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Etude analytique et experimentale du pilotage de stores vénitiens en vue d'obtenir des conditions de confort visuel optimales dans le cas du travail sur écran de visualisation

Abstract : In order to guarantee good visual comfort conditions to an office worker, it is necessary to take special care of the amount of light arriving on and surrounding the VDU screen. As far as daylight is concerned, the best way to control this remains solar protection. Nowadays, the automated control systems for Venetian blinds available on the market are often based on binary instructions. That is to say the blind will be lowered when a certain level of illuminance on the facade is reached. Though some automated control systems take the position of the sun in the sky vault into account, so that the slats are tilted in order to block direct sunlight penetration inside the room. As far as we are aware, no automated control systems for Venetian blinds feature instructions based on visual comfort nor behavioural nor indoor and outdoor environment considerations. The goal of the work that has been done here was to make up a model aimed to automatically control Venetian blinds in order to guarantee the best visual comfort conditions to an office worker, whatever the outside climate is, and at the same time to offer the maximum conceivable daylight. The model that we have built embodies photometric considerations resulting from an experimental optical characterisation of the luminous transfer function of a Venetian blind, which was deduced from measurements made inside test cells and scale models. The control instructions are based upon visual comfort criteria (veiling luminance not to be exceeded on the VDU screen and luminance uniformity in the visual field) and upon conclusions deduced from a field study that was conducted among a relatively short panel of subjects (eight) but during a long time span (eight months). We have registered every fifteen minutes, the way that the subjects were using their Venetian blinds as well as parameters such as the illuminance on the window, the veiling luminance on the VDU screen, the ambient temperature inside the office, the state of the artificial lighting and the configuration of the room. The results of this field study allowed us to validate the visual comfort criteria included inside the model in the first place, and then to adjust our model with the help of the behavioural observations deduced from the measurement campaign. The analysis of the results testifying to the real behaviour of VDU office workers allowed us to make several observations. Here are the main ones: •Recommendations concerning the luminance ratios in the visual field (1:3:10 rule), often mentioned in many lighting standards, do not seem to be validated as soon as a daylight source fulfils a part of the visual field. •Blackwell's recommendations concerning the veiling luminance on the VDU screen not to be exceeded appear to be too severe under daylight for bad to standard quality screens. •A motorized solar protection is used three times more often that a manually controlled one. •Office workers mainly leave their Venetian blinds totally lowered, with slats horizontally orientated or tilted towards the ground outside. •Users open their Venetian blinds at a lower vertical outside illuminance threshold than the one at which they close them. This hysteresis can reach the value of 30 klux. •Users position their Venetian blinds so that the closest window luminance remains under the mean value of 1800 cd/m2, during 75 % of the time. These observations, deduced from in-situ measurements, allowed us to adjust our model to match our subjects’ behaviour, as far as possible. Even if we have somehow observed some individual coherences, it is important to keep in mind that it is difficult to produce a universal model that is able to predict the way office workers will use their Venetian blinds. For example, the maximum window luminance acceptance thresholds can be five times higher from a subject to another, according to their sensibility to daylight or the configuration of his office. In order to compensate these individual differences, either the Venetian blinds control system must be able to be temporarily turned off by the office worker if so wanted, or the control instructions can be adapted to each individual.
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Yannick Sutter. Etude analytique et experimentale du pilotage de stores vénitiens en vue d'obtenir des conditions de confort visuel optimales dans le cas du travail sur écran de visualisation. Automatique / Robotique. INSA Lyon, 2003. Français. ⟨tel-02472529⟩

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