Electronics > Projects, Designs, and Technical Stuff

Human Eye -- Peak or Average Response

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jweir43:
Relatively dark room.  Plain jane 3mm red led.  1.5v fvd  13,5 volt DC supply. 

Two of these LEDs side by side.

One of them is connected to the DC supply through a 470 ohm series resistor.  If = 12/470 = 25 mA  Pd = 38 mW

One of them is driven by a square wave 13.5 volt peak, 240 ohm series resistor.  If (peak) = 12/240 = 50 mA   Pd (on the positive pulse) = 75 mW  Pd (on the negative pulse = 0 mW;   average Pd = 37.5 mW.

Neglecting the half a milliwatt difference, will one of them appear brighter to a human observer than the other?   And why?    :-//


Jim

wraper:
PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.

vk6zgo:
 Eyes, like most human sensors, have a logarithmic response, so I would guess, no difference.
You would need to swap the LEDs over to be sure.

Eyes can also modify their basic response by shrinking or enlarging the pupli, or even by blinking.(but that
should be the same for both cases)

The eye's spectral response is also important, as it means you can't directly compare the apparent brightness of, say, red & blue LEDs.(OK, I know both yours are red.)

David Hess:

--- Quote from: wraper on June 08, 2019, 01:53:51 am ---PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.
--- End quote ---

This depends on the LED.  High brightness LEDs can have lower efficiency below 100s of milliamps which is one of the advantages of multiplexed operation.

As far as the test, LED efficiency can be verified with a photosensor.

wraper:

--- Quote from: David Hess on June 08, 2019, 03:05:25 am ---
--- Quote from: wraper on June 08, 2019, 01:53:51 am ---PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.
--- End quote ---

This depends on the LED.  High brightness LEDs can have lower efficiency below 100s of milliamps which is one of the advantages of multiplexed operation.

As far as the test, LED efficiency can be verified with a photosensor.

--- End quote ---
If LED can work at 100s of milliamps (even peak), and especially have high efficiency at that current, it's high power LED, not simply high brightness LED.

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