Halogen lamps are mainly used for industrial image processing where large areas must be illuminated with incident light using spots or where a lot of light is required. A typical example are illuminations of large factory buildings, filling stations, outdoor areas and robot applications. In cold light sources for line scan cameras, too, halogen lamps are used as light source. This can be realised at quite low investment costs. The main competitor for large area lighting are fluorescent lamps which are, however, mainly available in tube shape. A halogen lamp, however, is typically a point light source combined with a reflector. Large tube lengths are also technically feasible, but the filament must be mechanically supported.
Typical properties used in industrial machine vision
- strong heat generation
- High power consumption
- high infrared emission
- Long switching times
- Highly sensitive to vibration
- rather large design, mainly available as spotlights
Functional principle of a halogen lamp
Electromagnetic light can be generated quite easily by heating a body. This happens in the halogen incandescent lamp by means of a live conductor in a protective gas atmosphere, in order that it does not burn up immediately. The brightness of the lamp depends on the melting point of the metal which is made glow.
Typically, a tungsten filament is used with an approximate melting temperature of 3420 degrees Celsius. Typical temperatures are approximately 2800 kelvin, far away from daylight with about 6000 kelvin. Halogen additives ensure that the material evaporated from the filament is transported back to the filament. In this way a halogen lamp is even "brighter" than a normal incandescent lamp and usually has a longer lifetime.
Such light sources always show a spectrum like a blackbody radiator in general approximation. This bell-shaped spectrum extends far into the infrared range and always unavoidably merges into heat radiation. A high heat emission is therefore typical for this type of illumination.
Important for industrial machine vision
- Coloured light cannot be generated by halogen lamps because of the typical emission spectrum. This is only possible by eliminating certain wavelengths by means of a colour filter.
- The durability strongly depends on power on/off cycles and voltage peaks, which can drastically reduce the service life. The cold filament has a lower electrical resistance, therefore much higher starting currents appear while the voltage remains constant, which can destroy the illumination especially when switching on. The typical service life is 1000 to several thousand hours.
- Vibrations from the industrial plant can also cause the rupture of the filament and reduce service life drastically.
- The brightness of the lamp continuously diminishes during operation. 1000 to several thousand hours of burning time do not ensure a robust industrial inspection over the entire service life of the lighting.