Friday, November 15, 2019

Black to the Future Redux

A bit more than a decade ago, Hue Angles presented an article called “Black to the Future” [1]. To further darken a black carbon surface, investigators in Rensselaer Polytechnic Institute and at Rice University roughened the surface by a carpet-like arrangement of carbon nanotubes (.01" long, 1/30,000 as wide) standing on their ends. The result was a surface with a reflectance as low as 0.045 percent (three times darker than any previous material) and a refractive index that could theoretically be as low as 1.01 [2]. We proposed possible uses for such a material in spectrophotometry:
1. Black surfaces for minimizing stray light in optical instruments

2. Light traps for suppressing unwanted diffraction orders

3. Gloss traps for removing specular reflection

4. Black calibration standards
Now, after more than a decade, how well did our wish-list work out? Not well, at least for commercial applications. At the ISCC topical meeting on black and white, held that very same year, it became clear that the carbon nanotube technology was too delicate and too expensive for our purposes. But the technology evolved and improved anyway, and new uses were found.
Beginning in 2014, Surrey NanoSystems issued a product called Vantablack, which reflects 0.04 percent of UV, visible, and IR radiation. Vantablack had the same mechanical vulnerability as its predecessors, so it did not find many applications on Earth. However, in space the substance could be undisturbed, and starting in 2015 helped capture stray light to enhance spaceborne imagery (e.g., tracking stars) without a large payload penalty. Also, back on Earth, it achieved an effect that was coveted by artists: three-dimensional objects covered with Vantablack would appear to be flat surfaces because not enough light was reflected to reveal the 3D topography. BMW even painted a car with Vantablack. By 2017, a version of Vantablack (S-VIS) became available in a spray-on form. The reflectance was understandably not quite so low in this form: 0.2 percent. But the material still served its various functions. Although Vantablack is not commercially available, Surrey NanoSystems has licensed the product.
Now there is a material that is still blacker. It emerged from laboratories in Shanghai and at Massachusetts Institute of Technology[3], and has a reflectance of 0.004 percent. The discovery was accidental, during attempts to grow carbon nanotubes on aluminum foil. To avoid the formation of oxides between the nanotubes and the foil, the investigators soaked the foil in salt water and moved it into a small oven where the nanotubes could grow without oxygen interference.
A popular article by Brandon Specktor [4] describes two implications of the new black technology. There is a $2 million diamond on exhibit in the New York Stock Exchange that has been covered with the material and is invisible on a background of similar black material. Specktor speaks poetically of the black material “eating” the diamond and that it is a “veritable black hole.” Indeed, as he suggests, we may soon be able to see real black holes if the new black material is deployed to optical instruments in space. But I don’t expect to see signs with the words “Schwarzschild radius” on any photos, though we have seen cartoons of other human-created follies (such as Pluto bedecked with the sign “am too a planet.”) Enough about black holes…
In summary, the last ten years have brought a factor of 10 reflectance decrease in the blackest black. We’ve achieved a decade in a decade. Stay tuned for the next decade.
[1] M H Brill, A Ingleson, and C McLellan, Black to the future. ISCC News # 434 (2008), 3-4.

[2] Z-P Yang, L. Ci, JA Bur, S-Y Lin, PM Ajayan, Experimental observation of an extremely dark material made by a low-density nanotube array. Nano Letters 8, No. 2 (Feb. 2008), 446-451.

[3] K Cui, B L Wardle, Breakdown of native oxide enables multifunctional, free-form carbon nanotube-metal hierarchical architectures. CS Appl. Mater. Interfaces 2019 XXXXXXXXXX-XXX:September 12, 2019;

[4] B Specktor, There’s a new blackest material ever, and it’s eating a diamond as we speak. Live Science, Sept 16, 2019,… 

Michael H. Brill

Friday, September 6, 2019

Reflection on “Dark Spectrum Part II”

As a new experiment for ISCC News, my column in Issue 487 (reproduced here), together with that of Carl Jennings, comprise an interdisciplinary dialogue within a single issue.

In the course of writing “Dark Spectrum Part II” for the current ISCC issue, Carl Jennings asked me for comments. In response, I began to think about the optics of Newton’s vs. Goethe’s experiment. My thought process changed through the dialogue, especially as it related to Figure 3 of Carl’s essay. This Hue Angles summarizes the essentials of our email discussion, which seems to reveal some heretofore unremarked differences between the experiments of Newton and Goethe.

