Photonic light machine

In the world of the photonic light machines brightness is an important subject. For example, flash tubes are used in skin treatment equipment. With this treatment a large amount of emitted light is required. For this treatment Xenon flash tubes are used in large light output pairs at short pulse widths. The lamps have a long thin cylindrical shape and radiate light in all directions. The house in which the lamp is mounted (the light head) is just one of the long sides open and protected from the environment by a cover glass. It is therefore obvious to use a mirror for the back radiated power and to it reflect towards the open side. This makes the design of the light head as important as the choice of the lamp, because there is more light emitted in the “wrong” direction than directly into the right direction. A properly designed mirror can thus achieve better results.

Sumipro has optimized some of these light machines optically and achieved a substantial extra light output.

The following information discusses the design choices for this type of light machine.

Lighting head

Some of these light heads (as an example) are shown in the pictures below:

Light head examples

The head is open on one side and the light is blasted onto the skin with an aperture of glass or quartz. The amount of light that radiates to the head depends on the size and shape of the opening in the housing and the extent to which the light is reflected from the back of the house. Often the interior casing is made of stainless steel bezel, as Xenon flash tubes must be water cooled. The reflection coefficients of stainless steel is only about 70%. With a false mirror form rays reflect several times in the housing and they will quickly extinguish. This means further warming up of the interior and so there is a need for more cooling.

Mirror Coating and shape

Given the (work-cooled) environment in which the mirror needs to operate, where no soiling or contamination may occur, the choice of used materials for the mirror is critical. Stainless steel is therefore recommended by the lamp manufacturers. As mentioned above, the reflection coefficient of this material is actually too low. To increase thus a coating on the mirror is needed. Depending on the reflection wavelength region, a suitable coating has be found.

The second aspect of great influence on the behavior of the mirror is of course the mirror shape. The optimal mirror to rebound light parallel from a source point is the parabola. See the following picture:

Schematic parabolic mirror

The rays that come from source point F bouncing to the rear as a parallel beam are then bounced forward. The optical focus of a parabolic F:  Z=aX2  is hereby given by: F=1/4a.

However, in practice it is often limited to depth (D) and width (W). The question then becomes what parabola and which focus on the geometric dimensions is the most ideal. The answer is that the parabola goes exactly through the points (B / 2, D) and (-B / 2, D). This applies to the parabola, for which: a= 4D/B2  . So the ideal focal point with given dimensions is given by: F=B2/16D

Practical alternatives

Besides the parabolic mirror, there are many less optimal forms that may often fit. As an example, in the table below the simulation results of various levels of data are shown with their extra light output compared to an interior housing without a mirror. It should be noted that the lamp itself is made of stainless steel with a reflection coefficient of 70% and that the mirrors had a reflectance value of 90%, which with the right coating or materials is obtainable. However, given the particular circumstances of  which the materials and coatings sometimes need to operate in (temperature and pressure), it is not always possible to get these high reflection coefficients. In these cases the shape of the mirror is even more important.

Simulation results:

Mirror Form

Additional light

No mirror, square stainless steel housing

N/A

Rectangular plate as mirror behind the lamp

11%

Curved circular mirror behind the lamp

11%

Given these results, the engineers at Sumipro have looked at the idea of the parabolic mirror, and let it fit in a narrow enclosure. One option is to approach it with parabolic parts of parabolas, which reaches at least a portion of the optimum required. See below for a parabolic mirror consisting of seven separate parabolic segments.

7-fold parabolic mirror and a trapezoidal shape

The mirror is hereby designed using the allowable angles of the light that is allowed to leave the mirror, the light source is then approached by a number of 7 source points. For each of the seven point sources a segment of the ideal parabola is chosen within the allowable angles. This mirror was found to conform quite well. With a good practical approach to achieve such levels, the engineers Sumipro managed to achieve an improvement of light yield of 70%.

Incidentally it should be mentioned, as well as an example that mirrors not only work with visible light, but also in UV radiation levels which are applied in practice by Sumipro successfully.

Conclusions

By applying the correct mirror shape and a high-reflection coating, the light output of each lamp in a certain direction is increased significantly. Form, material and coating of the mirror while specifically designed for the use of light, the housing has to fit the whole and the application for which the light source is intended. If they are consistent with each other dramatic improvements can be achieved in light output.