22.1 New products - LED technology: the future of lighting

24/03/10

What does LED mean? 

LED is the acronym for “Light Emitting Diode”, i.e. a diode which emits light when activated; in other words, a device that exploits radiative recombination of electrons and holes inside semiconductor materials, where excited electrons release their energy as photons (electroluminescence). Photons are not produced by 
the overheating of a substance such as in the fluorescent lamps (where a gas/vapor mixture is heated and excited by an electric current) or incandescent lamps (where a tungsten wire filament is overheated by electric current). Another relevant property of LED is to blink at very high frequency(>M-hz). The switching time is very short (~200 nanoseconds!)

What's a LED?

A LED is a diode made of a p/n junction; that is an interface between two types of semiconductor material, differently doped into p-type and n-type, inside a single crystal of semiconductor (silicon).
The first visible-light LEDs were of low intensity, and limited to red. LEDs covering yellow and green spectrum were introduced later. Early LEDs were strictly used as indicator lamps for electronic devices. The invention of the blue LED made possible to generate white light. By coating a blue LED with a phosphor material, a portion of the blue light could be converted to green 
and yellow and red light. The mixture of colored light, perceived as white light, could therefore be used for general illumination.

As depicted in the scheme below, the doped semiconductor is installed on a chip. 
Two golden wire filaments enable electric current to flow. The photons emitted by the semiconductor go to excite the phosphor laying on the surface. The quality and color of the emitted light depends on the type and quality of the phosphor. An additional plastic lens has the purpose to direct and intensify the light emission. It is easy to guess the solid package of LEDs can be designed to focus its light (the light emission cone is 120°wide). On the other side, the light emitted by incandescent lamps is scattered to any direction (360°); some devices like mirrors or reflecting metallic surfaces may be applied to reduce the dispersion in the surroundings. Anyway we can get better results with LEDs by using lenses that can focus the light into a 10° emission cone.


Why should we adopt LEDs technology?

First of all, LEDs technology offers 
high quality light for restoration and many other purposes. (for instance, LEDs are suitable for back-lighting for LCD televisions and lightweight laptop and smart-phone displays and light source for DPL projectors).

Many other 
advantages need to be underlined:
  • Consumption: very low energy consumption
  • Safety: as a low voltage device, LEDs are very safe for end users.
  • Lifetime: LEDs can have a relatively long useful life. One report estimates 100'000 hours of useful life, that means 11years. Moreover this technology is very cost-effective because minimum maintenance is required.
  • Size: LEDs can be very small and light.
  • Efficiency: LEDs emit more lumens per watt than incandescent and fluorescent light bulbs.
  • Environmental impact: these goods have low impact on the environment (LEDs do not contain mercury)
  • Operating temperatures range: between -40°C and +90°C


Here are the strength points of LEDs applications for our sector:
  • Absence of UV radiations (eyes do not get tired and painting layers are not degraded)
  • Absence of IR radiations (no risks of overheating the lightened surfaces)
  • LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile

In order to appreciate the advantages of the light emitted by LED is necessary to introduce Color Rendering Index (CRI).

Physically, objects can be said to have the color of the light leaving their surfaces, which normally depends on the spectrum of the incident illumination and the reflectance properties of the surface. Artificial light sources do not emit the wave-lengths of the visible spectrum. CRI is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source. Numerically, the highest possible CRI is 100; Typical LEDs CRI index may range from 65 to 95. (the updated general CRI is averaged over fifteen samples wave-lengths, not eight as before)

Other useful terms are:

Luminous flux_the total amount of visible light emitted by a source in a second and measured by the unit called lumen.

Illuminance_
 is the total luminous flux incident on a surface, per unit area and lux (lumen/m²) is the SI unit for measuring it. For example full moon on a clear night emits 1 lux flux.


Luminous efficacy_
It is the ratio of luminous flux to the absorbed electrical power. Its measure is expressed by lumen/Watt (lm/W). When choosing a specific device it is important to keep in mind different light sources with the same absorption can emit different flux. That means 'total energy for getting the same lighting is different'.
LEDs are characterized by luminous efficacy approaching 100 lm/W, much higher than that one of an incandescent bulb lamp (15 lm/w) or a halogen lamp (25 lm/W) or a fluorescent lamp (70 lm/W)

Color temperature_
This term is related to the hue of light. Color temperature of a light source is conventionally stated in the unit of absolute temperature, the Kelvin (K). Higher color temperatures are called cool colors (bluish white), while lower color temperatures are called warm colors (yellowish white through red). For an incandescent lamp 100W correspond to 2900K, and 40W correspond to 2650K. When changing the lamp intensity, the color temperature changes as well.
On the contrary, LED color temperature is a constant.

High color rendering index: a better future for retouching operations

Going further in speculating about possible applications of LED in the field of restoration, we are confident about the revolutionary introduction of high CRI systems for the retouching of paintings. In order to get a better understanding about this innovation, we should recall the explanation about the results of the interactions between photons and phosphor coatings. When properly modified they can emit a 
wide range of wavelengths and cover all the spectrum of visible light and work at high CRI. However overlaying of phosphor may reduce the efficiency because this element works as a filter. The reduction of the luminous flux is counterbalanced by a valuable gain in quality: that property is fundamental during the restoration of paintings: if we use bad light source, the natural light is likely to reveal the mismatching between original areas and retouched ones.

As previously discussed, CRI was formerly based on 8 levels of color, so constructors focused on this range to develop the rendering of their devices. Compared to the current reference scheme of 15 levels that old classification is not satisfactory. For example a halogen device labeled CRI 100 cannot help to perfectly discriminate blue from black or dark brown.

The graph shows how HCRI at 5000K is effective over a wide range of wavelengths despite the peak corresponds to blue frequency band. Moreover UV(<420nm)or IR (>700nm)components are not detected.

The restorer Daniele Piacenti, who gained a valuable experience as photographer too, is regarded as a promoter of HCRI devices for restoration. The first case study was related to a canvas by Matteo Rosselli, “portrait of St. John Baptist” (private collection). In that case the restorer had to cope with a relevant damage: a long horizontal tear crossing the azure background and the face of the portrait at the level of the lips. Slight chromatic differences would have been revealed when passing from the lab fluorescent lamp (5500K) to the sunlight. In order to get the best results, a HCRI LEDs light source was the proper solution. F&M Progetti S.r.l. (company that makes LEDs lighting) and C.T.S. S.r.l. were successfully involved in developing and providing the most effective LEDs lamp for that restoration job.

Learning from that precious work experience, it is now possible to supply the restorers with special lamps for high quality retouching operations. It is important to remind these lamps are low consumption devices and very safe for end users.

A new age for the light sources in our labs has just started.
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