The light color produced by LEDs depends on the specific semiconductor material used to make the chip. The most common chips utilize indium gallium nitride (InGaN) to produce blue LEDs and gallium-aluminum-arsenide-phosphide (GaAlAsP) to create orange, yellow, and green LEDs.
The broader spectrum produced by the phosphors makes up the rest of the visible light spectrum. The higher the CRI, the more faithfully the colors of objects are represented.
Light Emitting Diode technology
Light emitting diodes use a special semiconductor material to allow current to flow in one direction only. This makes them very efficient at converting electrical energy into visible light.
When an LED is forward-biased, the atoms in the n-type semiconductor material donate electrons to those in the p-type material. These electrons then fall into holes in the p-type material, which then releases electromagnetic radiation in the form of photons.
The p-n junction in an LED is heavily doped with specific semiconductor materials to produce light of different spectral wavelengths. This is what gives LEDs their characteristic color, and it’s what sets them apart from other lighting sources like lasers. The LED’s epoxy body acts like a lens, concentrating the photons emitted by the p-n junction into a single spot of light at its top.
Color Temperature
The color temperature of LED lighting is measured in Kelvin (K). Different color temperatures produce different shades of white. The color temperature of a light is an important factor in the ambiance created by the light.
Warm LED lights (2700K-3000K) are similar in tone to an incandescent bulb and are best for residential spaces or where a comforting atmosphere is desired. Cool LED lights (3000K-4900K) produce a bright white or yellowish tone and are ideal for kitchens, workspaces or vanities. Daylight (5000K and up) produces a bluish white light that is often used in commercial applications.
The LED spectral output is different from the smooth curve of an incandescent lamp shown above because it has an oblong shape due to the p-n junction structure of the semiconductor. This causes a shift of the emission peak with operating current.
Color Rendering Index (CRI)
CRI refers to the ability of a light source to render color accurately. A high CRI value is important because it allows people to see the colors of objects as they should look.
The traditional way to measure CRI is by comparing a test light source to sunlight or another reference illuminator with a perfect 100 rating. This process involves using a color calibration chart like the ColorChecker.
When shopping for LEDs, it’s best to consider those with a CRI above 90. This is a good choice for applications where accurate color rendering is critical, such as retail stores, art galleries and jewelry displays. A high CRI also makes for better quality lighting in homes and can help create a more comfortable living environment.
Full Spectrum vs. Narrow den am dat Spectrum
Many LED lights are advertised as full spectrum, however the spectral output differs from light source to light source. For example, some LED lights use different phosphors to produce different wavelengths of color that when combined create white light. This can result in a high CRI of over 80 and is often referred to as a broad spectrum light.
Other LED lights use a single type of phosphor for their entire die. These are typically monochromatic and do not meet the requirements for transmission fluorescence microscopy. Narrow spectrum LED lights tend to flood the canopy of a plant, ignoring lower leaves which can be problematic in some plants such as the Cranefly Orchid (Tipularia discolor). Narrow spectrum LEDs also lack wavelengths needed for photosynthesis which results in poor growth.
Applications
The most significant challenges faced in the fabrication of LEDs are maximization of light generation within the hybrid semiconductor materials and efficient extraction of this light to the outside environment. Due to total internal reflection phenomena, only a small percentage of the light generated isotropically inside the semiconductor can escape from the surface.
The emission spectra of different LEDs can be modulated by varying the band gap energy of the semiconductor material used to fabricate them. The most common diodes use a mixture of periodic table Group III and Group V elements, such as gallium nitride, SiC, ZnSe, or GaAlAsP (gallium aluminum arsenic phosphide), to produce the desired wavelength bands.
Many fluorescent microscopy applications require high-power LEDs with narrow spectral emission bands for efficient excitation of fluorophores. Modern LED lamphouses incorporate individually controllable modular LED modules to allow the user to select the required wavelength range for a given application.
