That is why jewellery stores like to light up their stores with multiple spot lights. In such a lighting condition, even bad stones will appear bright.

What is contrast?

AGS defines contrast as the light and dark patterns seen when observing a faceted diamond. It can be positive or negative in its optical effect. The dark patterns are caused by, but not limited to, reflection of the observer’s head.

Comparing the images below, the amount of white light eminating from a completely white image is surely more than that from a checkerboard because there is less light coming from the black squares.

No Contrast	Bright Contrast

But take a close look at the checkerboard on the right. By juxtaposing the small white squares with the black ones, each white square appear brighter and whiter. This may be an optical illusion, but a diamond's beauty is all about perception.

Low Contrast	High Contrast

In addition, humans are 'hard wired' to detect edges.  Contrast provides us with these edges. Humans find the image on the right to be more appealing compared to the left.  Researchers on diamond cut have pointed out that the presence of contrast enhances our perception of diamond brilliance.

Refer to the table on the right. Let us illustrate this with a couple of simulated images of diamonds.

Both diamonds are bright. However, I am sure you agree that the high contrast diamond looks brighter and more visually appealing compared to the low contrast one. This demonstrates the postive effect of contrast on the brillinace of a diamond.

Leakage Contrast

Although the main contributors to contrast are the dark patterns of a diamond caused by the reflection of the observer's head (reflector contrast), there is another important source of contrast. Some diamond cutters deliberately cut their diamonds with a little leakage around the edge of a diamond to enhance its edge contrast. This is best show with the following diagram:

Without edge leakage contrast, the edge of the diamond may appear white and uninteresting. It is generally agreed that some edge leakage contrast would improve the contrast brilliance and the beauty of the diamond.

Contrast Under Different Lighting Conditions

Next, we run the diamond in a simulator to see how it behaves under different lighting conditions:

Reflector Contrast observed in Hemisphere Lighting	Leakage Contrast observed in Office Lighting	Dispersion Contrast observed in Direct Lighting

The first observation is done in hemisphere lighting. This is akin to outdoor lighting on a cloudy day. Note that the reflectors (star and hotspots) contribute the most to the diamond's contrast.

The next observation is in office lighting, which is typically fluorescent lighting. Note that edge leakage is now working to give the diamond the contrast it needs.

Finally, we observe the diamond in direct lighting, like spotlighting. Under this lighting, fire/dispersion (to be discussed in the next page) is strong. Both leakage and reflectors work well to give contrast to the diamond. In fact, the reflectors alternate between darkness and fire when the diamond is moved.

As you can see, both the reflectors and edge leakage give the diamond contrast under different lighting conditions. Hence, to ensure that your diamond will perform in most instances, it is desirable to have both a good spread of reflectors and edge leakage.

Brilliance Redefined

In the past, we used to call the white light return of a diamond its 'brilliance'. However, in recent years, it is widely recognised that having good white light return is not enough to ensure that the diamond looks bright and beautiful. Having positive contrast is just as important.

As such, AGS redefined 'brilliance' as brightness with positive contrast effects.

We have only discussed the reflection of white light so far. Let us move on to Fire, where we deal with coloured light.