Infraredwavelength

In our laboratory tests we have found that LED heat sink temperature is generally equal to LED case temperature below approximately 60°C in lower power LEDs for 6-8W. Although there are some exceptions with older less efficient models with poor heat sink design. In general a poor heat sink design will have difficulty conducting heat away from the LED chip leading to a much higher LED case temperature compared with the LED heat sink temperature with a differential of up to 20°C.

In higher power LED light bulbs (10W+) such as 10W 500lm+ we measure case temperatures of approximately 70°C, whilst heat sink temperatures are approximately 62°C. What this tells us is that as the LED chip gets hotter the ability for the heat sink to transfer heat away from the LED chip is reduced. LED bulbs with a heat sink temperature of greater than 70°C are probably reaching the limits for safe LED functioning.

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The main harmful health effects of high IR exposure are to the eye. The cornea, iris, lens and retina are all highly sensitive to varying degrees of thermal damage. When the cornea absorbs IR radiation with conversion into heat, this is conducted to the lens. Aggregation of lens proteins after repeated exposure to extreme heat can cause lens opacities or cataracts, as are often seen in glass workers and iron and steel workers.

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Therefore, proper thermal management and heat sink design is paramount and will separate the “winners from the losers” in this emerging industry.

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Skin damage due to hyperthermia can occur but depends on the intensity and the duration of IR exposure. If the temperature of the skin is held at 44°C it takes several hours before irreversible damage occurs. This compares with less than a second at surface temperatures of 70°C. Long-term IR exposure of the skin without burning, such as after years of skin exposure to open fires, can cause a red-brown mottling of the skin. However, IR is not thought to play a role in initiating skin cancer.

Protection recommendations are aimed especially at the skin and relevant parts of the eye, which are at risk from excessive exposure to infrared radiations.

If the whole body is subjected to high levels of heat, increases in body temperature and physical heat-stress can result. Heat stress needs to be evaluated considering all contributing factors including air movement, temperature and humidity as well as the source of the heat.

Midinfrared

Bridgelux LM80 Report: 6500 Hour Test Data, Bridgelux RS LED Array BXRA -W3000, November 2, 2011. Innovative Circuits Engineering Inc.

Therefore, due to the importance of LED case temperature thermal management techniques in the form of passive and active cooling methods have been developed to maintain the temperature of the LED chip temperature within normal operating temperatures, typically between at 55°C and 85°C. On LED manufacturer data sheets this is typically referred to as LED case temperature (TS).

To avoid detrimental health effects from infrared radiation on the eye and skin, such as thermal injuries, ICNIRP provides guidance and recommends limits of exposure. Different limits are recommended depending on the wavelength bands and action spectra. The limits also depend on exposure duration and on the size of the source.

Infrared radiation (IR), also known as thermal radiation, is that band in the electromagnetic radiation spectrum with wavelengths above red visible light between 780 nm and 1 mm. IR is categorized as IR-A (780 nm-1.4 µm), IR-B (1.4-3 µm) and IR-C, also known as far-IR (3 µm-1 mm). Common natural sources are solar radiation and fire. Common artificial sources include heating devices, and infrared lamps used and in the home and in infrared saunas for health purposes. Industrial sources of heat such as steel/iron production also fall into the infrared region. Lasers are a special source of IR emitted over one or more extremely narrow wavelength bands.

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As mentioned the LED chip in your LED light bulb is sensitive to high temperatures such that overly high temperatures of > 85°C will significantly shorten the life of your LED bulb as well as the lumen output or brightness. For example, the lumen maintenance for a BRIDGELUX ES BXRA-W0802 with a case temperature of 55°C or 85°C is approximately 94% over 6,000 hours, whereas at a case temperature of 105°C the lumen maintenance is approximately 91% of the original brightness. This equates to a decline to 70% brightness by approximately 20,000 hours at 105°C versus approximately 40,000 hours at 85°C. At case temperatures greater than 105°C the LED will fail within a relatively short time, potentially approximately 2,000 hours.

IR penetrates the human skin and the eye to various depths ranging from several millimeters by IR-A to superficial absorption of IR-C. Humans have inborn protective aversion responses to pain from high heat and to the bright light that is often also present, so that potentially harmful exposure is avoided. Harmful health effects of IR are due to thermal injury of tissues mediated largely through water molecules but also through changes to protein structure.