I started off with the idea that Newton’s prism experiment passes collimated (uni-directional) light from the Sun through a hole in a light-blocking shade (like a window shade), and through a prism. The prism disperses the sunlight into a spectrum according to the various refrangibilities of the wavelength components of the light. Then, in one version of the experiment, the dispersed spectrum hits a screen, and is reflected as a multicolored pattern to the observer. Collimation is necessary because light from two directions incident on the same point will provide different banding, and the bands from multiple directions will superimpose to wash out the pattern. I was convinced that collimation, being essential to Newton’s experiment, also figured in Goethe’s experiment. The only difference, I thought, was that Newton looked at a narrow beam through a hole or slit, and Goethe looked at a broad beam with narrow blocking elements that would cast shadows the prism would refract differently according to wavelength. Accordingly, I reacted as follows to Carl’s Figure 3 and its caption (see below for figure):

Mike: The caption of Figure 3 states: “A pair of scissors against a bright white winter sky in Munich, through two prisms simultaneously. (Source: author).” A bright white winter sky is about as non-collimated as you can get, and on the face of it this seems incompatible with the color bands in Figure 3.  The only way to assure collimation is to position the prisms on the light path that includes the scissors and the eye. In that case, if the distance between the prisms is long enough, only light going nearly parallel in one direction through the first prism will intercept the second prism and hence get to the camera.

Carl: You discuss the color bands in the scissor image (Fig.3) as being incompatible with non-collimated light - but that is exactly the point - it happens when it shouldn't! None of the banding should happen, collimated or non-collimated, but the fact is it is there and is easily observable. Both prisms used in the photo were between the camera and the scissors, so no light was collimated. I found that two prisms made the banding more distinct, though it is observable with one, if you use a good prism.

Mike: I now think the paradox of color-banding with light from a white winter sky is not a paradox after all. Newton needed collimated light because Newton’s prism images the spectrum directly on a screen. In Goethe’s geometry, there is another element that must be in the optical train: a lens. A lens provides a point-to-point transfer from an object to an image (in respective object and image planes), whether or not the light is diffuse.  The plane of Goethe’s shadowing components was the object plane, the lens was in his eye, and the image plane was his retina (or a tangent plane thereof).  In your scissors example, the lens was that of the camera.  Of course, the eye’s lens is implicit in all these demonstrations, but it is physically essential in Goethe’s experiment in which the eye looks directly at the diffuse light through the prism(s). Newton’s experiment does not have the eye looking directly at the light through the prism, and no lenses are needed between the slit and the screen, so collimation of the spectrum-separated light is essential.

In other words, a lens (be it eye or camera) is essential for the diffuse white sky light to show bands when it passes the scissors (which should be in the object plane of the camera lens).  That role of the lens is essential to Goethe’s experiment. A lens is also part of Newton’s experiment because Newton used his eye to see the card-reflected spectrum, but the lens plays a different role here. It is a subtle point but should be understood.

Incidentally, Figure 3 suggests to me that, although a diffuse white sky exists in front of the camera, there must be very little light from behind the camera or there would be a white desaturating reflection from the front surface of the scissors.

The discreteness you have noted of the band colors---as opposed to their presence at all---is still a perceptual effect, as you have said before. I have no further thoughts on this matter now.

Carl: That is very interesting - I have never come across a description of boundary colors (even colorimetric ones, as in Koenderink or Bouma) that discuss the role of the lens. This is certainly a key feature to Goethe's phenomenological approach, but as far as I can tell does not exist in the literature.
One more question. Would sunlight passing through a hole in a window shade be already collimated? I ask because in Newton's own diagram of his experiment you can see that he has placed a lens in front of the prism, presumably to collimate the light.

Mike: Good question. The Sun is very far away (93 million miles), but it has a diameter of 0.864 million miles, which causes the Sun to subtend about half a degree of visual angle. The Sun’s rays depart from collimation by as much as ¼ degree.  Collimation is almost—but not quite—completed without the lens, and Newton obviously sought to do better.

Michael H. Brill

Figure 3. A pair of scissors against a bright white winter sky in Munich, photographed through two prisms simultaneously. (Reproduced with permission from Carl Jennings)