FIELD OF THE INVENTION

The invention relates to a method of enabling an artificial lighting system to automatically control a lighting level caused by daylight and by artificial lighting means. The method comprises sensing a lighting level and enabling the system to adjust the artificial lighting means in dependence on the sensed lighting level.

BACKGROUND ART

According to present lighting standards in use in The Netherlands, offices should provide an illuminance of about 400 lux for reading and writing on a horizontal working plane. Drawing rooms should have an illuminance of about 800 lux. The recommended colour temperature should be between about 3300 and about 5000 K. Existing artificial lighting control systems have been developed to create a specific lighting environment, such as one complying with above standards, and/or to save energy consumption. Apart from the obvious aspects involved in adequate lighting of an environment, the lighting level also affects the human physiology. It is well known that the human physiological system is regulated by a mechanism that is commonly referred to as a biological clock. A plurality of physiological functions show a periodicity with a cycle of 24 hours. This repetitive behaviour is also called a circadian rhythm. It also is well known that lighting can dramatically affect this circadian rhythm. The mood and behaviour of an individual thus turn out to show a dependence on environmental lighting. Lighting optimized with regard to a person's preferences is found to contribute in a positive way to his or her sense of well-being and, hence, to his or her performance or behaviour. See for more background information, for example, U.S. patent 5, 163,426.

OBJECT OF THE INVENTION

The above serves to emphasize the importance of regulating the aggregate

lighting level stemming from daylight and artificial lighting sources combined, and in particular the control of lighting systems in domestic environments, offices, plants, public buildings and places, hospitals, ocean liners, aircraft, etc.

In lighting engineering, lighting design practice and light modelling, working plane illuminance is considered the dominant parameter. Typical known daylight dimming systems are designed to primarily keep a working plane illuminance, i.e. , the aggregate effect of daylight and artificial lighting typically on a horizontal plane, substantially constant.

It is an object of the invention to provide a method of automated lighting control that takes into account the relevant ergonomics more distinctly than known systems based on standards such as those mentioned above.

SUMMARY OF THE INVENTION

To this end, the invention provides a method as specified in the preamble, characterized in that the artificial lighting means is adjusted substantially in dependence on the sensed lighting level associated with light incident on a substantially vertical plane.

The invention is based on the insight gained by experiments conducted to study user behaviour and response in window zone offices lit by a combination of daylight and artificial lighting. The experiments have led to the conclusion that the experimental data do not agree with conventional lighting engineering practice and standards, and with daylighting theories and techniques. This is believed to be at least partly due to the fact that horizontal working plane illuminance is conventionally considered to be the dominant parameter in determining the required lighting levels. Also, artificial lighting standards were primarily developed excluding daylight contributions. The experiments demonstrate that vertical plane illuminance or vertical plane luminance and spatial illuminance ratios or spatial luminance ratios that involve the vertical plane illuminance and luminance, respectively, determine the lighting levels preferred by the tests persons. The sensing may comprise sensing an illuminance (incident light) or a luminance (scattered or reflected light) of the substantially vertical plane. The artificial lighting means may be adjusted substantially in proportion to the sensed lighting level.

DESCRIPTION OF THE DRAWING

The invention is explained in further detail by way of example and with reference to the accompanying drawing, wherein Fig. 1 schematically shows an office interior lit by daylight and artificial lighting.

DETAILED DESCRIPTION

Fig. 1 schematically pictures an office 100 with a desk 102, a chair 104, a window 106 provided with blinds 108, and lighting units 110, 112, 114 and 116. Lighting units 1 10-116 and blinds 108 form parts of a system to control an aggregate lighting level caused by daylight entering through window 106 and the light from units 1 10- 1 16. Blinds 108 typically reduce daylight levels in the room by 20 - 50% , but may still allow for a "view out".

Conventional lighting standards relevant to offices specify the lighting level of light incident on a working plane, i.e., a substantially horizontal plane such as the top surface of desk 102. Since working plane illuminance is the dominant parameter used in lighting engineering/design practice and daylight modelling, conventional automated control systems may typically sense the luminance or illuminance with regard to the horizontal plane with appropriate sensors, e.g., integrated in the ceiling or in the horizontal plane itself. The results of the experiments conducted by the inventors show that despite the fact that most of the time the daylight desk illuminance is well above the 400 lux level specified in many office lighting standards, the test persons added artificial light even with daylight levels in the 2000 lux range. Interesting is the finding that in the January-June period the test persons added, on average, 800 lux, independent on the daylight level and time of the year. When looking more closely at the preferred illuminance under different weather types different effects can be distinguished. On overcast days the added artificial lighting level stays roughly constant around 1000 lux for both situations with increasing daylight levels. On clear days a decrease in artificial lighting level is noticed from 1200 to 500 lux with an increasing daylight level from 0 to 2000 lux. However, when the daylight level increases above 2000 lux, the added artificial Iighi increases in the situation without blinds 108 but decreases with blinds. On mixed days a similar pattern is detected as on clear days with on average higher artificial lighting levels.

In other words, with blinds 108 the brightness of the windows is reduced, which results in the addition of less artificial light from units 110-116 than without blinds. These results also indicate that the horizontal working plane illuminance, e.g., at the top surface of desk 102, is in fact an irrelevant parameter most of the year and that balancing of the spatial brightness ratios is far more important and that this is related to weather type. Simulated or real daylight dimming systems that are designed to keep a constant working plane illuminance stemming from daylight and artificial light combined, arguably are far too simplistic and are in fact working against the fundamental properties of the eye and human preference and response. Vertical planes and spatial illuminance and luminance ratios appear to be far more important to the creation of optimum luminous environments using a combination of daylight and artificial light.

According to the present invention, sensors 118 and 120 are arranged to sense the luminance or illuminance of relevant vertical planes, here the walls. Preferably, sensors 118 and 120 are located at eye-level, e.g. , with regard to a seated person so as to sense light representative of that perceived by the person. Sensors 118 and 120 are coupled to a control system (not shown) that controls the lighting level contribution of units 1 10-1 16 in accordance with the person's individual preferences determined in advance. Preferably, the control system also controls daylight entrance. For example, daylight entrance is governed by controllable daylight entrance means such as blinds 108 or, alternatively, window panes whose transparency can be varied (not shown) to allow adjusting the entering daylight in accordance with the person's individual preferences. To this end, a daylight sensor may be located inside the office and close to window 106, e.g. , integrated with the glass panel of the window. Preferably, sensors (not shown) are provided to sense the horizontal luminance and/or illuminance e.g. , with respect to the top surface of desk 102. This enables the control system to control the spatial luminance or illuminance ratios, e.g., vertical versus horizontal contributions.

The control system preferably is to be customized or re-programmed so as to meet any individual's preferences. The control system typically may include a rule-based controller such as a fuzzy controller to allow for a high degree of user-fπendhness regarding programming and to allow for a high degree of versatility.