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Dec 21

An inside-out snowman

I made an inside-out snowman from two integrating spheres. Below is a picture and line drawing to show the insides. This is also a personal tribute to the work of Rachel Whiteread, who amongst other things produced an inside-out house, and a library as a memorial to the victims of the holocaust.

The snowman’s body is made from two integrating spheres joined at two open ports. The arms are tubes that we use to pass gas into and out of the larger cell. The hat is made of pieces of SpectralonTM (see below) and the scarf is an electrical cable that I found in the lab.

Un-lit Snowman

Photo of the inside-out snowman. The integrating spheres each have a spherical cavity within the cuboid.

snowman schematic

Schematic diagram of the inside out snowman made from two connected integrating spheres.

These devices are made from SpectralonTM, which is has a very high (~99%) diffuse reflectivity. When this material is wrapped around on itself to form a cavity, any light entering  the cavity is able to bounce back and forth randomly, many times, until it is absorbed by the sidewalls, the gas we want to analyse, or a photodetector. For an ideal diffuse scatterer or Lambertian surface, the radiance L (in W m‑2 sr‑1) from a given point is a constant in any direction. For a cavity with spherical geometry, it follows that the irradiance at the surface E (in W m‑2) received from that point is then constant over the entire sphere[1][2]. Therefore, after only a few passes across the cell to remove the local effects of launch geometry, the irradiance is perfectly uniform over the surface, and a detector placed at the surface is able to make a representative sample of the irradiance everywhere else. This makes integrating spheres ideal for use in measurement of parameters such as the total emission from light sources.

integrating sphere

How light behaves in an integrating sphere, showing two examples of random light paths from laser to photodetector. In fact there is an infinite number of potential light paths.

 To prove that the snowman works properly as an integrating sphere (or rather, integrating snowman), we performed the following experiment. Here, a laser pointer is being aimed into the lower integrating sphere port. When the beam enters the cell, you can see the port in the upper integrating sphere glow red.

What if HAL made a snowman

When the light enters the lower sphere of the snowman, you can see a red glow in the viewing aperture of the top sphere.

We have been using integrating spheres in our gas detection work, as they provide a convenient multipass cell, increasing the light’s interaction length with the gas and improving our signal to noise ratios. Compared to other multipass gas cells, they are also spectacularly easy to align. Although they have been used in this context before, we have been unpicking the science behind this application. We worked out the nonlinearity in system response that results from the fact that there is no single optical pathlength within the cell[3][4] (it has a surprisingly simple analytical answer) and investigated the limit in performance caused by laser speckle in the sphere[5].

References


[1]       Labsphere Inc. A guide to integrating sphere theory and applications. Labsphere, North Sutton, NH, USA, 1998.

[2]       P Elterman. Integrating cavity spectroscopy. Applied Optics 9 (9), 2140‑2142, 1970.

[3]       J Hodgkinson, D Masiyano and R P Tatam. Using integrating spheres as absorption cells: pathlength distribution and application of Beer’s Law. Applied Optics 48 (30), 5748-5758, 2009.

[4]       J Hodgkinson, D Masiyano and R P Tatam. Using integrating spheres with wavelength modulation spectroscopy: effect of pathlength distribution on 2nd harmonic signals. Applied Physics B 2012, DOI 10.1007/s00340-012-5166-7

[5]       D Masiyano, J Hodgkinson and R P Tatam. Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 2: Integrating spheres. Applied Physics B 100 (2), 303-312, 2010.