By Richard Cadena
Editor: PLSN (Project Lights and Staging News)

"Our audience has become an incredibly wired, techno-literate audience whose fundamental approach to entertainment has changed."
- Nicolas Butterworth, chief executive of MTV's Internet arm

Dichroic filters are amazing in their simplicity and efficiency. They are an integral part of automated lighting technology - color filters that allow robotic lighting to change color on command. They are made of glass covered with a metallic coating, but they are able to withstand tremendous heat produced by high-powered lights focused in optical paths no larger than a quarter or half dollar. They stand up to the abuse of stagehands tossing them carelessly in a roadcase and traveling thousands of miles across the earth, many of them on the pothole-strewn highways of third-world countries. But dichroic filters always look as good as the day they were manufactured - highly saturated, vibrant and intense with color. What is it that makes a dichroic filter one of the most important components of automated lighting?

Dichroic filters are made of thin-film coatings applied to a glass substrate. A thin-film coating is a very fine layer - a couple of microns or a couple of thousandths of an inch thick - of a dielectric material that is applied by evaporating crystals onto glass substrate. Several of these layers make up the coating of a typical dichroic filter. It can range from as few as three or five layers to as many as 30 or 40 layers. The process is carried out in a vacuum chamber and is similar to the process used to make semiconductor chips.

The principle behind the magic is the science of bending light, or something called refraction. Every substance has a certain index of refraction, which is an indicator of how fast light travels through it. When light passes through an interface of two materials with a different index of refraction, it "bends" or changes direction. Every wavelength, which is contained in the incident white light, will bend to a different degree causing the white light to scatter into its component colors. That principle allows the dichroic filter designer to pick and choose which wavelengths to pass and which to reject by manipulating individual wavelengths.

Figure 1. Example of white light refracting and scattering into component colors.

Because some of the light is redirected back towards the source, several good things happen. First and foremost, the quality of the color produced by the dichroic filter is significantly "better" than that of a gel. The resulting color can be very saturated and pure. It will have only the desired wavelengths and very little else. The only light with a more pure color is coherent light or laser light.

Secondly, the wavelengths that are rejected are not dissipated, but only redirected away from the exit path of the filter. By contrast a gel absorbs the unwanted and dissipates them in the form of heat. Any first year physics student knows that energy cannot be created nor destroyed, but it can change forms. A gel changes light energy to heat energy. The heat has to be absorbed and dissipated by the polyester gel, which eventually destroys the gel, causing it to wrinkle, fade, and burn. How long a gel lasts before it fades or burns depends on the color. Some gel filters have to be replaced every show or every few shows. A dichroic filter, on the other hand, absorbs no heat. It reflects unwanted colors and passes the rest. It never burns, fades, or wrinkles. As long as it doesn't take a direct hit and break, a dichroic filter will last longer than the life of the fixture.

Perhaps the most significant characteristic of the dichroic filter is that it is very efficient. Depending on the color, it can pass as much as 60% to 90% or more of the incident light in the desired wavelength. Compared to a gel or colored glass a dichroic filter can produce two or three times as much light from the same light source.

Figure 2 - Graphical representation of the spectral
energy distribution of dichroic filter and gel filter.

The real success story in the realm of dichroic filters has been the boom of automated lighting and the great success of the glass gobo. Dichroic filters do best in a sealed environment - surrounded by expensive hardware that fetches a high price, that is. Automated lighting that is used on the concert and theatrical stage fits the bill. It seems we are willing to pay big bucks for dichroic filters as long as they pan, tilt, mix, focus, and frost.

Glass gobos, which are nothing more than dichroic filters onto which an image has been etched, have also proliferated in the current entertainment lighting environment. More and more entertainment lighting companies are willing to cough up extra money to get better and more colorful patterns with their lighting fixtures.

As recently as ten years ago, most gobo patterns were made of steel or aluminum. Before the advent of moving lights, you simply dropped a pattern into the "gate" of a leako and it projected a single image in black and white. Or more correctly, in light and shadow. With a good quality instrument you could project a nice clean image with sharp contrast, and by adding a gel or dichroic filter you could make it one color plus black. The problem with metal gobos is that they restrict your design by forcing you to create bridges to isolated areas. For example, the center of a tunnel effect with a metal gobo pattern needs three bridges to hold the centerpiece in place.

