WP's Photo

Heat with light bulbs and measure temperature with a laser beam

Measuring device, instrumental set-up

Entirely touch-free temperature measurement with an ellipsometer. Reference dimension was the change of polarization of a laser beam caused by change in temperature. Twice it run through a 1 mm (0.04 inch) thin quartz pane, which could be heated by halogen lamps. Its refraction angle of the inner total reflection changed with temperature. The 180° turn of the beam after the first run through the glass pane and the entry to the detector after the second run on the way back were done by prisms.
A calibration curve with polarization change over temperature and respective time was the goal.

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Measuring temperature with an ellipsometer

Extraordinary effort! - Why? Thermocouples need a good contact to the measuring object surface. Bad then if the surface pressure is limited. Together with a low heat conduction capability of the measuring object or its surface (e.g. due to coating) the thermocouple moreover acts like a cooling fin or under fast temperature changes like a thermal load. The deviation can easily reach values beyond ±10 °C (± 18 °F) or in the two-digit percent range.
And pyrometers? They measure the heat radiation of an object and calculate the temperature out of the Stefan-Boltzmann Law, see below, and the area of the measurement spot. This radiation property, however, strongly depends of the object material and is incorporated by the correcting factor emissivity 0 < ε ≤ 1. Shiny metal surfaces provide an ε much lower than 0.1, the black body (approximately e.g. carbon black) a value of 1. Usually at least 0.7 is preset. Besides one should not underestimate the distortion caused by external irradiation (e.g. reflections and other sources in the field of view analog to a video camera) and standing waves in transparent layers.

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The light bulb as heating element

Electrical light bulb - the better stove: seven 500 W halogen bulbs easily achieve 1 200 °C (2 192 °F) within seconds. In industry and especially in semiconductor processing this setup is often used for rapid thermal processing or annealing (RTP, RTA).

The main connection between irradiation and heat is given by Planck's law of radiation. In wave-length form (wave-length λ in m) it provides for the temperature depending spectral irradiation energy density of a black body (the object with the highest possible emissivity)

r(λ, T) dλ = 2 π h c²/λ⁵ (exp (h c / (k T) λ^-1) -1)^-1 dλ

with
Planck constant h = 6.6262 10^-34 Js
speed of light c = 2.99792 10⁸ m
Boltzmann constant k = 1.3807 10^-23 J/K
absolute temperature T in Kelvin

Efficiency of sun and light bulbs in comparison

Depicted in the figure is the irradiation of a black body after Planck with the temperature of the sun surface (about 6 100 °C, 11 012 °F), that of a filament of a standard light bulb (about 2 800±200 °C, 1 472±392 °F) and that of a halogen bulb (about 3 500±300 °C, 6 332±572 °F).
The visible light region is highlighted in yellow.

Integrating over all wave-lengths yields with the Stefan-Boltzmann law the temperature depending irradiance

R(T) = ∫ r(λ, T) dλ = σ T⁴

with
Stefan-Boltzmann constant σ = 5.670 10^-8 W/(m² K⁴)
absolute temperature T in Kelvin
(Putting the area A [m²] of the black body as multiplier into the formula above yields the radiation power.)

When (numerically) integrating over the visible light wave-length region 380 to 780 nm of the light bulb (temperature of the filament 2 500 K) one calculates a efficiency of about 6%. With the halogen bulb (temperature of the filament 3 200 K) one receives an efficiency of about 16%. Taking in account the low sensitivity of the human eye at the etches of the visible region, here especially the infrared, the »literature values« of about 3% for a 100 Watt bulb and maximal 8 to 10% for halogen bulbs, resp., seem to be comprehensive.

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Fluorescent tubes, energy saving tubes and LEDs

Emitting of the gas light media is not done by a heated source, but by electron pumping, see Franck-Hertz experiment. In the in principle comparable fluorescent tubes and energy saving tubes free electrons accelerated by the mains voltage hit electrons circling around their nucleus and with sufficient energy kicking them on a higher orbital. After the relaxation time of 10^-8 seconds these electrons return to their former orbital by emitting an ultraviolet photon with energy hf. The opaque special coating of the glass tube converts these UV irradiation to visible white light. These lamps show a more or less continuous spectrum, thus covering all wave-lengths.

The semiconductor material of light emitting diodes (luminescence diodes, LEDs) is directly electrically pumped. Recombination emitted photons with the energy hf are closely arranged around a frequency or a wave-length, resp., about some ten nanometers, thus they are monochromatic.

Measured spectrum of a red super-bright LED

The spectrum of the 640 nm GaAlAs super-bright Kingbright L-53SRC-E LED measured with a (grating) monochromator in arbitrary units shows good accordance with the data sheet values - i.a. the maximum is found at about 660 nm. (Maybe due to the not well calibrated measurement setup ;-)
The completely depicted section lies in the monochrome red. That is the efficiency improvement compared to bulbs (incl. halogen) with their very wide spread spectrum shown above.
By the way: in this plot the spectrum of a laser LED would degenerate to a single small (but very high ;-) impulse spike instead of the bell-shaped curve.

For white light one has to bundle a red, a green and a blue LED, either in separate housings or integrated in one and the same. (Often there are two blue LEDs due to their comparatively low efficiency.) A discontinuous spectrum is the result.
There is a similar possibility compared to fluorescent tubes. Often an efficient blue (UV) LED illuminates a special yellowish coating, which then together emit white light with a halfway continuous spectrum. Thus the often circulated »tails« into the UV section cannot be excluded in general.

The technical development does not pause. More and more powerful, intelligent connected and well designed LED lamps are available. And for cars there are laser headlights already.