Revelation of form is concerned with the level of 3-dimensionality you want the audience to see. Revealing the form of an actor, dancer, or part of the set will make them stand out from their background and allow the audience to see more or less detail. In some situations you might want to create a "flat" look, whereas in others, e.g. such as ballet, you might want to create a more sculpted look to let the audience see the dancers move through the space.
The four pictures below show the same object. Depending on the direction (angle) of light, the object has a completely different dimensionality and more or less detail is revealed.
Picture A: Here the light hits the object from the back, thus causing a black silhouette of the object with no details visible.
Picture B: Here a front light was added, this allows the audience to see a three-dimensional shape.
Picture C: Here a second front light was added. This allows the audience to see a three-dimensional shape. Adding this third light allows for the details to become clearly visible.
Picture D: Here you see an example of too much light. This causes the details and form to become less visible.
The four pictures below show the same object. Depending on the direction (angle) of light, the object has a completely different dimensionality and more or less detail is revealed.
Picture A: Here the light hits the object from the back, thus causing a black silhouette of the object with no details visible.
Picture B: Here a front light was added, this allows the audience to see a three-dimensional shape.
Picture C: Here a second front light was added. This allows the audience to see a three-dimensional shape. Adding this third light allows for the details to become clearly visible.
Picture D: Here you see an example of too much light. This causes the details and form to become less visible.
Source : (P.326 Parker, W, O, Wolf, R, C, Block, D. Scene Design and Stage Lighting Ninth Edition. Boston Wadsworth. 2009)