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How do high power LEDs with forward current of 350mA compare to bright 5mm LEDs with forward current of 20mA? If forward voltage is the same (same color) does that mean light output of 17.5 of 5mm LEDs equal 1 power LED? Which type is more efficient?

And one more question, in datasheets why measure of light output of power LEDs are given in lumens (luminous flux) but standard 5mm LEDs are given in mcd (luminous intensity)?

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  • \$\begingroup\$ giving luminous output in lumens, candela, and radiant flux are all common. It's just an issue with the industry of light. \$\endgroup\$ – KyranF May 30 '16 at 4:34
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    \$\begingroup\$ Also, "I want a..." is not the best way to start a question asking for help. \$\endgroup\$ – KyranF May 30 '16 at 4:34
  • \$\begingroup\$ @KyranF thanks for your comment, I edited the text. \$\endgroup\$ – ahmadx87 May 30 '16 at 6:52
  • \$\begingroup\$ read more about luminous efficacy (lumens per watt) and about light emitted is sometimes spec'd in steradians, usually related to the viewing angle of the LED and if any optics are used \$\endgroup\$ – KyranF May 30 '16 at 17:35
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in datasheets why measure of light output of power LEDs are given in lumens (luminous flux) but standard 5mm LEDs are given in mcd (luminous intensity)?

Luminous flux measures the total visible light emitted by the LED. It's a useful spec if you're going to use the LED to illuminate a room.

Luminous intensity measure the intensity of visible light emitted in a particular direction. It's a useful spec if you want to use the LED as an indicator and need to know if it will be visible against a bright background, or how bright it will appear when viewed directly, compared to other LEDs, possibly of other colors.

Since high power LEDs are often used to illuminate other objects, while 5 mm LEDs are often viewed directly (for example, when used as panel indicators), it makes sense to specify them differently.

Which type is more efficient?

You'd have to compare the output luminous flux (or intensity, depending how you will use the device) with the input power. Since the two devices aren't specified equivalently, you might have to experiment on samples of each type to find out.

If forward voltage is the same (same color) does that mean light output of 17.5 of 5mm LEDs equal 1 power LED?

Only if the efficiencies are the same.

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    \$\begingroup\$ It seems appropriate that a question about light intensity should be answered by Photon. \$\endgroup\$ – Mark May 30 '16 at 5:34
  • \$\begingroup\$ Thanks for your Answer @The-Photon . "Only if the efficiencies are the same" this is exactly what I want to know. I know three 1W LEDs are more efficient that one 3W LED (from the same manufacturer). I think theoretically 17.5 low power LEDs should be much more efficient. Is it true? \$\endgroup\$ – ahmadx87 May 30 '16 at 7:03
  • \$\begingroup\$ @ahmadx87, like I said in my answer, "Since the two devices aren't specified equivalently, you might have to experiment on samples of each type to find out." \$\endgroup\$ – The Photon May 30 '16 at 15:17
  • \$\begingroup\$ It also depends on how you want to define efficiency, in terms of total light output or in terms of light received by a viewer in a particular direction looking at the device directly. \$\endgroup\$ – The Photon May 30 '16 at 15:18
  • \$\begingroup\$ I want to use them in something similar to bar led lights (wallwashers). Maybe a viewing angle of up to 45 degrees. Standard LEDs are attractive in that they are much easier to drive and heating is not that much an issue. \$\endgroup\$ – ahmadx87 May 31 '16 at 9:27

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