Story of Lighting : By Jaydeep shah

LED Lights & It's Measurement Units (Jaydeep Shah)\

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This is Story of Light :

Many times , things are not easy to understand , specially in case of science  & Technology.The concept of electro luminescence lies at the heart of the development of LED lights. In 1907, Henry Joseph Round observed this phenomenon in Silicon Carbide. However, the yellow light emitted was too faint to be of any real use. No further research was conducted into this phenomenon then. In 1920, Bernhard Gudden and Robert Wichard Pohl used Zinc Sulphide with Copper to replicate the phenomenon.

Many technical person also not understand basic definition and terms related of LED lights. Many times when customer buy bulbs or Tubelights they look at Watt "W" , but not Lumens.😕 ????

I have tried here to explain basic difference in definition . So Let's start :

1) Luminance Flux: (Lm, lumens) :

Total amount of radiation emitted by given light in all direction that is visible for human eyes.

Sensitivity of human eyes varies for different wavelength (e.g : Light sensitivity for green light is higher compare to red or blue).

Unit : lumens - Lm

2) Solid angle / St. radian : 

St.radian is the SI unit of solid angle.

Spherical surface area divided by radius of sphere = solid angle of the cone inside sphere.

In geometry, a solid angle (symbol: Ω) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The point from which the object is viewed is called the apex of the solid angle, and the object is said to subtend its solid angle from that point.

3) Luminance intensity : (candela) : 

Luminous flux within given solid angle from light source.

It is measurement of amount of light emitted by light source into certain direction measure at 1 meter distance.

1 cd = 1 lm / st.radian.

If light source emitted L lumens into small st.radian angle B; then Luminance intensity = L/B.

Luminance intensity also depend on optics ;which make light more or less focused.

4) Illuminance : (lux) 

It represents total amount of light reaching a particular illuminated surface area.

1 LUX = It illuminated 1 m*m surface area uniformly by Lm flux.

So basically amount of light that cover require surface.

Flux = F ; Surface area = S

Lux E = F/S

5) Luminance (cd/m*m): 

Brightness of lit surface or object as perceived by our human eyes.

All these things in single image :

Light intensity decrease with increase junction temperature after certain range.

So it is all about basic terms of Lights, You can also download it in soft copy (.pdf ) , Just click below link :

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You can also download it from:

- scribed.com

- stuvia.com

- slideshare.net.com

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Thanks : (Author details)

Jaydeep Shah

Electronics & Communication Engineer

Ahmedabad - India