The balance between brilliance and fire (dispersion) is governed largely by the combination of the table size and the crown angle. The relationship between the table size and the crown angle determines the crown height.
Crown too Steep
With a steep crown angle, light travelling up from the pavilion will strike the crown interface, just within the critical angle. At this angle light leaving the stone is bent the most (refraction). The more light bends the more it is separated into its spectral hues.
While this crown angle produces more fire, much of the light will travel in a direction away from the viewer and will not be seen.
Crown too Shallow
With a shallow crown angle, light travelling up from the pavilion will strike the crown interface, within the critical angle, nearly perpendicular to the interface. As the light exits the stone it is only bent slightly away from the normal (refracted).
The light travels in the proper direction to be viewed, but it is not bent enough to produce any significant amount of fire (dispersion).
Antwerp Ideal Proportions
Most experts agree that a crown angle close to 34.5° is the best compromise between optical theory and beauty.
With this crown angle, light leaving the stone is bent enough for the light to be separated and still travel in the direction of the viewer.
Maximum brilliance and Maximum fire is impossible to achieve since optically they work against each other.
The Antwerp Ideal displays the best balance between brilliance and fire.
Crown angle also affects the way light enters the stone. The size of the table relative the other crown facets also contributes to scintillation.