Speaker
Description
The use of lenses and specular reflecting surfaces (mirrors) to direct light in optical instruments is commonplace. However, light may also be directed within a closed hollow volume having diffuse internal reflecting surfaces and a small exit hole – if the volume is round, this is called an integrating sphere. This is a critical component in the homogenisation of light and used when determining the efficiency of luminescent materials and light sources. Modern commercial integrating spheres employ Teflon-based materials for the internal diffuse reflecting walls, which produces excellent optical characteristics but at high cost. Barium sulphate powder has been identified as an eminent diffuse reflecting material and the aim of this work was to assess whether effective diffuse reflective coatings could be formed by dip-coating a substrate in a suspension of barium sulphate powder in a solution of polystyrene dissolved in toluene. For the solution, 50.6 g of polystyrene pellets (192 000 g/mol) was dissolved in 200 ml of toluene. Substrates of glass slides and sand-blasted steel sheets were dip-coated using a withdrawal rate of 300 mm/min. This was repeated after 15, 30, 45 and 60 g of barium sulphate powder had been mixed into the solution using an ultrasonic bath, while greater loading was not considered to produce a solution suitable for dip-coating. Scanning electron microscopy showed that the barium sulphate particles had the form of irregular plates varying from 0.1 - 2 μm in size. Using the mass deposited for the pure polystyrene films, their thicknesses were estimated near 20 μm, so the films were significantly thicker than the size of the powder particles. Loading the polystyrene solution up to 45 g of barium sulphate had only a small effect on the total diffuse transmittance for the glass slide samples, although the direct transmittance was strongly reduced. Only at the greatest loading of 60 g was a significant decrease in the diffuse transmittance observed (to 57% at 550 nm), together with a substantial increase in the diffuse reflectance (to 31%). Dip-coating the steel plates, even to the maximum barium sulphate loading, had little effect on their diffuse reflectance. This is ascribed to the fact that the diffuse reflectance of the coating was comparable to that of the steel plate itself. The conclusion is that dip-coated films of barium sulphate in polystyrene do not produce effective diffuse reflectance coatings since barium sulphate has a similar refractive index (~1.64) compared to polystyrene (~1.60), which reduces the scattering of the light considerably.
| Apply for student award at which level: | Honours |
|---|---|
| Consent on use of personal information: Abstract Submission | Yes, I ACCEPT |