Dichroic Glass in Kaleidoscopes: Colour From Interference

By Rocco (Roi) Ramon - licensed architect, founder of Studio Yabaye

Most coloured glass gets its colour by removing light. A red pane absorbs the green and blue part of the spectrum and passes what is left. Dichroic glass works differently. It carries a coating that absorbs very little light. The coating sends one part of the spectrum through the glass and reflects the rest. One piece therefore shows two colours, one in transmitted light and another in reflected light, and both change when the piece is tilted.

This article explains what dichroic glass is, how thin-film interference produces the colour, where the technology came from, how the Roman Lycurgus Cup relates to it, how a dichroic piece behaves inside a kaleidoscope, and what to know about handling it.

What dichroic glass is

The word comes from the Greek for "two colours". The Corning Museum of Glass defines dichroic glass as glass that is one colour when seen by reflected light and another colour when light shines through it. The museum's definition covers two different things: glass that has this property in its body, in some cases from minute quantities of colloidal gold, and glass that receives it from a dichroic coating applied to the surface.

The modern material sold to glass workers is the second kind. It is a sheet of clear or black glass with a stack of very thin layers on one face. A manufacturer of the material, Coatings by Sandberg (CBS), describes the process this way: quartz crystal and metal oxides are vaporised by an electron beam gun in a vacuum chamber, and the vapour condenses on the surface of the glass. Reference descriptions list oxides of titanium, chromium, aluminium, zirconium and magnesium, together with silica, as the usual layer materials.

Two figures from the manufacturer give the scale:

• Layer count. CBS states that its colours have as many as 30 layers.

• Total thickness. The whole stack is approximately 35 millionths of an inch, which is about 0.9 micrometres - less than a thousandth of a millimetre.

CBS describes the result as an interference filter permanently adhered to the surface of a piece of glass. The glass is the carrier, and the coating does the optical work.

Thin-film interference in plain language

Light is a wave. When two waves of the same wavelength overlap in step, they add up. When they overlap out of step, they cancel. Interference is the name for this adding and cancelling.

A dichroic coating is built from alternating layers of two materials with different refractive indices. At every boundary between two layers, a small part of the light is reflected and the rest continues. With many layers there are many weak reflections, all travelling back toward the viewer.

Whether those reflections add up or cancel depends on the layer thickness compared with the wavelength. Each layer is a fraction of a wavelength thick. For one band of wavelengths, the reflections from all the boundaries leave the stack in step and reinforce each other, so that band is reflected strongly. For other wavelengths the reflections are out of step and cancel, so that light passes through.

Why the two colours are complements. The layers are transparent oxides, so very little light is absorbed. Whatever is not reflected is transmitted. If the coating reflects the blue end of the spectrum, the transmitted light is white minus blue, which the eye reads as yellow. A coating that reflects red transmits cyan. Apart from the small absorbed part, the reflected and the transmitted colour add back to the light that arrived.

Why the colour shifts with angle. The condition for reinforcement depends on the path that the light travels inside each layer. When the light arrives at a tilt, the difference in path between the reflections from successive boundaries becomes shorter. The band that reinforces therefore moves to shorter wavelengths. Edmund Optics states the same rule for interference filters in general: as the angle of incidence increases, the transmission curve shifts to lower wavelengths. In practice a coating that reflects red when viewed straight on moves toward orange and yellow as it is tilted, and its transmitted colour moves with it.

The same vacuum-coating family of processes is used to put the reflective film on a front-surface mirror. The difference is the purpose: a mirror coating is meant to reflect the whole visible spectrum, and a dichroic stack is tuned to split it.

From optical filters to the glass studio

Dichroic glass was not developed as an art material. The coating is an optical interference filter, and CBS notes that the technology used to make such filters, vacuum thin-film deposition, had existed for many years before artists used it. The same type of filter is used in lighting and projection equipment, where it has a practical advantage: unwanted wavelengths are reflected instead of absorbed, so the filter heats up less than an absorbing filter and its colour does not bleach.

