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LED

A light-emitting diode (LED) is an electronic light source. Luminescence from an electrically stimulated crystal had been observed as early as 1907. The LED was introduced as a practical electronic component in 1962. All early devices emitted low-intensity red light, but modern LEDs are available across the visible, ultraviolet and infra red wavelengths, with very high brightness. LEDs are based on the semiconductor diode. When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm²) with integrated optical components to shape its radiation pattern and assist in reflection.

Color temperature

Color temperature is usually expressed in Kelvins and is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, and other fields. The color temperature of a light source is determined by comparing its chromaticity with that of an ideal black-body radiator. In simple terms, color temperature determines the way we perceive a light source. For example, a usuall incandescent bulb operates at about 2500 Kelvin. That temperature makes the light seem a little yellowish and is termed as 'warm white'. As we go higher on the Kelvin scale reaching 4000 Kelvin, the light would look 'pure white' such as a T8 fluorescent tube. Going even higher, we will start seeing a small amoumt of blue light. That is usually termed as 'cool white' and has a color temperature of 5000-5500 Kelvin or higher.

Power consumption

Power consumption is the total amount of energy that we spend to heat water, keep the lights on, operate our TV set and basically all electric devices in our house. It is expressed in Watts. Our electricity bill is also expressed in KiloWatts per hour. We pay based on the amount of power we consumed over a period of time. Suppossing that the KW/h in your area costs 0.09 €, we can calculate the consumption for one month period of a light bulb 25W. When a bulb 25W is kept on for an average of 5 hours a day, then we have: KW/h = 25W / 1000. So we consume 0.025 KW/h every hour of operation. For 5 hours we have 0.025 KW/h * 5 Hrs = 0.125 KW/h for every day. Now, supposing that a month has an average of 30 days, we have: 0.125 KW/h * 30 Days = 3.75 KW/h. So, if we keep this bulb on for 5 hrs every day for a total of 30 days, we consume 3.75 KW/h which costs 3.75 KW/h * 0.09 € = 0.337 €!

Luminous intensity

Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit. Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function.

Lumens

A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m². A single fluorescent light fixture with an output of 12000 lumens might light a residential kitchen with an illuminance of 500 lux. To light a factory floor with area dozens of times that of the kitchen would require dozens of such fixtures. Lighting a larger area to the same level of lux requires a greater number of lumens.

Lux

The lux (symbol: lx) is the SI unit of illuminance and luminous emittance. It is used in photometry as a measure of the apparent intensity of light hitting or passing through a surface. It can be measured by using a Lux-meter. Lux intensity decreases as we get further from a light source. It can only be a reference parameter when comparing two almost same type of light sources emmiting at almost the same wavelength but does not indicate in any way the overall light intensity of the source.

Efficiency

A light bulb might have 2% efficiency at emitting light yet still be 98% efficient at heating a room. (In practice it is nearly 100% efficient at heating a room because the light energy will also be converted to heat eventually, apart from the small fraction that leaves through the windows. An electronic amplifier that delivers 10 watts of power to its load (for example a loudspeaker), while drawing 20 watts of power from a power source is 50% efficient. (10/20 × 100% = 50%)

Efficacy

In lighting design, "efficacy" refers to the amount of light (luminous flux) produced by a lamp (a light bulb or other light source), usually measured in lumens, as a ratio of the amount of power consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output devided by input which for lighting relates to the watts of visible power as a fraction of the power consumed in watts.

Wavelength

Wavelength is extracted from electronics and has got to do with the frequency of an electromagnetic signal. Wavelength is actually the distance that an electromagnetic energy signal can travel over one single cycle of its' frequency. Let's take the FM band that we all use when listening to the radio. That band extends from 87MHz to 108MHz. So a signal that is transmitted on 108MHz contains 108 million cycles per second. Every cycle takes almost 9.26 nano seconds! Taking into consideraton that electromagnetic signals travel at 300000 Km per second, we can calculate the distance that one cycle of the 108MHz signal will travel! That gives us: 300000000 m / 108000000 = 2.78m. So, a signal having a frequency of 108 MHz will have a cycle of 9.26 nano seconds and will travel during that period over a distance of 2.78 meters! Light is actually electromagnetic energy but having a huge frequency that lies in the Tera Herz band. Wavelengths calculated for that band give us color blue having a wavelength of 450–495 nm and a frequency of 606–668 THz, color red having a wavelength of 620–750 nm and a frequency of 400–484 THz, etc!