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Daylighting simulation
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Daylighting simulation

Introduction

Direct sunlight is poor for visual comfort in offices or homes. Areas exposed to direct sunlight are too bright, with illuminances over 10000 lx, making most computer displays unreadable. The contrast between lit zones and dark zones is generally too high, requiring the eye to constantly adapt when looking at different places of the same room : this can be an important cause of eye fatigue.
People will generally react by closing the curtains or masking the sun light by any other mean. Then, the indoor lighting will be too low for comfortable work, and they will switch on artificial lights.

Day-lighting in bright sunlight is a main concern in architecture : using special designs and materials, it is possible to improve significantly the visual comfort. These techniques can be realistically simulated with Ocean. Unlike traditional building light simulation software, no model simplification is necessary : it may work with fully detailed CAD models and several millions of polygons. The same software and model may be used for producing detailed illuminance mappings, as well as realistic renderings for visualization.

Simulation of direct sunlight

Visualization

The following image is a simulation from our urban test scene, the camera being placed at the third floor of the glazed office building. It shows direct sunlight entering the room through the double-glazing windows, and hitting a first desk, while another one was placed in a darker zone of the room. Walls and ceiling are covered with a white paint having an albedo of 0.75. The floor is made of smooth concrete. The sun elevation is 57°, while its azimuth is at 45° to the window orientation, resulting in an angle of incidence of about 54°. The dynamic range of the image was compressed using a Reinhard algorithm.

Direct sunlight with no special daylighting system

The first desk has an illuminance of about 17000 lx. This is too much for comfortably reading at a computer display (emittance of about 2500 lx). This is also too much for looking at darker areas of the room without several seconds of eye adaptation.

The second desk has an illuminance of about 460 lx. While this is close to the standards for reading (500 lx), the high contrast of the room will be very uncomfortable, the user will be blinded for several seconds if he looks at the first desk.

Contrast analysis

This image was generated with the same scene, but with color encoding of the luminance values. The two desk areas differ in average luminance by about two decades : the eye must adapt when passing from one area to the other.

Office luminance in cd/m^2 - direct sunlight

Simulation of a day-lighting system

There are many possibilities for addressing this direct sunlight issue. We will focus on a simple one which consists in adding reflecting blades in the window for redirecting sunlight to the ceiling.

Blade geometry and material

The blades are horizontal sheets of smooth mirror-like steel, located inside the double glazing spacing. They are 16mm wide and as long as the windows (115cm). They are very thin : we modeled them with zero thickness in this scene, but in the real world, they would be for instance 0.2mm thick. They are placed vertically every 15mm inside the window. The bottom side (facing downwards) was paint in black to avoid double reflections, which would lower the efficiency of light redirection.

Results

Office exposed to direct sunlight, with lamellas inside the glazing for redirecting light to the ceiling

The visualization image shows that light is redirected very efficiently to the ceiling for this sun orientation. Nearly no direct sunlight passes through the blades.

The illuminance of the first desk is now about 1500 lx, which is a reduction of 91%. This is a good value for both reading and working comfortably on a computer screen. However, this could have been achieved by simply closing the curtains.

The best result comes from the illuminance of the second desk : it is now 408 lx, which is only a 12% reduction. Closing the curtains would have reduced this by around 91%, leading to an illuminance of about 40 lx, and forcing the user to work under artificial lighting by a bright sunny day.

Contrast analysis

We can see that the two desk areas have average luminance that differ by less than a decade. Less adaptation to is required, thus reducing eye fatigue.

Office luminance in cd/m^2 - direct sunlight with daylighting system

Other day-lighting experiments

Dichroic film blades

In the following picture, we experimented replacing the steel blades by polymer foils with a dichroic coating. This coating separates light into two colored components : the reflection is orange-red, while the transmission is cyan.

Day-lighting with dichroic lamellas

While the day-lighting effect is not as efficient as with the steel blades, this gives an interesting creative lighting effect

Diffuse blades

This time, we replaced the blades by white transluescent diffuse material.

Daylighting with diffuse transluescent blades

The effect of diffuse blades is less interesting than for reflective blades : the illuminance of the desk close to the window is less reduced, while the illuminance of the darker one is too low for reading.

 

Conclusion

In this simple day-lighting example, we demonstrated the broad possibilities of the Ocean software for lighting design. The exact nature of its calculations allow making quantitative analysis of day-lighting performance. As it is possible to work on detailed CAD models, with complex materials based on measured spectral data, testing virtually any day-lighting system design is possible.

For more information, contact info@eclat-digital.com

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