Different Shades of Laser Light

The observant visitor of ISE 2015 could not avoid noticing that the presence of projectors with Solid State Illumination are here, especially of those with Laser Phosphor Light is growing steadily.

The replacement of professional projectors with light output of 4,000-20,000 lm with traditional light sources as UHP and Xenon has already commenced. In only a few years this process will be completed. At this point it is worth reflecting on the criteria of quality. How do the Laser Phosphor machines of the manufacturers differ? What has to be observed using equipment for fixed installations or in rental & staging? What has to be considered when purchasing SSI projectors?

Laser Diode Arrays

In order to achieve the demanded light output of the projectors, arrays of 20 to 60 laser diodes are installed. Right now blue laser diodes with 1.6 W are used, some designs already have 3.5 W. Of course the dimensions of the diode arrays have a great influence on size, noise as well as the heat emission of the projector depending on the required cooling method of the arrays. Here there is a clear advantage for units with liquid cooling. The control of the diodes again will inuence the redundancy of the projectors. With a serial control, a failure of a laser diode will result in a significant loss of light output, whereas with a parallel sub array control, the loss of light output will be marginal.

In Laser Phosphor Projectors blue will be generated directly from the laser diodes, i.e. we can expect sometimes a distinct speckle in the Blue. Also here the various machines currently on the market differ. Effective reduction of speckle can be achieved by using optical diffusors or using different wavelengths of the used blue laser diodes, which has influence on the colour position of blue and the colour space of the projector.

Phosphor Wheels

The construction and the set up of the Phosphor Wheel within the projector have considerable effects on the performance of the projector. Starting with the consistence of the applied phosphor coating and its matrixing leading to different properties of the emissions, which affect colour space, colour temperature and uniformity of the projector. The substrate of the phosphor wheel has to warrant efficient heat dissipation. At the same time its coating has to provide a high degree of reflection. Only when these qualities are combined in an ideal way the projector will have long term stability at a constantly small drop in light output. Also here big differences exist of the Laser Phosphor Projectors, which can currently be found on the market.

Thermal Quenching

Lang article 2A basic property of phosphor used in projectors is the so called Quenching. It means that the efficiency of emission of phosphor degrades to a certain extent significantly at increasing temperature. In order to warrant long term stability, homogenous brightness and colour reproduction, phosphor has to be used in the highly efcient area of its characteristic line. This can lead to promising results when laser diodes and phosphor wheels are dimensioned correctly. Also here there are different methods to resolve this in the market. Systems that are well dimensioned show a hardly measureable deviation of brightness, colour temperature and colour space amongst projectors of the same production series, whilst badly dimensioned projectors differ considerably.

We can assume that a laser phosphor projector does not need to be a good projector by nature. Only the manufacturers, whose construction of laser phosphor has been planned with care and vision, will be able to provide reliable solutions for 20,000 lumen SSI next year – with all profitable advantages for the user of Laser Phosphor Projector technology. The potential certainly exists.

Markus Ries – Direktor der LANG Academy