Rabbit Hole - Polarising Slides 101

Polarising slides are a bit fiddly to make but great fun once you have mastered the basics. Beautiful natural prismatic colours and your hands stay clean!

What is polarised light?

I’m not sure if modern school curriculums have been updated but in science, when I went, we were told that light travels in straight lines. Later on, at advanced level, you learn that light has a waveform and that the wave form is vertical, horizontal and every degree of angle in between. Polarising filters, are like a grid, and as their name suggests, polarise light into a single linear waveform. If a second polarising filter is placed in front of the first it will only pass that single waveform if the grids are aligned. If they are at right angles to each other no light will pass. If the filters are rotated the reduction from light to dark is gradual, like a dimmer.

If certain materials, described as birefringent, are placed in between the filters they behave as prisms and allow the light to escape the second filter in spectral form i.e. the colours of the rainbow.

And if the second filter is rotated the colours change through the spectrum.

Materials that have birefringent properties include:

Cellophane (but not Mylar - used by itself and also in layers to enrich the colours of other materials)
Sellotape (although the glue can look a bit messy, sometimes)
Polythene (clear plastic bags stretching and/or scratched)
Plastic (thin, clear and moulded, like food packaging - cup cakes are a favourite. For obvious reasons, double the enjoyment!)

One of the scientific applications of polarised light is examination of Perspex (plastic) models to check for stress and weak points.

Then come crystal chemicals. The two that I have had most success with are

Acetanilide - Beautiful tree lined alpine mountains and icy plains.
Cholesterol Acetate - Clusters of spinning rainbow circles.

These I discovered in Bob Beck’s ‘Light Show Manual’, published in 1966 which also lists several other chemicals. This publication is viewable on-line. In particular, the Psychedelic Light Show Preservation Society’s FaceBook group which has a lot of other useful information on the original analogue art form.

Along with other light show pioneers I owe a debt of gratitude to Bob Beck. Making polarising slides at home in my late teenage years and selling them to the burgeoning companies in London provided much needed pocket money at the time. Even when I first joined Krishna Lights in London, the wages were pitiful, often parts in kind if there was no cash around at the end of the week!

Before moving on I must say that birefringent is one of my favourite words. Pre internet when we used things called books like dictionaries and encyclopaedias it was the test of a good dictionary if it was included.

Later on, as my company Optikinetics grew, we published our sales literature in some foreign languages, French, Spanish and German. Birefringent appeared in the copy accompanying the polarising effects wheels that we offered at the time.

The German for it is doppellichtbrichtenden, or similar, that's not how it Googles now, the licht bit is not there. According to our German distributor at the time it translates to ‘double broken ended light’. Which is as it says on the tin.

Polarising Filters

These come in rigid and flexible form. Both are coatings on a substrate. Glass in the case of rigid and a non-birefringent plastic for the latter.

Flexible polarising film is considerably less expensive than rigid. Flexible being used commercially and glass for scientific applications.

Even so small quantities are relatively pricey. At Opti we used a flexible display grade material, Ref HN42. In the 1970s a 100 foot (circa 30m) roll, around 20 inches (50cm) wide cost £90.00. I have checked on line and similar material is offered by a variety of companies, including our old friends Edmunds Optics, formerly Scientific, from the U.S.

Other equipment you will need

A light box - useful too at gigs, when they return post COVID. I would recommend one with a glass top that is replaceable so that you can cut material with a modelling knife/scalpel.

A polarising spinner - this is just a disc of plain polarising film sandwiched between thin flat glass of the same diameter. This can also be used on your projector when showing your handiwork. This can be turned by hand when making effects or if it has a hub a spindle or screwdriver makes it easier to twiddle. It is important to use glass for your spinner as it is not birefringent, unlike plastic which is.

Polarised sunglasses - an addition or alternative to the spinner above. Good neck exercise as you will have to keep nodding and twisting your head. Now you know why Polaroid sunglasses have that little card dangling from them with a small round window of polarising film to check they are genuine.

Cellophane - already mentioned above for enhancing the colours of birefringent effects. Three layers, in the same orientation - as it has a grain - I have found to be best. To achieve this cut it into strips, with a width slightly in excess of the diagonal of your slide and three times that in length. Zig-zag fold it into three, hold and rotate it over the slide you are making whilst viewing it through polarised light to select the best colour range. Cut off the excess and clamp your slide together.

Slide mounts, with cover glasses, preferably 2” x 2” full frame to give the maximum image size. These are quite difficult to find nowadays. 35mm slide mounts will suffice, or you can just use 2” square glass cover glass and edge tape it together. I’ve not done this. I imagine it would be tricky but not impossible. You could pre-tape two glasses together as a hinge.

I’ve slipped into ‘how to’ whilst listing equipment required.

An obvious item would be a slide, or effects, projector with either gate or front-of-lens motor to rotate the polarising spinner. Variable speed control would be a bonus.

Making polarising slides

Place a 2” x 2” piece of polarising film on the illuminated light box then stretch some polythene over it. Using a spinner or Polaroid sunglasses check the result. The trick in making a good composition, I found, means stretching the polythene but not letting it tear, or to have folds when it is flattened. Having said that, like all light show effects, there are no hard and fast rules. Experimentation is the name of the game. And both those ‘flaws’ can sometimes look good. Add the three layers of cellophane, rotated to their optimum position slap the top slide glass on, trim off the excess, mount in the slide frame. Turn the lights out and see your creation.

Scratching polythene requires you to have finger nails or an instrument to emulate them. Good scratched ‘poly’ slides always reminded me of oriental script, cherry blossom etc..

