The light spectra of some lamps

An investigation of some light spectra

I made a spectroscope according to the conception of Arvind Paranjpye (Inter-University Centre for astronomy and astrophysics-India) and adapted by Christophe Carteron (Club Astronomique de la Région Lilloise - France). I used a DVD and made this box with a split in the direction of the lamp and an opening to see the spectrum on the DVD. The photographs were taken without tripod (only by hand) with a Canon PowerShot SX120 IS.

1. Spectrum of incandescent light bulb

The spectrum of a black body is continuous. The higher the temperature, the shorter the wavelength of the peak. Your light bulb filament can be considered as an approximation of a black body. The light seems to be white. The spectrum of this light is a mixture of different wavelengths from 300 to 700 nm and shows all colours of the rainbow. The currently used incandescent tungsten wire has a temperature of approximately 2700 Kelvin and thus produces a relatively large proportion of radiation in the infrared wavelength range. Due to the comparatively low temperature, it is not necessary to use quartz glass for the bulb. The use of ordinary glass, also called "soft glass", is possible. Due to the relatively low temperature, the UV emission is very low, and also the soft glass absorbs an additional portion of UV radiation. (Karl Schulmeister e.a., Optische Strahlung: Ultraviolett-Strahlungsemission von Beleuchtungsquellen, AUVA, Wien, Oktober 2011)

A nice colour rendering is essential in our life. A clear evaluation of the complexion and skin colour of a patient is of primary importance to the medical staff and other people to make a good diagnosis. The best color is daylight. The spectrum of incandescent light bulbs leans more to the red (such as the setting sun) but is complete. Natural daylight and incandescent bulbs both have a CRI (Colour Rendering Index) of 100. CRI tells you how accurately colours appear under the bulb's light. It is a measure of the level at which colours are represented in comparison to a natural light source (e.g. sunlight).

We conclude that the incandescent light bulb has a high quality of light. During generations it was an essential part of life.

2. Spectrum of halogen (reflector) lamp

A halogen lamp has about the same features as an incandescent light bulb. The CRI is 100 as well.

Quartz halogen bulbs (incandescent and halogen, or also called quartz tungsten halogen lamps) are, in principle, light bulbs, in which through a chemical cycle, the evaporated tungsten from the filament - with the help of halogen such as iodine or bromine in the bulb - is fed back to the filament. Thus, higher temperatures of the winding than in conventional incandescent light bulbs are possible (about 3000 K). Due to the higher temperature, the use of soft filament glass (see above bulb) for the bulb, however, is not possible and it is necessary to use quartz glass. Again, however, quartz glass - in the undoped state - has a substantially higher transmittance for ultraviolet radiation than soft glass. For lamps that can be operated in open luminaires, it is necessary that the quartz glass has a UV-filter. (See Karl Schulmeister e.a.)

We conclude that the halogen lamp has also a high quality of light.

3. Spectrum of compact fluorescent light

This light has a very different origin. A typical compact fluorescent lamp consists of a phosphor coated glass tube with electrodes at both ends. The tube is filled with mercury vapour. The mercury atoms become excited and when they return to the ground state, they release photons of light in the ultraviolet region of the spectrum. The ultraviolet photons collide with the phosphor coating inside the bulbglass, causing it to fluoresce and emit visible light in a range of colours. The light of a CFL is white, but, as can be deduced from the spectrum, it is not complete!

In principle, these are lamps in which the glass tube) is folded appropriately to make the lamp compact. In the bends, it may come to a thinner glass, which makes that increased UV emission is possible. (See Karl Schulmeister e.a.)

What we see are the emission lines of metals used in the lamp. When the electrons leave their excited status, they emit that energy back in the form of a photon with a specific wavelength depending on how high the jump was. This is the reason why you get specific emission lines on your spectrum, depending on the specific behaviour of different metals. In several CFLs I found about the same lines. It can be a sign that most CFLs use the same phosphors. Between the lines we find black spaces what means that colours are missing. Even after the manufacturers have managed to give CFLs the form of a light bulb, the spectrum remains incomplete. Your skin tone will be different and unnatural.

The spectrum of each CFL is in fact a fingerprint of the metals found in the tubes of the lamp. From the lines we can deduce which metals are used.

Source of the labelling
number colour origin of the colour wavelength
1 (not visible here) violet but can hardly be seen mercury 404.66
2 violet mercury 435.83
3 blue terbium ~ 485-490
4 green terbium ~ 543-544
5 green mercury 546.07
6 green likely terbium or mercury ~ 578 for Tb or 576.96 for Hg
7 green mercury or terbium 579.07 for Hg or ~ 580 for Tb
8 yellow possibly terbium or europium ~ 580
9 yellow likely europium ~ 587
10 orange likely europium ~ 593
11 red likely europium ~ 598
12 red europium ~ 611
13 red likely terbium ~ 625
14 red likely europium ~ 630

Source table

Terbium and europium are rare earth elements whose compounds are lowly to moderately toxic.

Together with other shortcomings (UV radiation, electrosmog, power factor, environment polluting extraction and processing of mercury and rare earth elements, etc.), the CFL is a very bad lamp that you should avoid with all means. It is very shameful that such a lamp has been promoted to substitute the incandescent light bulbs. The lamp with the best quality of light has been banned. It was the most unhuman decision taken by the European Commission. Why have we to use that awful lighting in our houses? Have the dark ages entered in Europe since September 2012? Because the CFLs are NOT a substitute for the incandescent light bulbs - they even don't meet the standards enforced by the EC - and because LEDs have also specific shortcomings and are too expensive, the incandescent lamps have to be available again without delay. The incandescent light bulb is an unequalled product. The rights of the consumers have to be restored immediately. Consumers have been badly informed about the performance of CFLs. This is a reason to question the whole ban process even now.

4. Spectrum of a LED lamp

This item will be investigated in the near future.

The EU has even lowered the CRI and PF requirements for CFLs and LEDs (household lighting)!


Last update May 1, 2013