FAQs
Lighting Display Holograms
The mounting and lighting of a display hologram are closely linked and should be considered together.
Various light sources can be used. The two factors to consider are that the light is as much a 'point source' as possible, that is it is not diffuse, and for some specialist applications that it is as monochromatic (single colour or wavelength) as possible. By far the most common way of lighting a display hologram is to use a low voltage (12V) Tungsten Halogen spotlight with a high reflective (NOT diffuse) reflector. These are very common and available in most lighting retail outlets. They are available in 25, 30, 50 and 75 Watt powers. The higher the power, the brighter the hologram will appear.
These lights use low voltage so a transformer is required to reduce the input voltage. Many types are available that have an integral transformer. If a number of lights are required, a larger transformer can be used to drive several lamps. Alternatively, a track lighting system with its own integral solid state transformer may be more suitable. Some of the very latest Halogen lamps can now operate directly from mains voltage.
Ideally, a hologram should be illuminated with the conjugate of the beam that was used to make the hologram. A hologram is usually made with a diverging beam, so ideally the illumination source would be a converging beam. This is the opposite of conventional lighting where the light diverges from the source. For technical reasons during hologram manufacture it is rarely possible, without great expense, to make the hologram with a converging reference beam so that perfect illumination can be achieved with a common spotlight.
However, for most applications this is not a problem. In reality, only portraits show this effect to any large degree, because the eye is so tuned to recognising faces. A 'half way house' solution would be to partially collimate the white light source using a small lens in front of the light. Although expensive, these are commercially available and are commonly used in shops. Alternatively they can be made to suit specific applications. The vast majority of display holograms are known as white light display holograms, as the illumination source is standard incandescent light (such as a spotlight). For greater clarity and depth, a more monochromatic light source may be required.
The simplest way to achieve this is by filtering a conventional bulb, either by buying a dichroicly filtered Halogen bulb, or by physically placing a filter over the light. The problem here is that the transmissive wavelengths of the filter must precisely match that of the reflection hologram. This can be difficult because the replay wavelength of a reflection hologram will change with ambient temperature and humidity. Sodium and Mercury Vapour Lamps emit light in quite narrow bands of the spectrum and can be used to light holograms. Usually, additional filtering is required to allow a very narrow band of the spectrum for the sharpest image.
An ideal light source is the laser itself. While lasers have in the past been prohibitively expensive and delicate for display purposes, modern diode lasers are approaching a price level similar to that of high quality incandescent lamps. They are also far more rugged than the old gas lasers. Diode lasers for displaying holograms are more powerful cousins of the lasers found in CD players. They can be purchased with integrated optics for spreading the beam, are small and can be run from batteries if necessary. In the near future laser lit holograms may become more common because of these factors.
