Tungsten-Halogen Lamp; Optical Configuration - THORLABS SLS302 User Manual

Stabilized Tungsten-Halogen Light Source
2.2.

Tungsten-Halogen Lamp

A tungsten-halogen lamp is an advanced form of an incandescent lamp. The structure of a tungsten-halogen lamp
is shown in Figure 2. It consists of a tungsten filament sealed in a quartz envelope filled with inert gas with traces
of halogen mixed in.
During operation, a current flows through the filament, which will heat up to around 3000 K. Tungsten emits
visible and infrared light at this temperature, creating a light source that can be approximated by a blackbody
radiator.
One problem with a tungsten filament is that it evaporates at the working temperature, and then condenses at
the inner wall of the quartz envelope thus gradually blackening it, reducing the lamp's brightness.
Since the lamp is filled with inert gas, tungsten atoms evaporated from the filament will hit the gas atoms and
then bounce back to the filament, thus lowering the overall rate of evaporation.
The halogen in the gas mixture will also react with tungsten deposited on the inner surface and form tungsten
halide, which evaporates more easily back to the gas mixture. When tungsten halide gets near the filament, it
dissociates at the high temperature of the filament, creating a gradient in the concentration of tungsten. Such
flow is called the "tungsten-halogen cycle". It effectively "pumps" the tungsten back to the filament, prolonging
the life of the light source.
2.3.

Optical Configuration

The SLS302 tungsten-halogen light source has a 150 W halogen lamp. The lamp is mounted inside the lamp housing,
at the back focal point of the collimating lens, with the long axis of the filament parallel to the plano surface of
the lens, giving a collimated output beam.
To increase the output power, there is also a silver-coated concave mirror at the rear of the lamp housing with its
center of curvature aligned with the center of the lamp filament, as shown in Figure 3. This configuration focuses
the light back to the filament position so that it can be collimated by the front lens.
Mirror
Rev C, March 20, 2024
Quartz Envelope
Figure 2 Structure of a Tungsten-Halogen Lamp
Radius of Curvature
Figure 3 Basic Optical Configuration of SLS302 Light Source
Chapter 2: Product Overview
Gas Mixture
Tungsten Filament
Lens
Lamp
Back Focal Length
Page 3