Review of ILC results for fritted glass Jacob C. Jonsson Windows and Daylighting Program Windows and Envelope Materials Group Building Technology and Urban Systems Department Building Technologies Program 0

Clear fritted glass on 6 mm White fritted glass on 6 mm Frit samples in the ILC

Results Complex ILC – Fritted glass Front < Back 1.Longer path length gives increased absorption 2.Increased internal angle for leaving the sample 3.Side-shift of light missing the port 4.Side-loss of light exiting the sides of the sample Front > Back 5.Rough exit interface leads to light trapping

Results Complex ILC – Fritted glass Beam area/Entrance port area: Box 18: 0.20 Box 20: 0.15 Top 3: ~0.30 Bottom 3: ~0.40 Not only ratio but acceptance angle, so larger port with same ratio give better result Sphere diameter: Box 18: 270 mm Box 20: 150 mm Top 3: 220 mm & 150 mm Bottom 3: 150mm & 75 mm

Diffuseness also instrument dependent Ranking is similar as in total transmittance Smaller beam to port ratio results in less loss of light scattered at large angles which gives larger b-factor Less difference for almost fully diffuse sample

1.Large angle scattering results in TIR, side loss 2.Intermediary angles miss the port, side shift 3.Enters sphere in detector field of view, high impact 4.Enters sphere like reference beam 5.Enters sphere and strikes baffle, lower impact Simple geometry Surface scattering sample with scattering surface toward the light source Jonsson J.C., Roos A., Smith G.B., Light trapping in translucent samples and its effect on the hemispherical transmittance obtained by an integrating sphere. Proc. SPIE 5192, L. M. Hanssen Ed., (2003)

International Commission on Glass (ICG) TC10 – pursues a method using standard diffuse samples, targets two problems: reduces effect of the sample’s scattering distribution and gives a good reference Investigation of center-mount accessory for absorption measurements in addition to the R, T measurements Status of research projects

Increasing port significantly increases measured transmittance Clearly dependent of scattering Larger port is still shielded from detector Constant beam size increase port size Clear frit White frit

Different ways to measure transmittance 1. Sample, here a fritted glass with the fritted side towards the sphere 2. Standard Labsphere port plate, easy to remove 3. Part of the sphere 0. Aperture plate (not included by Labsphere) to reduce beam size

One sample - 6 results

Using a specular reference for a specular sample seems straight-forward Combined with knowledge of sphere response there is a harsher general truth for diffuse samples Use a reference so that light travels the same path from source to detector Applies to scattering samples Relative: S ds /S ref ≈ Tscatt/R wall

Detector sphere response Nilsson A., Jonsson A., Jonsson J.C., Roos A, Method for more accurate transmittance measurements of low-angle scattering samples using an integrating sphere with an entry port beam diffuser. Applied Optics, 50 (2011) Milburn D. I., Hollands K.G.T., The directional response of an integrating sphere detector system. Optics Communication 115 (1995)

Pilato P., Rossi G., Roucour J., Simons J., Rose-Wilson H., Spectrophotometric determination of visible and solar parameters of sand-blasted glass panes and translucent glass laminates, Rivista della Stazione Sperimentale del Vetro, 5, 2003 Simulation of light entering a sphere Sample Mie scattering No absorption Instrument d sample thickness RLB – light beam radius RTP – sphere port radius

Results Complex ILC – Fritted glass Curious peaks in NIR, inverse effect of the spectralon absorption

Not seen in specular component Relative: Sds/Sref ≈ Tscatt/Rwall Answer:

Correction possible b( ) = T diff ( ) /T total ( ) Tcorr ( ) = b ( ) *RS pectralon ( ) *T total ( ) + (1-b ( ))*T total ( ) This correction is much smaller than the spread between submissions

First order error too small – 6mm thick sample 25mm port 150mm sphere The relative intensity of light scattered from a single pixel at the front of the frit, after passing through the scattering interface, that gets transmitted into the sphere. Total internal reflection limits the radius of the light entering the sphere port. Center of beam to the left, extreme corner to the right. Even for Lambertian, 1 st bounce has 67% of the pixels fully captured. Ratio between light captured and light transmitted is But the transmittance in the first bounce is only 0.42 relative to what left the fritted surface.

Gaussian distribution might not do it– 6mm thick sample 25mm port 150mm sphere Division of the light leaving the fritted surface inside the sample for a given Gaussian distribution. Left of the blue solid line is how much hits the specular reflectance port The blue dashed is showing how the solid blue gets refracted leaving the material Left of red solid line is the amount captured by the sphere all in all Right of the red solid is amount that is captured by total internal reflection. In this case a factor of is exluded from the first order analysis.

First order error too small – 6mm thick sample 25mm port 150mm sphere For Lambertian scattering total amount of light captured is This is before considering the Fresnel components of absorption and reflectance at the exit interface. A haze value of 0.9 is low enough to result in a distribution where no light scatters outside TIR. a) We have to look at second order effects for Lambertian cases. B) Try different distribution

1.The reflected light has interacts with the scattering surface and can be scattered back towards the port. 2 nd order effect needs to be included Surface scattering sample with scattering surface toward the light source