Metal gobos also tend to warp, making it difficult for them to exhibit uniform focus from the center to the edge. Nor are they able to project gray scale or multi-colored patterns. It's also difficult to render fine detail with metal gobos. In fact, the finer the lines and bridges, the faster they will warp and burn out. Depending on the design and application, metal gobos need to be replaced relatively quickly.

Figure 3 - Metal gobo on left shown with telltale bridges
and single color glass gobo on right with floating tunnel.

In the late 1980's and early '90's, manufacturers increased the production of dichroic filters for the entertainment lighting market. With lithographic etching techniques they are able to produce gray scale images with any artwork. Multi-step etching and coating processes can produce multi-color and full color patterns. Glass gobos don't require bridges to hold isolated pieces like tunnels. The projections are much more effective and true to form.

Glass gobos provide lighting designers with the best possible image projection, clarity of the image, and color palette. Like dichroic filters, they are a very important part of the overwhelming majority of lights shows in production today. Lighting designers are demanding more glass gobos in their lighting fixtures and manufacturers are responding. A number of glass gobo manufacturers are proliferating around the planet.

The manufacturers of the fixtures that are projecting these images have increasingly improved the optics to the point where the resolution of the images has become very important. Many fixtures now have achromatic lenses, improved lamp sources with better color balance, and short arc lamps that allow for much smaller optics. These improvements have rendered lower resolution gobos more unacceptable because you can see the steps of resolution better.

By the same token, manufacturers of glass gobos have risen to the challenge. They are producing amazingly creative and beautiful multicolored and photo realistic gobos with very high resolution. Companies such as Rosco (www.rosco.com) and Apollo Design Technology (www.internetapollo.com) have increased resolution of some of their products to 10,000 to 12,000 dots per inch while devising new ways to marry dichroic filters, textured glass, and both photographic and abstract images.

So how are these projected images used? The majority are going into automated lighting where they are put to work in various concert tours, television, Broadway shows, theatre productions, industrial shows and nightclubs. They are also very popular in sporting events like professional hockey, basketball, and wrestling - just about any sporting event that takes place indoors. Many of the projections are abstract in design, very colorful, and artistic. Many others, however, are used to project corporate and team logos. Some teams even sell gobo projections to advertisers who pay enough to cover the cost of the glass gobos and a portion of the lighting rig.

The demand for glass in automated lighting keeps growing. It is not uncommon for dichroic manufacturers to run three shifts per day to meet the demand. To help speed the production process manufacturers are automating as much of the process as possible. One of the most critical areas of dichroic and glass gobo production, and the area that presents the biggest chance of producing a bottleneck, is glass cutting. Glass has to be prepped for coating beginning with cutting and finishing the edges. Then once the glass is coated, it has to be cut to size for the application. There are a great variety of shapes and sizes needed to build an automated light. The simplest color changers use round or trapezoidal dichroic filters of various sizes. More complicated color mixing mechanisms use larger round shapes with mounting holes cut in the center. They sometimes have a small tab that works with a photo detector to detect the position of the color wheel. Glass gobos are usually round and vary in size from .75 inches to a couple of inches in diameter. The edge finish is more important with glass gobos since they are user replaceable.

In the future, more and more retail shops will use automated lighting or fixed gobo projection to draw attention to their brands. Entertainment lighting is becoming more accessible because they are getting smaller, brighter, cheaper, and have lots more effects. As the world becomes more and more desensitized to extraordinary events, advertisers will work harder to grab the attention and the imagination of the masses. Automated lighting - not just ordinary automated lighting, but automated lighting loaded with glass and effects - will become a more powerful tool in the arsenal of the advertiser.

About the author -
Richard Cadena is the technical editor for Pro Lights and Staging News magazine (www.plsn.com). He writes a regular monthly column on lighting technology in both PLSN and in Club Systems International magazine (www.clubsystemsinternational.com) and draws from over 15 years of experience in the entertainment lighting industry. For more information about automated lighting, contact the author at .