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Discharge Lamp Spectrum
In the spectral images here the wavelength of
light decreases from left to right. Long wavelength red is shown on the left
progressing to short wavelength blue on the right.
Atomic spectral lines are generated from
specific electron transitions which take place inside the gas atoms within the
lamp arc. The emission (light) is generated every time an atom transitions
between an ionised to stable state. These are unique 'finger prints' of the
gasses which can be used to identify the light source.
The images shown below were taken with a diffraction grating and a digital camera. Positioning, focusing and preventing light saturation in the camera were difficult to control. As a result the images have limitations.
'Clicking' on the spectrum image takes you to the lamp that emits it.
Image 1 is of Neon. This is one of the more difficult spectrums to capture as the light intensity is a fraction of the output generated by the rest of the HID (High Intensity Discharge) lamps. The source used was the neon sign question-mark on the main page.
Image 2 is the output from a 250W MBF/U lamp. The red lines are due to the fluorescent coating on the outer bulb converting UV to a visible red. This red emission attempts to improve the light output to a more white appearance.
Image 3 is a 250W MB/U lamp. This is a high pressure mercury lamp without the fluorescent coating. The line emissions are purely from the mercury vapour and the light has a greenish-blue appearance.
The 400W lamp shown here has mercury and some other elements added to the arc to produce a bright green light. By adding different halides various colours and colour temperatures are possible.
MIT 70W Blue Halide
Similar to the above but the lamp in Image 5 emits mostly blue light.
HPI-T Plus Lamp
Figure 6 is from a white mercury halide source. This produces a cool white light with good colour rendering qualities. It can be seen from the spectrum that there is a good balance of Red/Orange, Green and Blue outputs. This is fundamental for a white light to be produced.
SOX Sodium Lamp
Figure 7 is of sodium vapour. Almost all of the light output from a sodium lamp is contained in just two yellow lines. These are so close together that my diffraction grating set-up could not resolve them and just one fat line is shown here.
SON/T HP Sodium
Figure 8 is of the light output from a high pressure sodium lamp. Right away, you know there is more than just sodium in the arc tube due to the extra visible lines. Mercury is the additional element. Vapours operating at high pressures only create line broadening due to Doppler-shift as the atoms speed up in the higher temperatures. Blending the sodium and mercury improves the light output colour and still keeps the lamp reasonably efficient.
Cadmium vapour Spectral Source
Figure 9 is of a Cadmium spectral source. This lamp is designed for laboratory use and the electrodes are shielded to prevent light emission from the glowing electrodes being observed.
Xenon car head Lamp
Figure 10 is from a Xenon car driving lamp. Like the mercury halide, these lamps are not just xenon gas. There are additives to improve the light colour and intensity. This lamp has a colour temperature of 4300K.
Hydrogen Spectral Source
The simplest atom's spectrum
Krypton Spectral Source
Krypton spectrum from a didactic Geissler tube.