Dichroic Disk in Infrared Visible Light
Natural ventilation and daylighting are increasingly employed in modern buildings, underground spaces and tunnels to minimise energy consumption and release of harmful emissions to the environment. Innovative daylighting techniques including lightshelves, prismatic glazing, holographic films and light pipes have facilitated the effective use of daylighting in a wide range of buildings. In addition to bringing energy savings, these daylighting technologies also help to create healthier interiors for occupants. Natural daylight has also been found to relieve seasonal affective disorder, cholesterol problems, chronic fatigue, jet lag as well as benefiting people in shift work and computer VDU work.Natural ventilation techniques have also been widely used and include passive stack systems in a wide variety of buildings across Europe. Until now, daylighting and natural ventilation techniques have been developed independently and form separate systems. Integration of these technologies would reduce system costs and payback periods as well as make natural ventilation and daylighting more attractive to owners and users of buildings.
One method for the integration is the use of concentric channels for both daylighting and natural ventilation. As shown in Fig. 1, the central channel, or the light pipe, will guide sunlight and daylight into occupied spaces while the outer channel, or the ventilation stack, enables passive stack ventilation. By constructing the light pipe using dichroic materials, the infrared part of the solar radiation is allowed to be transmitted to the stack but the visible light is reflected downwards within the light pipe towards the room interior. The heat gain to the interior can be reduced and the thermal stack effect strengthened.
A dichroic material is usually manufactured on a glass or plastic base in which alternate layers of transparent materials are laid. The amounts and values of the wavelength transmitted or reflected depend on the thickness and refractive index of each layer. Examples of dichroic materials include magnesium fluoride/zinc sulphide and tantala/silica oxides. The dichroic material used in the tests reflects the visible light while transmitting the infra-red radiation, at very high efficiencies. Thus, it is referred to as a “Cold Mirror”.
Experiments were set up to determine the infra-red and visible-light transmission/reflection characteristics of the material. This information then formed the basis of a detailed analytical study of the effect of dichroic material on light transmission in the light pipe and enhancement of air flow in the passive ventilation stack.
Tests were carried out in a specially constructed room with black internal surfaces to obtain the characteristics of the dichroic material in terms of infra-red and visible light transmittances and specular reflectances for visible light, as detailed below.
The dichroic material for testing was obtained from Sycamore Glass Components, USA. The material has a nominal reflectance of 90% between the wavelengths of 420 and 630 nm and a nominal transmittance of 85% on average between the wavelengths of 750 and 1200 nm. The 750–1200 nm band carries most infra-red energy in the solar radiation. The substrate is Borofloat, a borosilicate glass that can handle heat. The high cost of the dichroic material currently purchased from Sycamore precludes the construction and testing of actual light pipes lined with the material. However, recent advances in material and manufacturing technologies indicated that dichroic materials’ cost would soon be dramatically reduced to allow large-scale applications in buildings.
The light source used for the tests was manufactured by General Electric (GE) lighting. The lamp is Halogen TAL 100 mm with an integrated metal reflector and constant colour coatings providing consistent light quality and high intensity. The power of the lamp was 50 W and the beam had a peak intensity of 55,000 cd and beam spread of 4°. The colour temperature of the lamp is 3000 K and with a rated average life of 3500 h. The environmental chamber within which the optical experiments were carried out is a wooden structure measuring 3×3×3 m, with a single door and no other openings. The walls, door, floor and ceiling were covered in a black matt paper to reduce the amount of secondary reflection, which may affect readings, but only to a negligible amount. The test rig was on a platform inside the chamber and the equipment was operated from outside the chamber without the presence of the researcher inside the chamber. This also reduced the amount of unwanted secondary reflection. All experiments on the dichroic material were carried out under these strict conditions to ensure that accurate results were obtained.
The tests for the dichroic study were performed on a platform situated at the centre of the environmental chamber. Fig. 2 shows the schematic layout of the typical arrangement for the measurement of transmitted infra-red and visible light through a dichroic-coated glass disk. Steel rods and clamps were used to fix a 0.5 m long black tube horizontally on a table covered with matt black paper. The position and the horizontal levelling of the tube were checked regularly with a spirit level. The light source was placed at one end of the narrow black tube and the dichroic disk positioned on a spectrometer at the other end of the tube. The spectrometer allowed accurate adjustment and measurement of the angle between the dichroic disk and the light beam. The irradiance of infra-red and illuminance of visible light were measured both before and after the dichroic disk is positioned in place. These data were then used to calculate transmittance values.
The tube was coated on the inside and the outside with a matt black paint to ensure that the light rays meeting the dichroic disk are very close to being parallel. This allowed the accurate determination and adjustment of the incidence angle used in the tests. It was anticipated that, as the experiment continues, the tube would get hot due to the radiated heat from the lamp which may cause errors in the infra-red measurements. The lamp operating time and the corresponding extent of this effect were monitored and the information was then used to plan the test procedures. Measurements were taken during a short period of a few seconds between cooling intervals of about 15 min to control the temperature of the tube and to prevent the tube from heating up. This eliminated the error due to the radiated energy from the tube. The instrumentation was controlled and measurements undertaken from outside the chamber. The photometer used was a Hanger Universal Photometer/Radiometer model S3 fitted with sensors for the measurements of visible light illuminance and a special detector for the measurement of infra-red irradiance. It incorporated silicon-diode photocells with approximately the spectral sensitivity of the human eye (CIE). It was capable of measuring illuminance in the range 0.01–200,000 lux. It has an accuracy of ±3% and a virtually perfect cosine correction curve. The photometer was connected to a remote photocell via a flexible lead, making it easier to obtain readings without blocking incident radiation reaching the photocell. The infra-red remote sensor (SD7) had a spectral response to wavelength in the range of 700–1150 nm. It had an absolute sensitivity of 2 nA/W/m2, with an accuracy of ±3%.
Tests to determine the visible-light specular reflectance of the dichroic disk were carried out in overcast-sky conditions. Prior to each test, a luminance range check was carried out to ensure that the overcast-sky condition was present at the time of the test. The overcast condition is assumed to occur when the average horizon luminance does not exceed half the luminance of the zenith. The tests were carried out in a large room, with large windows open to the outside, using the technique of Fontoynont and Berrutto. A piece of matt white paper (0.2×0.3 m) is attached to the window glass. The dichroic disk is positioned on the floor at a distance from the sheet of paper to allow light from the paper to strike the disk surface at a required angle of incidence on the disk to be established. The specular reflectance at a specific incidence-angle was determined by measuring the luminance “Lr” of the image of the paper reflected off the dichroic disk and the luminance from the paper, “Ls”. The ratio (Lr/Ls) of the two values is the specular reflectance of the dichroic disk at the specific angle-of-incidence. The test was repeated for a range of incidence angles of 10 to 80° at 10° intervals.
Tags: visible light
