Optical Illusions, Lighting Effects

It should be obvious by this time that the lighting of objects or of a scene can alone produce an optical illusion, and that it can in still more cases contribute toward an optical illusion. Furthermore, there are many cases of optical illusions in lighting due to brightness and color. Many effects of lighting have been described elsewhere with detailed analyses of the underlying principles, but a condensed survey applying particularly to optical illusions will be presented here.

Several times in the previous chapters, the comparison of low relief with intaglio has been mentioned. Examples of these as related to lighting are found in Figs. 70 to 73. Fig. 70 represents a bas-relief lighted from above and Fig. 71 would ordinarily be taken to represent a bas-relief lighted from below. However, the latter was made from a photograph of the mold (intaglio) from which the bas-relief was made and Fig. 71 really represents an intaglio lighted from above.

Similarly Fig. 72 represents the bas-relief lighted from the left and Fig. 73 ordinarily would be taken to be a bas-relief lighted from the right. However, Fig. 73 was made from a photograph of an intaglio lighted from the left. These amply demonstrate the effect of lighting as an influence upon the appearance of objects and they indicate the importance of correct assumptions in arriving at a correct judgment. In these cases the concealment of the light-source and the commonness of bas-relief as compared with intaglio are the causes for the optical illusion or the error in judgment.

Fig. 74 a. - A disk (above) and a sphere (below) lighted from overhead.
Fig. 74 b. - A disk and a sphere lighted by perfectly diffused light.

Certainly in these cases the visual sense delivers its data correctly.

In Fig. 74 the upper object is a disk and the lower is a sphere. In a Fig. 74 the lighting is due to a source of light of rather small physical dimensions directly above the objects.

Fig. 70. - A bas-relief lighted from above.

The same objects illuminated by means of highly diffused light (that is, light from many directions and of uniform intensity) appear as in b. Both objects now appear as disks. It is obvious that under appropriate lighting a disk might be taken for a sphere and vice versa, depending upon which dominates the judgment or upon the formulation of the attendant assumptions.

Fig. 71. - An intaglio lighted from above.

Incidentally an appearance quite similar to that of a, Fig. 74 is obtained when the light-source is near the observer; that is, when it lies near the line of sight.

Fig. 72. - A bas-relief lighted from the left.

Somewhat similar to the confusion of intaglio with bas-relief is the confusion of the two hemispherical objects illustrated in Fig. 75. The one on the left is concave toward the observer. In other words, both could be hemispherical shells - one a mold for the other. Under the lighting which existed when the original photographs were made they could both be taken for hemispheres. The lighting was due to a large light-source at the left, but if the object on the left is assumed (incorrectly) to be a hemisphere convex toward the observer or a sphere, it must be considered to be lighted from the right, which is also an incorrect assumption. Obviously, if the direction of the dominant light is clear to the observer, he is not likely to make the error in judgment. Incidentally the object on the right might be assumed to be a sphere because a sphere is more commonly encountered than a hemisphere.

Fig. 73. - An intaglio lighted from the left.

The same objects are represented in Fig. 76 lighted from the left by means of a light-source of relatively small dimensions; that is, a source subtending a relatively small solid-angle at the objects. In this case the sharp shadow due to the edge of the hemispherical cup (on the left) is likely to cause the observer to inquire further before submitting his judgment. The more gradual modulation of light and shade as in the case of a sphere or a hemisphere convex toward the observer is not present in the case of the cup. This should be sufficient information for the careful observer to guide him, or at least to prevent him from arriving at the definite conclusion that the left-hand object is a hemisphere with its convex side toward him. Furthermore it should be noted that we often jump at the conclusion that an object is a sphere even though we see with one eye practically only a hemisphere and with two eyes hardly enough more to justify such a conclusion. However, spheres are more commonly encountered than hemispheres, so we take a chance without really admitting or even recognizing that we do.

Fig. 75. - A concave hemispherical cup on the left
and a convex hemisphere on the right lighted by
a light-source of large angle such as a window.

The foregoing figures illustrate several phases which influence our judgments and the wonder is that we do not make more errors than we do. Of course, experience plays a large part and fortunately experience can be depended upon in most cases; however, in the other cases it leads us astray to a greater extent than if we had less of it.

Fig. 76. - The same as Fig. 75, but lighted by a very small light-source.

The photographer, perhaps, recognizes more than anyone else the pitfalls of lighting but it is unfortunate that he is not better acquainted with the fundamentals underlying the control of light. Improper lighting does produce apparent incongruous effects but adequately controlled it is a powerful medium whose potentiality has not been fully realized. The photographer aims to illuminate and to pose the subject with respect to the source or sources of light so that undesirable features are suppressed and desirable results are obtained.

Finally his work must be accepted by others and the latter, being human, possess (unadmittedly, of course) a desire to be "good looking." Lighting may be a powerful flatterer when well controlled and may be a base revealer or even a creator of ugliness.

Incidentally, the photographer is always under the handicap of supplying a "likeness" to an individual who perhaps never sees this same "likeness" in a mirror. In other words, the image which a person sees of himself in a mirror is not the same in general that the photographer supplies him in the photographic portrait. The portrait can be a true likeness but the mirrored image in general cannot be. In the mirror there is a reversal of the parts from right to left. For example, a scar on the right cheek of the actual face appears on the left cheek in the mirror. Faces are not usually symmetrical and this reversal causes an individual to be familiar with his own facial characteristics in this reversed form. This influence is very marked in some cases. For example, suppose the left side of a companion's face to be somewhat paralyzed on one side due to illness. We have become more or less oblivious to the altered expression of the left side by seeing it so often. However, if we catch a glimpse of this companion's face in the mirror and the altered expression of the left side now appears upon the right side of the face, the contrast makes the fact very conspicuous. Perhaps this accounts for the difference which exists between the opinions of the photographer (or friends) and of the subject of the portrait.

All the optical illusions of brightness-contrast may be produced by lighting. Surfaces and details may appear larger or smaller, harsh or almost obliterated, heavy or light; in fact, lighting plays an important part in influencing the mood or expression of a room. A ceiling may be "lifted" by light or it may hang low and threatening when dark, due to relatively little light reaching it. Columns may appear dark on a light background or vice versa, and these illustrate the effects of irradiation. A given room may be given a variety of moods or expressions by varying the lighting and inasmuch as the room and its physical characteristics have not been altered, the various moods may be considered to be optical illusions. It should be obvious that lighting is a potent factor.