The definition and analysis of optical parameters of LED

LED luminaires are this kind of device which can transmit, distribute, and change the light distribution of light source. They are becoming the main products of a new generation of lighting market with its high efficiency, energy saving, safety, long service life, compact, and other technical characteristics. They effectively promote energy-saving environmental protection industry of high-speed development. The usual parameters of light which associate with photometry are light intensity, luminous flux, illuminance and brightness. Although these four quantities are related, they are different and can not be confused. Just as the physical quantity like pressure, gravity, pressure, quality, they represent different parameters and effects.

1. Light Intensity( I, Intensity)
Unit Candela (CD), this is the inherent properties of point source. It is to represent the convergence ability of light.
Definition: The luminous flux which light source emits to the unit solid angle in fixed direction is defined as light intensity.

Explanation: The light intensity is in terms of the point source, or the size of the light emitting body is relatively small when compared with the irradiation distance. This quantity is to show the convergence ability of the emitter in space. Light intensity represents the radiation capability of light in different directions, also popular to say that the intensity of the light is the degree of the intensity emitted by the light source. It can be said that the light intensity is the description of how “bright” the light source is, because it is a common description of the optical power and convergence capability. Light intensity is higher, light will looks more bright, and under the same conditions, the being lighted object will be more bright. Therefore, in the earlier time, people often use the this parameter to describe flashlight. The common mistake is that people often use the intensity to express the “brightness “. The “Brightness” which we talked above all be added with quotation marks, because this is our conventional brightness, not photometry strict sense of brightness in photometry. We will have a detailed description for the brightness.

2. Luminous Flux(F, Flux), the unit is lumen, that is lm.
It is the inherent attribute of the light source, and the total energy of the light source in the unit time, namely the light power
Definition: the amount of source emitted light in the unit of time is called luminous flux of the light source

Explanation: similarly, this quantity is in terms of the light source to describe the size of the total amount of light source, and is equivalent to light power. Higher the Light flux is, more light will be emitted. Compared with the mechanical unit, luminous flux is equivalent to the force, and the intensity is equivalent to pressure. To make the irradiated point looks more bright, we should not only improve the luminous flux, and also need to increase the convergence, actually is to reduce the area, so as to get greater strength. As we may know the luminous flux is also the artificial quantity. So it is not completely natural for other animals because this definition is completely based on the response of the human eye.

3. Light effect, unit is LM / W, that is how much luminous flux can be converted with 1w power.
As for the luminous efficiency of electric light source is another related topic, that’s how much luminous flux can be transformed in the end with 1W power.The human eye responses differently to light of different colors, this feeling determines the conversion relationship between luminous flux and light power. Because human eye is most sensitive to the 555nm light, if half of the light converted to 555 nm light, and the other half turn into heat loss, then the efficiency is per watt 34.15 lumens. For the other colors of light, such as red light with 650nm, 1W power is equivalent to only 73 lumens. This is because the human eye is not sensitive to red light. For white light, that’s depends. Because there is a lot of different spectral structure of white light. For example, there is large difference between LED white light and white light on the TV. The incandescent lamp is even hard to reach 1W=20lm, and the rest light are all turned to heat or infrared. That’s due to the different spectrum.

4. Light Illuminance (E, Illuminance), unit is LX (formerly known as lux). To reflect the light shines or not from another point of view.
Definition: the light illuminance generated when 1 lumen evenly distributed on the surface of 1 square meter.

Explanation: Light illuminance is in terms of the lighted location, but it has nothing to do with the been lighted object. A luminous light, uniformly gives light to an object wiht 1.2m long, then the illuminance is 1 lx. People can use the illuminance photometer to measure the illuminance. In order to protect eye and make life and work convenient. The illuminance has different provision in different places. In fact, the illumination is the most easy parameter to measure(compared with other parameters), the illumination can also been tested by the integrating sphere.

5. Brightness (L, Luminance), unit is nits, namely nt. It reflects the light bright or not when the emitting surface or the reflection of light into the human eye.
Definition: the unit area of the light source in the normal direction, the light flow within the unit solid angle, which also called the light intensity of the unit area light emitting surface, that’s candela / square meter.

Explanation: This is the most confused conception that can be easily understood. The brightness is in terms of light source not to the point source. In addition, the brightness of the light emitting surface is independent of the distance, but is related to the direction of the observer. Many people are confused about the concept of illuminance and luminance, actually the amount of total incident light in a unit area on the surface can be described as illumination; if illustrate the reflective total light in a unit area of surface from human eyes, then it can defined as brightness.

The usual test for luminaire are intensity, light effect, brightness and illuminance, these parameters are also the general requirements of laboratory. According to the LM-79, clause 9.3.1, there are only two kinds of goniophotomter can be acknowledged. Namely, central rotating mirror type goniophotometer and circular motion reflecting mirror type goniophotometer. They are all the rotating mirror vertical distribution system which can automatically test 3D light intensity distribution curve. They can realize C-γ、A-α and B-β measurement solution and fully meet the CIE, IESNA, GB and many other international standards and in accordance with the LM-79 clause 9.3.1.

Usually engineer need use Integrating Sphere to test a lighting luminaire. The integrating sphere is used to provide a testing environment which can let the light evenly distributed. Lisun provide different diameter integrating sphere work with spectroradiometer to apply in measuring all kinds of lamp testing. The CIE-127-1997 and IES LM 79-08 suggest using the array-typed spectroradiometer and integrating sphere to test solid-state lighting products(SSL). The LMS-9000A combined the LED optical maintenance test function which is meet LM-80.

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