For both the above try different thicknesses of polythene

Cellophane can be cut and placed in rigid formal geometric patterns or just placed randomly in an abstract fashion. The added three layers may or may not be necessary as you choose.

Sellotape lends itself to formal patterns. Strips radiating from a central point can look like the sun’s rays. This effect was used towards the end of The Beatles ‘Yellow Submarine’ animation film.

Plastic packaging, cut out interesting bits add cellophane and project.

Making crystal slides from the two chemicals above requires heat and, in this day and age, probably some safety precautions. The two chemicals appear to still be available on line. I didn’t attempt to buy any so there could be restrictions or controls on there use. I’m sure the vendors would appraise you of any current regulations regarding their use.

Certainly I would recommend a mask, there's plenty of those around at the moment!, and adequate ventilation.

Both come as powders which melt to a clear liquid with heat. Place a slide cover glass on a hot plate and put a small amount of powder on it. When it melts put a second, pre warmed glass on top and press them together. Remove from the heat and allow to cool. Acetanilide produces better colours the thinner it is so applying pressure whilst it is cooling achieves this. Interesting patterns can be achieved by letting different areas of the slide cool at different rates. To achieve this we used to keep steel nuts, of differing sizes, in a fridge and then place them on the surface of the cooling slide.

Cholesterol Acetate is more tricky to work with. If it is squeezed whilst cooling the structure disappears altogether. Yet if it is too thick the resultant image is cloudy. Producing your first successful CA slide is a wonderful moment.

After cooling both appear like frosted windows until the magical polarising film and cellophane are added.

As cited in Bob Beck’s aforementioned manual Cecil Stokes invented a device called The Auroratone which incorporated crystal slides with musical accompaniment in 1940. It was used therapeutically to help military veterans suffering from shell shock. As ever, more on this on-line.

In the early 1980’s I developed for Optis the Crystal Pulse. This comprised an acetanilide crystal wheel, turning on a very slow speed motor, mounted between fixed crossed polarisers and a liquid crystal shutter. The shutter was fed with an audio source through a circuit that responded to significant changes in amplitude. The orientation of the liquid crystal changed instantaneously on the ‘attack’ but the ‘delay’ was dampened such that the next attack usually occurred before the delay had returned to its starting point.

The resultant on screen image mimicked the audio precisely and like Cecil Stokes invention was very relaxing to watch.

One of our clients who worked with deaf children projected the image of the Crystal Pulse, connected to an amplifier and microphone, down onto the surface of a kettle drum. The children sat around the drum touching its sides. As the drum was banged the children could see and feel it.

Etched polarising slides

As polarising film is applied to the surface of a substrate so it is possible to partially or totally remove it.

At Krishna Lights etched slides were made by hand using a craft knife to scratch an image on a piece of polarising film. A second piece of polarising film cut at 90 degrees to the first had a complementary image scratched onto it. The two pieces of polarising film were the placed on top of each other. Sometimes a square of colour filter was also added behind and the whole placed in a slide mount. When projected with a polarising spinner in front the two images cross fade from one to the other. Simulating two projectors with a fader wheel in front of them.

By far the best exponent of this style was an Australian girl at KL called Kaylin. Unfortunately all my examples of her work were lost.

At Opti’s we subsequently discovered that HN42 polarising film could be chemically etched, in a similar fashion to that employed in making PCBs (Printed Circuit Boards). An ink resist is screen printed onto the surface of the polarising film. It is the dipped into a warm bath of soda crystals (sodium carbonate) to remove the polarising film from its backing substrate where it is not protected by the resistant ink.

As we used transparent coloured printing inks we could either leave the colour in place or remove, as you would to reveal the copper tracks on a PCB, if we wanted black and white imagery.

Slide artwork was arranged with image pairs at right angles to each other and then sheets of polarising filter were cut from a roll, maintaining the same polarity, and printed in a variety of colours. After etching they were cut and intermixed in pairs from differing coloured sheets.

A progression of this was ‘Nouveau’ a polarising effect wheel made by this process but with a split fountain rainbow print. The artwork was again arranged with the two images at ninety degrees so that when they were cut out and assembled the rainbows ran across each other.

Another way of animating slides using polarising filter is to use small pieces of it stuck to black an white film slides. For instance pieces stuck behind a slide of stars causes them to randomly twinkle. Getting more technical pieces of polarising film can be cut such that the resultant animation is more precisely controlled. Imagine a circle of twelve spots arranged as the numbers of a clock. By cutting polarising film in a similar pattern and placing each one accurately behind each spot the illusion of rotation is achieved.

In the above example the black and white slide is placed lamp side with the polarising film in front of it. This is so that there is nothing between them and the spinner. If the slide was in between it would itself be polarised and produce slight colouring.

The same principle is applied if a colour filter is being added to a black and white etched polarising pair. So that the colour filter is not itself polarised.

Have fun

Neil Rice - October 2020 (Ono Yasumaro’s, Krishna Lights, Optikinetics, Liquid Light Orchestra, Odd Light Show)

The following materials are used in the video sequence below (in order of appearance):

Clip 1:

Cholesterol Acetate

Clip 2:

Chemically Etched - A single colour filter has been added to the rear of the slide so that it is not polarised.

Clip 3:

AKA 'The Flasher' - Chemically Etched slide as 2 above but without a colour filter being added.

Clip 4:

Animated polarising slide. A black and white slide of the Optikinetics logo has had strips of polarising film cut at regular intervals of 22.5 degrees stuck in sequence behind each line. The slide is behind the polarising film so that it is not polarised. (A pride and joy, treasured item!)