ON LUMEN & WATT OF LED

Four common descriptions of light’s physical quantity:

1. Luminous intensity is a measurement of the wavelength-weighted power emitted by a light source in a given direction.  The SI unit of luminous intensity is the candela, abbreviated cd.  Some people still used the candle to express the luminous intensity, which was outdated; however, in 1940 (some saying in 1948) the word “candela” has been used. And the “candle”, abolished after 1968, became just a candle.
2. Luminous flux is the quantitative expression of light source emitting per unit time and its standard unit is lumen (Lm).
3. Illuminance is the total luminous flux incident on the 1m2 surfaces, and the standard unit is lux (Lx).
4. Luminance, the luminous intensity of light per unit area in a certain direction, describes that the luminous flux is emitted for a unit solid angle.  The unit is called nit (nt).

Generally, the luminous intensity is a way to measure candela of LED flashlights.  In early days the brightness of LED was low, so the unit of measurement was micro candela, abbreviated mcd. Later on, many of them came out with over a thousand and over ten thousand mcd’s, therefore 10,000mcd=10cd.

With the same tube core, the mcd of 5mm LED is lower than 10mm LED due to the good concentration and small light spot. In fact, their luminous flux is apparently the same.

A norm of the measurement of luminous efficacy is how many lumens are emitted by electric power per watt.  In theory, the maximum possible efficacy is 683lm/W (and it needs the 0.54 micron wavelength light in olivine).  An incandescent light bulb is about 10lm/W and a good white LED should be possible to achieve above 200 lm.  Due to the phosphor material which is added around the LED, the LED which is originally blue turns into white.  Therefore, there are two peaks in the emission spectrum of the white LED.  A bad manufacture process could possibly make the LED blue.

Measurement of luminous intensity: Firstly, find a digital light meter and make upward the sensor on the floor.  Secondly, prepare a flashlight or a LED to illuminate it.  Lastly, the maximum data you get is cd and multiply it by 1,000, which will be mcd number.

The conversion between the wattage and the mcd.

Luminance has the light character and wattage has electric character.

Wattage is the unit of electric power of electromagnetic wave from the visible and invisible light emitted by LED and the lumen refers to the unit of electric power of the visible light.  Therefore, luminous efficacy means the ratio of the lumens and the electromagnetic wave emitted by the LED.

The lumen, in the solid angle, is the luminous intensity and its unit is candela. Generally speaking, the luminous intensity of the LED means the maximum of the luminous intensity.  In order to describe its character of brightness, there is still a technical parameter, beam divergence.

The candela is the unit for the luminance, which is candela per square metre and is also called nit (nt).

There is no connection between the wattage and the luminance.

With 1W white light, our LED is about 13~18 lumens and the LED of America is 25 lumens. It just sees who can make more light by transforming the electricity.  The reason that the LED can economize on electricity is that the LED’s efficiency of changing the electricity into the light is more than other light materials, which can saving the energy source.

關於LED的流明及瓦數

描述光的常用物理量有4個,他們是:

1、發光強度,為一光源在給定方向上的發光強度,單位candela,即坎德拉,簡稱坎、cd。有人仍然用燭光來表示發光強度,那太老了,要知道1940年(又一說1948年)已經採用新燭光了,只不過“燭”=candle罷了。1968年以後燭光被廢除。
2、光通量,光源在單位時間內發射出的光量稱為光源的發光通量,單位流明,Lm。
3、光照度,1 Lm(流明)的光通量均勻分佈在1m2表面上所產生的光照度,單位勒克斯,Lx。
4、亮度,單位光源面積在法線方向上,單位立體角內所發出的光流,單位尼特,nt。

衡量手電筒和LED一般用發光強度,但早期的LED“亮度”低,因此都用毫cd來衡量,即mcd,後來出來了上千、上萬mcd的,單位也不改了,因此10000mcd=10cd。

同樣管芯的LED,5mm的mcd值就沒有10mm的mcd值大(我這裡避免使用亮度一詞,因為意義太容易模糊了),原因是10mm的聚焦好、光點小。實際它們發出的光通量顯然是一樣的。

衡量發光效率的一個指標,就是每瓦的電功率可以發出多少流明的光通量,理論極限是683流明(而且需要是0.54微米波長的黃綠色光)。白熾燈是每瓦10流明左右,好的白色LED應該可以達到200流明以上。白色的LED,本來就是籃色的LED,在管芯周遭加上了螢光粉。因此,看看白色LED的發射譜線,有兩個峰值。製程不好偏籃是很正常的。

發光強度的業餘測量︰找一個照度表(即Lx表,我就有一個),探頭放在地上向上,在暗處把手電筒或LED距離1m向其照射,得到的Lx最大讀數就是cd值,乘上1000就是mcd值。

瓦數和亮度之間的換算(1W=?mcd)
亮度是光特性,瓦數是電特性。

瓦數是描述LED發出的可見不可見光所有電磁波的功率的單位,流明專指可見光的功率單位,所以發光效率是指LED發出的可見光的流明數與所有電磁波的瓦數之比。

單位立體角內的流明數就是光強,單位堪德拉(candela)。一般說LED的光強是指光強的最大值,為更好的說明發光特性,還有一個技術參數擴散角。

單位面積的堪德拉(candela)數就是亮度,單位堪德拉每平方米,也稱為尼特(nt)。

W數跟亮度沒有關係。
如1W大功率白光,我們的是13–18流明,而美國的是25流明.只不過看誰的電能轉換成光能多一些而已。LED省電的原因在於它的電能轉換於光能高於其它發光材料,因此節能。

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