An infrared heater or heat lamp is a heating appliance containing a high-temperature emitter that transfers energy to a cooler object through electromagnetic radiation. Depending on the temperature of the emitter, the wavelength of the peak of the infrared radiation ranges from 750 nm to 1 mm. No contact or medium between the emitter and cool object is needed for the energy transfer. Infrared heaters can be operated in vacuum or atmosphere. One classification of infrared heaters is by the wavelength bands of infrared emission.
Short wave or near infrared for the range from 750 nm to 1.4 μm; these emitters are also named "bright" because still some visible light is emitted; Medium infrared for the range between 1.4 μm and 3 μm; Far infrared or dark emitters for everything above 3 μm.
History German-British astronomer Sir William Herschel is credited with the discovery of infrared in 1800. He made an instrument called a spectrometer to measure the magnitude of radiant power at different wavelengths. This instrument was made from three pieces. The first was a prism to catch the sunlight and direct and disperse the colors down onto a table, the second was a small panel of cardboard with a slit wide enough for only a single color to pass through it and finally, three mercury-in-glass thermometers. Through his experiment Herschel found that red light had the highest degree of temperature change in the light spectrum, however, infrared heating was not commonly used until World War II. During World War II infrared heating became more widely used and recognized. The main applications were in the metal finishing fields, particularly in the curing and drying of paints and lacquers on military equipment. Banks of lamp bulbs were used very successfully; though by today's standards the power intensities were very low, the technique offered much faster drying times than the fuel convection ovens of the time. After World War II the adoption of infrared heating techniques continued but on a much slower basis. In the mid 1950s the motor vehicle industry began to show interest in the capabilities of infrared for paint curing and a number of production line infrared tunnels came into use.
Elements The most common filament material used for electrical infrared heaters is tungsten wire, which is coiled to provide more surface area. Low temperature alternatives for tungsten are carbon, or alloys of iron, chromium, and aluminum (FeCrAl, trademark and brand name Kanthal). While carbon filaments are more fickle to produce, they heat up much more quickly than a comparable medium-wave heater based on a metal alloy filament. When light is undesirable or not necessary in a heater, ceramic infrared radiant heaters are the preferred choice. Containing 8 meters (26 ft) of coiled alloy resistance wire, they emit a uniform heat across the entire surface of the heater and the ceramic is 90% absorbent of the radiation. As absorption and emission are based on the same physical causes in each body, ceramic is ideally suited as a material for infrared heaters. Industrial infrared heaters sometimes use a gold coating on the quartz tube that reflects the infrared radiation and directs it towards the product to be heated. Consequently, the infrared radiation impinging on the product is virtually doubled. Gold is used because of its oxidation resistance and very high infrared reflectivity of approximately 95%.
Types Infrared heaters are commonly used in infrared modules (or emitter banks) combining several heaters to achieve larger heated areas. Infrared heaters are usually classified by the wavelength they emit: Near infrared (NIR) or short-wave infrared heaters operate at high filament temperatures above 1,800 °C (3,270 °F) and when arranged in a field reach high power densities of some hundreds of kW/m2. Their peak wavelength is well below the absorption spectrum for water, making them unsuitable for many drying applications. They are well suited for heating of silica where a deep penetration is needed. Medium-wave (MWIR) and carbon infrared heaters operate at filament temperatures of around 1,000 °C (1,830 °F). They reach maximum power densities of up to 60 kW/m2 (5.6 kW/ft2) (medium-wave) and 150 kW/m2 (14 kW/ft2) (carbon). Far infrared emitters (FIR) are typically used in the so-called low-temperature far infrared saunas. These constitute only the higher and more expensive range of the market of infrared sauna. Instead of using carbon, quartz or high watt ceramic emitters, which emit near and medium infrared radiation, heat and light, far infrared emitters use low watt ceramic plates that remain cold, while still emitting far infrared radiation. The relationship between temperature and peak wavelength is expressed by Wien's displacement law.
Metal wire element Metal wire heating elements first appeared in the 1920s. These elements consist of wire made from chromel. Chromel is made from nickel and chrome and it is also known as nichrome. This wire was then coiled into a spiral and wrapped around a ceramic body. When heated to high temperatures it forms a protective layer of chromium oxide which protects the wire from burning and corrosion, and causes the element to glow.
Heat lamps
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