Simulating the “Death Ray”

Simulating the “Death Ray”

How to turn a skyscraper into a solar furnace

The new London’s skyscraper at 20th, Fenchurch street, nicknamed the “Walkie Talkie”, and still under construction, recently became the city’s latest attraction. Its huge, south-facing concave facade reflects light into a focused beam of light, and has been reported to melt cars, damage shopfronts, and even fry eggs. So we could not resist in testing how Ocean could predict this phenomenon!

Simulating with Ocean

Modeling a scene with SketchUp

We started from our standard urban test scene, which contains two small detailed buildings and some blocks, and added a large building to the north, approximately 90 meters high and 38 meters wide. Its south facade was made concave, with a curvature of about 100m, and faceted in 2.5m x 2.5m flat double-glazing panels.

sketchup
A “Fryscraper” quickly modeled with sketchup

Setting the facade glass materials

Modern glass facades are made of double glazing. The outer glass pane is generally coated with a thin film that reflects solar heat outside of the building, in order to reduce air-conditioning costs and improve comfort.

As we do not know exactly which coating was used at 20th Fenchurch street, we started the simulations with PPG Solarban R100, a high performance low-E reflective coating. It is specified by the manufacturer to reflect 32% of the visible light, and 41% of the solar heat. These values will likely have a strong impact on the intensity of the focused beam.

The inner glass pane is a standard float glass.

Both glass panes were not made perfectly flat, but instead, have the following imperfections:

  • Roller-wave distortions of amplitude varying between 0.1mm and 0.2mm, to simulate the glass tempering process
  • Glazing misalignment of about 0.25° (which corresponds to ±2.5mm tolerances when positioning the glazing in its frame or mount)

Adding a reference probe surface

We added two 30cm white boards to the scene. One is on the building wall, another laying on the floor. They are modeled with a perfectly neutral lambertian material with 18% reflectance.

These probes will allow calculating the amount of light incoming on the scene, by measuring their luminance. We could also have set up a “lightmap” instrument in Ocean, but this would require a separate simulation for rendering the image and measuring incoming light.

Modeling the sunlight

The environment lighting was set to the Preetham skylight model with a clear sky (low turbidity : 2). It was modified to extend the wavelength range of direct sunlight to the 295-2500nm range. We also use an beta version of Ocean, modified for working outside the visible light wavelength range and include the infrared. These features will be available in an upcoming release.

The sun elevation was set to 41°, corresponding approximately to one hour after solar noon in London, on September 5th.

Results

deathray-reference
Reference image with direct sunlight exposure.
1/160s, f/16, 200ISO

Reference image

We first made a reference image, with the sun hitting the building at the same incidence of 41°.

The probes are then measured in Ocean’s interface:

  • Vertical (wall) probe
    • Luminance : 13700 cd/m²
    • Illuminance : 76300 cd/m²
    • Radiance : 121W/m²
    • Irradiance : 676 W/m²
  • Horizontal (ground) probe
    • Luminance : 13300 cd/m²
    • Illuminance : 74000 cd/m²
    • Radiance : 118 W/m²
    • Irradiance : 660 W/m²

These are classical values for direct sunlight.

 

Death ray simulation

deathray-ppgr100
Simulated skyscraper death-ray image
1/160s, f/16, 200ISO

The concave skyscraper has been added. The sun was moved to the opposite direction, and hits the facade indirectly after being reflected and focused on the skyscraper. The image on the right shows the simulation results.

The probes are measured again :

  • Vertical (wall) probe
    • Luminance : 61800 cd/m²
    • Illuminance : 343600 cd/m²
    • Radiance : 668 W/m²
    • Irradiance : 3708 W/m²
  • Horizontal (ground) probe
    • Luminance : 51500 cd/m²
    • Illuminance : 286200 cd/m²
    • Radiance : 558 W/m²
    • Irradiance : 3102W/m²

The luminous values are 4.5 times higher, and energy values 5.5 times higher than our reference simulation for direct sunlight! Taking into account the angle of incidence on the vertical probe (41°), this is equivalent to more than 4.9 kW/m² at normal incidence!

Frying eggs

In comparison, this plancha/teppanyaki device boasts a 8.9 kW/m² power per unit area ratio. So a perfect absorbing black board would heat as much as the cooking device with the knob on 5/9.

plancha-teppanyaki

Conclusions

This short test shows that skyscraper “death ray” effect are well simulated by Ocean. Its unbiased algorithm ensures that the simulated beam is the exact solution of geometry optics for the given model. The spectral calculations allow a precise estimation of light and energy amounts in the scene, even with high-performance coated glass.

In this study, we tested only one type of coated glass. We could use the tool to better understand the impact of high-performance solar control glazings on the effect. Glass flatness likely has a strong impact too, as imperfections will reduce the beam focusing.

Ocean’s simulations could be as well used for designing shading structures preventing that kind of phenomenon.

About Ocean

Ocean is a high performance light simulation software, designed to predict visual appearance and illumination on complex CAD models, such as fully detailed buildings. It is developed by Eclat-Digital, a company based in Nantes, France. Eclat-Digital provides software licenses, as well as optical simulation services.