Cooling (18 W – 60 W Systems) cont’d
It is equally important not to directly cool the arc chamber
(the center of the bulb). This may also cause erratic
performance and shortened life.
Figures 14 and 15 show the critical regions of the lamp
and the optimum temperature ranges. To help you design
the S ¯olarc lamp into your equipment, Welch Allyn can
provide specially prepared lamps with Type K thermocouples
attached to the exposed end of the arc tube (anode seal)
and embedded at the cathode seal. Using these sensors,
thermal management systems and operating temperatures
can be monitored and optimized.
While the operating temperature at both ends of the
tube is important, the thermal characteristics of the lamp
construction actually make the exposed end of the arc
tube the most vulnerable. The reflector tends to conduct
heat away from the near end. For this reason, it is wise
to carefully distribute the airflow to the lamp.
Some air must be directed across the reflector in order to
prevent adverse effects. The designer must also allocate
some airflow across the bulb tip without directly cooling
the arc chamber itself. This may require careful design
since the reference surface for the lamp is the front face of
the reflector. Drawing air across the front of the reflector
and directly cooling the tip of the arc tube and anode seal
can accomplish this. (Refer to Figure 3.) S ¯olarc lamps
have been incorporated in Welch Allyn proprietary products
using forced-air cooling at flows ranging from 9 to 20 cfm
(ft
3
/min) (0.25 to 0.57 m
3
/min), depending on external
environment and chassis restrictions.
Many of the same considerations apply to a single-ended
lamp, except that the application may be complicated
by the user’s own optical design.
The ballast should reside in a well ventilated housing.
Forced-air cooling is highly recommended, but not a strict
requirement. The power field effect transistor (FET) heat
sink (largest heat sink on PC board) located adjacent to
the input power connections must be maintained below
90˚C. See Figures 1 and 2 for the power FET location.
For optimum temperature measurement, position and
adhere a thermocouple on the reverse side of the FET
heat sink at the same height as the FET. Increase airflow
requirements by 1 cfm for every 2˚C rise above 25˚C.
Do not allow the temperature of the heat sink to rise
above 125˚C. Additional heat sinking is possible by
screwing more thermally conducting material to the top
of the heat sink. Use a #2 screw and thermal compound
to ensure proper conduction.
S ¯olarc lamps are typically mounted within dichroic-
coated reflectors for visible applications. For fiber optic
illumination, typical elliptical reflectors are utilized where
the arc is positioned at the internal reflector focal point
(F1) and light emitted from the lamp is reflected and
redirected to the external focal point (F2). The majority
of reflected light is focused at the F2 position within a
defined solid angle. The angular distribution of the light
emitted from the reflector is a function of the ellipse
geometry and the radiation emitted from the arc source.
For maximum transmission through fiber optics, it is
critical to match the reflector angular distribution to the
fiber optic acceptance cone angle (otherwise known as
numerical aperture—NA). The NA of the lamp must
match the NA of the fiber for optimal performance.
The angular distribution of the lamp coupled with larger
bundle diameters can impact the optical performance. A
light depression is typically observed when the angular
distribution propagates through the fiber optics. In most
applications, it is desirable to tilt the lamp’s optical axis
relative to the fiber optic opto-mechanical axis to eliminate
this propagated depression for uniform projected
illumination as viewed from the fiber optic distal end.
Welch Allyn typically sets this angle at about 12 degrees.
This tilting of the lamp can also be used to provide
additional thermal optimization. Tilting the lamp’s
connector downward allows the reflector’s top to
open slightly, allowing the chimney effect to exhaust
more efficiently.
10
OPTICAL PERFORMANCE OPTIMIZATION
Figure 3: Lamp Cooling