The aerospace connection is documented by NASA. A 1993 article in NASA's Spinoff publication describes dichroic glass as a technology in which extremely thin films of metal are vacuum deposited on a glass surface, with very thin layers of metal oxides applied one at a time in a specific order and thickness. According to that article, such coated glass was used to shield spacecraft instruments from cosmic radiation and to protect human vision from unfiltered sunlight in space. CBS states that dichroic glass was originally created for the aerospace industry for satellite mirrors. That is the manufacturer's statement, and I found no independent source for it.

The same NASA article records the move into art glass. It names Murray Schwartz, a former aerospace engineer, who founded a business called KROMA to produce dichroic glass for windows, mobiles and jewellery.

The Roman precedent: the Lycurgus Cup

Articles about dichroic glass usually cite one early object: the Lycurgus Cup in the British Museum (museum number 1958,1202.1). The museum dates it to the 4th century. It is a Roman cage cup, 158.8 mm high, with figures cut from the glass wall showing the myth of King Lycurgus. The glass is green in reflected light and red when light is transmitted through it.

The cup fits the Corning definition of dichroic glass, but the mechanism is not the one described above. There is no coating and no layer stack. The colour comes from extremely small particles of gold and silver dispersed through the body of the glass. Published analyses, as summarised in reference sources, report roughly 330 parts per million of silver and 40 parts per million of gold, in particles about 70 nanometres across. Particles of that size scatter and absorb specific wavelengths, which gives one colour to light bounced back from the glass and another to light that passes through.

The distinction:

• Lycurgus Cup: metal nanoparticles inside the glass. The effect is a property of the glass body.

• Modern dichroic glass: thin-film interference in a coating on the surface. The glass under it is ordinary clear or black sheet.

Very few other Roman glasses with this property survive, most of them as fragments. The cup is a precedent for the visual effect, and it is not an ancestor of the coating technology.

How dichroic glass differs from ordinary coloured glass

Mechanism. Ordinary coloured glass contains metal compounds dissolved or dispersed in the melt. They absorb part of the spectrum. Dichroic glass separates the spectrum by reflection and transmission, with very little absorption.

Number of colours. An absorbing glass has one colour, the same from both sides and in both reflected and transmitted light, only darker or lighter. A dichroic piece has two, and CBS notes that both shift with the angle of view.

Angle. The colour of absorbing glass depends on thickness, not on direction. Tilting it only lengthens the path through the glass and deepens the colour slightly. Tilting dichroic glass changes the hue.

Sides. Absorbing glass is the same through its whole thickness. Dichroic glass has a coated face and an uncoated face.

Light efficiency. An absorbing glass turns the removed light into heat. A dichroic coating returns it as a reflected beam that can land somewhere else.

How a dichroic piece behaves in a kaleidoscope

A kaleidoscope looks at a small group of objects at the end of the mirror system - in an object cell or on a wheel - and repeats that view by reflection. The general optics are covered in how a kaleidoscope works. A dichroic piece in that position behaves differently from a piece of absorbing glass in three ways.

Two colours at once. Light arriving from behind the piece reaches the eye as the transmitted colour. Light arriving from the viewer's side of the piece, or bounced onto it from neighbouring objects, returns as the reflected colour. A piece on a clear base shows mainly the transmitted colour against a bright background. A piece on a black base transmits nothing, so it shows the reflected colour only.

Colour change with movement. When a wheel turns or the pieces in a cell tumble, each piece changes its angle to the incoming light and to the eye. Because the reflected band moves to shorter wavelengths with tilt, the piece changes hue as it moves. An absorbing piece in the same position changes position in the pattern and keeps its colour.

Dependence on direct light. The effect depends on a directed beam. Under diffuse light, rays reach the coating from many angles at once, so the colours are less distinct. Under direct, strong light the transmitted and reflected colours are more saturated and more clearly separated. This matches the general viewing rule for kaleidoscopes: point the object end at a direct, preferably strong light source.

The mirror system then repeats whatever the piece is doing. A single dichroic fragment that flips from one hue to another does so in every sector of the image at the same moment. Mirror layouts are compared in two-mirror vs three-mirror kaleidoscopes and in types of kaleidoscopes.

