Sorry to have been so quiet recently, y’all, but it was vacation time. The Castor-Perrys visited the big theme parks in Orlando and had a great time. What’s more, filters played a big part in our enjoyment! Let me explain.
I’m an electronics guy, and so most of what I write (technically at least) here and elsewhere involves fun with electrons. The filters that I talk about are generally electrical filters, whether passive LC, or active, or switched-capacitor, or calculation-based. But I’ve also been interested in optics and light for most of my life – I started wearing spectacles when I was three years old, and haven’t, well, looked back.
But electrical waves aren’t the only things that can be sorted into frequency bands with filters. Any time you wear tinted sunglasses, place a sheet of coloured glass in front of a stage spotlight, or even peer through a crumpled sweet wrapper (Quality Street in the UK would be the obvious example; I’m not a US candy aficionado ), you’re using an optical filter – a system whose optical attenuation depends on the frequency of the incoming light.
There’s been a quiet revolution in the technology behind 3D projection systems in the last few years, and filters have played a big part in this. I hadn’t noticed this until our local cinema installed a new laser projection system and started handing out there rather fancy-looking specs for 3D films. So I read up on what’s been happening – and got some real filter joy from it!
Really old-school 3D projection – with those disposable paper specs that have one reddish and one greenish plastic eye film – was a crude approximation in which each eye only receives a subset of the entire color gamut of the image. The brain (of a person with binocular vision, anyway) manages to stitch it together, and the impression is of a fully coloured scene with some 3D effects. It’s pretty poor, really – especially if, like me, you don’t have binocular vision, in which case the colour distortion is terrible.
More recently the standard has been to use specs with sheets of plastic Polaroid material in front of each eye. One of the eyes receives predominantly vertically polarized light, the other gets a horizontally polarized feed because the sheet has been rotated by 90 degrees relative to the first one. Both sheets pass all the colours in the image, producing greater colour fidelity. But the Polaroid sheets lose significant signal amplitude, and it’s hard to increase the light output of standard cinema projectors to make up for this. Another constraint is that you have to keep your head vertical in order to preserve the orientation between the Polaroids on your eyes and the matching polarizers on the projectors, or you get “double vision”. And mass production plastic Polaroid doesn’t completely eliminate light with the ‘wrong’ polarization. So again, image crosstalk can be quite a problem if your brain is doing all its work one eye at a time.
Filters to the rescue!
With the advent of so-called 6P 3D, optical projection technology has reached a new peak. The system relies on the fact that the eye – just like most camera sensors – really detects only three colours, and infers hue from the relative proportions of the signals from its three receptor channels. What this means is that you could send light of two somewhat different frequencies to the eye (or your camera), and as long as those frequencies predominantly excite only, say, your red sensor, they will both look like the same red colour .
So, the way the 3D image is constructed is with a light source that has six very precisely defined wavelength components – two reds, two greens and two blues. Each eye gets one of each colour, the separation being done by a beautifully elegant multilayer optical filter with multiple passbands. All you need to do is to get the basic amplitude relationships between each eye’s R, G and B components right and the images will have exactly the same colour balance – even though they are being constructed with light of differing wavelengths. A fantastic exploitation of the limitations of our three-colour visual system.
This is clearly shown in the frequency response (actually, wavelength response) of the filters created for the two eyes, in this image borrowed from the website of Iridian, a major supplier of precision optical filters:
Check out their website at Iridian Optical Filters– they are clearly on top of this optical filter technology and show several other interesting applications, including spectacles to protect pilots from green laser pointer light.
Now, you know I’m a filter guy, and I can’t begin to express how overjoyed I was to see just how precisely defined and delineated the frequency bands of those two filters are. This kind of filtering is difficult enough to do as an electrical filter, so the idea that you can do it by controlled deposition of many layers of carefully chosen dielectric materials in sub-wavelength thickness, at a price that fits the business model of a cinema or theme park ride, is, well, uplifting for me!
We saw quite a few theme park rides that used this approach for the 3D projection. Now you can tilt your head freely. And the separation between images is perfect, as far as I can detect.
In the very latest projection systems, the six required wavelengths of light are created by tunable lasers, sometimes even installed in a ‘light farm’ that can pump high intensity coloured light to all of the theatres in a multiplex.
Next time you’re at a 3D presentation, check out your specs. Optically-filtered ones have distinctly different hues in the reflections from each eye’s “lens” (they are curved, but don’t have any optical power) – and that’s another filtering message, loud and clear. You might ask – what happens to the light at wavelengths that the filters won’t pass? Well, it’s not absorbed, it’s reflected – just as in the case of an LC passive filter. So these optically filtered specs look rather like cool mirrored sunglasses, because they are reflecting a lot of light! And when you look at incandescent, fluorescent and LED lights through these glasses, you can see significant differences in colour balance if you concentrate on just one eye at a time. I spent much of my ride queuing time with my 6P 3D glasses on, checking out the way they filtered the various light sources around differently from one eye to the other. Probably looking more than a little weird to everyone else. But I’m a filter nerd. And I got my filter joy, even on holiday!