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Constancy

Why objects appear the same despite changes in lighting

Fixed objects produce retinal images of
varying shapes, sizes and brightness. Due to changes in lighting, distance or perspective, this indicates that mechanisms must exist to identify these objects and their properties and to perceive them as being constant. There is no single, simple explanation for the way perception works. Optical illusions provide an opportunity to examine the performance and objectives of perception.

Brightness

Brightness

The perception of brightness of the grey field depends on the environment - in bright surroundings, an identical grey appears darker than in dark surroundings.

Brightness

The fact that a medium grey area will appear light grey if it is bordered in black, or dark grey if it is bordered in white. This can be explained by the fact that the stimuli perceived are processed directly - brightness is perceived as a result of the lightness contrast between the grey area and the immediate surroundings. What we are considering here is a visual impression that is based exclusively on sensory input which is not influenced by any criteria of order linked with our intellectual processing of this information.

Luminance gradient

Luminance gradient

The continuous luminance gradient across the surface of the wall is interpreted as a property of the lighting. The wall reflectance factor is assumed to be constant. The grey of the sharply framed picture is interpreted as a material property, although the luminance is identical to the luminance in the corner of the room.

Three-dimensionality

Three-dimensionality

The spatial impression is determined by the assumption that light comes from above.

Three-dimensionality

By inverting the picture, the perception of elevation and depth is reversed.

Three-dimensionality

The spatial form of an object can be recognised by the gradient of the shadows.

Changing luminance levels may arise from the spatial form of the illuminated object; examples of this are the formation of typical shadows on objects such as cubes, cylinders or spheres.

Wall structure

Wall structure

The lighting distribution on an unstructured wall becomes a dominant feature.

Wall structure

The same lighting distribution on a structured wall is interpreted as background and not perceived.

Wall structure

Light distribution that is not aligned with the architectural structure of the space is perceived as disturbing patterns that do not relate to the space.

Wall structure

Irregular or uneven luminances can result in confusing lighting situations. This is evident, for example, when luminous patterns created on the walls bear no relation to the architecture. The observer’s attention is drawn to a luminous pattern that cannot be explained through the properties of the wall, nor as an important feature of the lighting. If luminance patterns are irregular, they should, therefore, always be aligned with the architecture.

Beam of Light

Beam of Light

Beam of Light

The visible pool of light determines whether it is perceived as background or as a disturbing shape. Light distribution that is not aligned with the shape of the picture is perceived as a disturbing pattern.

Perception of colour

Perception of colour Perception of colour

Perception of colour Perception of colour

The perception of colour, similar to the perception of brightness, is dependent on neighbouring colours and the quality of the lighting. The necessity for us to be able to interpret colours is based on the fact that colour appearances around us are constantly
changing. A colour is therefore perceived as being constant both when viewed in the bluish light of an overcast sky or in warmer direct sunlight - colour photographs taken under the same conditions, however, show the distinct colour shifts that we must expect under the particular type of light.

Perspective

Perspective

In this case the perspective results in an optical illusion. The vertical line to the rear appears to be longer than the line of identical length in the foreground.

Our misinterpretation of lines of the same length shows that the perceived size of an object does not depend on the size of the retina image alone, but that the distance of the observer from the object is significant. Vice versa, objects of known sizes are used to judge distances or to recognise the size of adjacent objects. From daily experience we know that this mechanism is sufficient to allow us to perceive objects and their size reliably. Therefore, a person seen a long way away is not perceived as a dwarf and a house on the horizon not as a small box. Only in extreme situations does our perception deceive us: looking out of an aeroplane, objects on the ground appear to be tiny; the viewing of objects that are considerably farther away, e.g. the moon, is much more difficult for us to handle.

Size

Size

Constancy with regard to perception of size. Due to the perspective interpretation of this illustration, the luminaires are all perceived as being the same size in spite of the variations in size of the retina images.

To allow for the perception of size, we have a mechanism that balances the perspective distortion of objects. It guarantees that the changing trapezoidal and ellipsoidal forms in the retina image can be perceived spatially as being normal, rectangular or round objects by being aware of the angle at which the object is viewed.

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