Handling: the coated side and fusing compatibility

Finding the coated side. The UK supplier Warm Glass gives a simple test. Hold the sheet at an angle to the light. If the colour runs to the very edge of the glass, the coated side is facing up. If it does not, the coating is on the other face.

Cutting. CBS and Warm Glass both recommend scoring on the uncoated side. Warm Glass states that this gives a smoother and cleaner break.

Fusing compatibility. Glass that is fused together in a kiln must have a matching coefficient of expansion (COE), or the piece cracks as it cools. CBS states that the dichroic coating does not change the COE or the integrity of the glass. Compatibility is therefore set by the base sheet: a coating on COE 90 glass is fused only with COE 90 glass, following the firing schedule of the glass manufacturer.

Coating orientation in the kiln. Warm Glass describes two results. Fired with the coated side on top, the coating has a matte surface. Fired between two layers of glass, or under a clear cap, the surface is smooth and glassy. The same source gives one firm rule: never fire with two dichroic surfaces facing each other.

Exposed coatings. A coating left on the outer face is a layer less than one micrometre thick. Where abrasion or a chipped edge removes it, the colour is lost at that spot.

Checklist

• Identify the mechanism: coating on the surface (modern dichroic) or particles in the glass body (Lycurgus Cup type).

• Check both colours: view the piece against a light source, then with the light behind the viewer.

• Tilt the piece and confirm that the hue changes, not only the brightness.

• Find the coated side before cutting, and score on the uncoated side.

• Match the COE of the base glass to every other glass in a fused piece.

• Do not fire two coated faces against each other.

• Judge the colour under direct, strong light.

FAQ

What is dichroic glass?

Dichroic glass shows one colour in reflected light and a different colour in transmitted light. The modern material is clear or black sheet glass carrying a stack of very thin layers of quartz and metal oxides, deposited in a vacuum chamber. A manufacturer describes up to 30 layers with a total thickness of approximately 35 millionths of an inch.

How does dichroic glass work?

The coating is a stack of thin transparent layers with different refractive indices. Light reflected at each boundary overlaps with light reflected at the others. For one band of wavelengths the reflections reinforce each other and are returned. The remaining wavelengths pass through. This process is called thin-film interference, and it absorbs very little light.

Why does dichroic glass change colour when it is tilted?

Interference depends on the path the light travels inside each layer. At a tilt, the path difference between successive reflections becomes shorter, so the reflected band moves to shorter wavelengths. Optical filter manufacturers state the same rule: as the angle of incidence increases, the transmission curve of an interference filter shifts to lower wavelengths.

Is the Lycurgus Cup made of dichroic glass?

By definition yes, by mechanism no. The 4th-century Roman cup in the British Museum is green in reflected light and red in transmitted light. Its colour comes from gold and silver nanoparticles dispersed in the glass body, not from a surface coating. Modern dichroic glass uses thin-film interference in a deposited layer stack.

What is the difference between dichroic glass and coloured glass?

Ordinary coloured glass absorbs part of the spectrum and passes the rest, so it has one colour from every direction. Dichroic glass reflects one band of wavelengths and transmits the remainder with very little absorption. It therefore shows two complementary colours, and both shift when the viewing angle changes.

How does dichroic glass look inside a kaleidoscope?

A dichroic piece on a clear base shows its transmitted colour when lit from behind and its reflected colour when lit from the viewer's side. As the wheel or cell turns, the piece changes angle and its hue shifts. The mirrors repeat that change in every sector. The colours separate most clearly under direct, strong light.

About Studio Yabaye

I am Rocco (Roi) Ramon, a licensed architect and the founder of Studio Yabaye. I build kaleidoscopes and teleidoscopes with front-surface mirror systems in several configurations, each housed in a brass tube, cut, fitted and hand-finished in the studio. Each object wheel is cast in crystal-clear resin and holds coloured elements such as beads, glass stones, shells, brass rings, pigments and dried flowers. Each wheel is poured and composed by hand, so no two pieces are alike. The studio is based in Israel and ships worldwide. The current range is at all products, and questions can be sent through the contact page.

Related reading: How a kaleidoscope works - Oil-filled vs dry cell kaleidoscopes - Types of kaleidoscopes - Kaleidoscope glossary