Two paradigms about human perception of light and glare suit with the planning of lighting conditions: Engineers currently work with a stimulus-response approach relying on criteria which express the visual capacity of the lighting conditions The second approach is the model of cognitive perception which is often applied by architects. Both paradigms relate to psychological research areas.

The stimulus-response-model refers to the psychological paradigm of the "behaviourism" which was the major theory of psychology in the 1960s. It supposes that the behaviour is determined entirely by environmental conditions. According to this model the behaviour of people can be directed by "arrangements" of environmental stimuli. Representatives of this theory suppose that what we become is mostly the result of environmental influence, not of inheritance.

This theory formulated the methodical principle that only the directly observable behaviour may be the research object, while internal mental events of the human beings are not accessible.

Although only few psychologists represent the radical behaviourism which denies any inner life, its theoretical and methodical principles have influenced the thinking so lastingly that they are still noticeable.

The cognitive model supposes that the neural information processing determines how an individual will behave. The processing logically assumes the input of information from the environment, but human action is not seen as a direct reaction to this input. The active process of the cognition or information processing is in between and the person selects actively the information which he needs to react. People do not react to reality as an objective material world, but how it displays to them as a subjective reality. The individual constructs his own interpretation of the world as an internal image — called mental model or schema — which does not correspond to the objective description (Gardner, 1989).

The experiences and perceptions are stored in schemata (or scripts if they describe a course of events) and are activated in the processing. Schemata are general conceptual frames or knowledge structures and contain assumptions about certain subjects, people and situations. Schemata are knowledge parcels about the complicated generalisations of our experiences with structures in the environment. One has for example schemata of kitchens, bedrooms and exam celebrations. Schemata (and scripts) do not contain all details of our different experiences, but represent our "average experiences" which we gain through interaction with our environment (Rumelhart, Smolensky, McClelland, Hinton, 1986).

This can be illustrated by an experiment conducted by Brewer & Treyens(1981). In this experiment test persons were led to a room which was said to be the office of the investigator. After some minutes they were brought to another room where they were
instructed to note down what they remembered from the first room. The test persons recalled very well equipment which is typical for an office schema, but did not recall objects which are not typical for an office schema. They also listed objects which are quite usual in offices but had not been present in the test room.

According to discomfort glare the main notion for the stimulus-response-system is that a stimulus activates via a net of „internal switches" a response of the human being. This procedure is linked with reaction times. The elaboration of the Discomfort Glare Indices (DGI) fits largely on the concept of stimulus-response. During the visual operation luminance distribution and color rendering of the surrounding are the stimuli. The responses are behaviour, feeling and willingness to perform. Current intentions and concentration determine „internal switch" positions. The crucial category for this concept is the visual capacity which is linked with the maximal speed of perception which is reachable in difficult visual tasks. The time between stimulus and response as well as the accuracy of the response is the measure which expresses the match between the light conditions and human abilities.

Explaining discomfort glare with the cognitive approach one must consider that mental concepts of potential environment composition are available for human beings as a matter of visual experiences. Approved cognitive theories show that humans rely on existing cognitive schemata — which they elaborate in their life time — and are therefore able to reduce the necessary cognitive capacity to a minimum. Sensorial information from the eye — often only cue information of a complex perception — leads to the choice of a suitable mental concept from all stored patterns and brings this one to perception. In an objective physical discomfort glare condition the perceived information of the eye might not be the discomfort glare but the computer screen which activates an existing mental schemata that appraises work environment as negative. In the same condition the cue information might be for another individual that sun is shining outside and activates a schemata of summer holiday and leads to positive appraisal. In comparison to stimulus response systems this procedure reacts in negligible time and is able to operate as well if sensorial information is missing because it sets hypothesis about the environment composition.

Current cognitive psychology assumes that human perception and assessment of environmental conditions is directed by subjective schemata of world constructions. From this finding one has to state clearly that the stimulus-response approach for the elaboration of the assessment of Discomfort Glare is using a mechanical idea of man which is disproved by psychological research of the last 20 years.