# Pulsed illumination and perceived brightness **URL:** \ **Category:** Factual Questions\ **Created:** [February 7, 2003, 8:44pm UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556 "2003-02-07T20:44:50Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 7, 2003, 8:44pm UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/1 "2003-02-07T20:44:50Z")
I read something on some electronics site or other (I can probably find it again if anyone is interested) to the effect that a pulsed light source with a total of 50% duty cycle, provided the frequency was high enough, would appear just as bright as a constant one, while using only (about) half the energy (assuming perfect square waves and perfect linear emitter with instantaneous response etc etc). It sounds like bunk to me; after all the total output in terms of photons is halved, we would just perceive it as half as bright, wouldn’t we? ---
**Author:** ![hobbes730](https://avatars.discourse-cdn.com/v4/letter/h/97f17d/32.png) [@hobbes730](https://boards.straightdope.com/u/hobbes730)\ **Post date:** [February 7, 2003, 10:53pm UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/2 "2003-02-07T22:53:26Z")
I don’t think it’s bunk; I’ve actually employed such a technique before to extend battery and LED life. I don’t know the physics behind it though. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 7, 2003, 11:08pm UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/3 "2003-02-07T23:08:27Z")
I don’t doubt that it would extend battery life; it’s the biology rather than the physics that baffles me; half the amount of photons is reaching the eye, but is causes the same amount of stimulation? if this is true, is it perhaps because the ‘dark’ phase of the cycle is happening while the rods/cones are ‘resting’ after having received a stimulus? ---
**Author:** ![danvanf](https://avatars.discourse-cdn.com/v4/letter/d/13edae/32.png) [@danvanf](https://boards.straightdope.com/u/danvanf)\ **Post date:** [February 8, 2003, 2:36am UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/4 "2003-02-08T02:36:10Z")
When a light bulb works the filiment is so hot, small pieces come off. Since the wire takes time to cool down. As long as your off time is short enough, the wire should still fizzle in about the same way. (There is a word for that fizzle, I just don’t remember it) Lumens are not directly porportional to RMS or even average voltage. I dunno if it’s bunk or not, but it sounds possible to me. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 8, 2003, 3:25am UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/5 "2003-02-08T03:25:38Z")
**danvanf** , are you perhaps confusing “incandesce” with “effervesce”? ---
**Author:** ![danvanf](https://avatars.discourse-cdn.com/v4/letter/d/13edae/32.png) [@danvanf](https://boards.straightdope.com/u/danvanf)\ **Post date:** [February 8, 2003, 4:09am UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/6 "2003-02-08T04:09:25Z")
Na but I am reading Incandecent light rather than light source. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 8, 2003, 4:52am UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/7 "2003-02-08T04:52:15Z")
[http://www.caves.org.uk/led/foot4.pdf](http://www.caves.org.uk/led/foot4.pdf) > [@](#): > > **Is a flashing light more efficient?** > It is well-known that the apparent brightness of a fast-pulsed light source is equal to the integrated value of the actual illumination. In other words, if you viewed it through a shutter that opened and closed 100 times a second, and it was open for 30% of the time, then the light would appear to be 30% as bright. This is the principle of persistence of vision and is formally expressed in Talbot’s Law for frequencies greater than about 10Hz. > **LEDs** > Expressed another way, this says that if were to drive an LED with 100mA pulses on a 20% duty cycle at a frequency high enough for ‘persistence of vision’ to apply, then (for the same LED efficiency) the apparent brightness would be as if we drove it with 20mAt continuously. Early LEDs were more efficient at converting electrical energy to light energy when driven at a higher current, so a pulsed LED, as in the example above, would actually give a brighter light than a continuously-driven diode. However this is not true of modern LEDs, and it is certainly far from true of white LEDs. Sounds reasonable to me. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [February 8, 2003, 5:30am UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/8 "2003-02-08T05:30:04Z")
While reading a printed article on telescope eyepiece design, I ran across the curious claim that a 50% decrease in the brightness of an image from the center to the edge of vision is not normaly perceptible. That sort of visual filtering might leave room for some slop in the actual versus percieved brightness of pulsating illumnation. ---
**Author:** ![something\_clever](https://avatars.discourse-cdn.com/v4/letter/s/3bc359/32.png) [@something\_clever](https://boards.straightdope.com/u/something_clever)\ **Post date:** [February 8, 2003, 5:44am UTC](https://boards.straightdope.com/t/pulsed-illumination-and-perceived-brightness/153556/9 "2003-02-08T05:44:47Z")
well… i have a ‘night light’ by the guys at color kinetcs [www.cksauce.com](http://www.cksauce.com) that uses two sets of rgb leds set end to end and run with varying pulsewidths (thats what they said) to use less energy and create all colors in the spectrum including white… if you are talking about lighting large spaces though, i think flourescent lighting is still the best option right now… cfl lights for your house last longer and use mostly 1/4th the energy of normal lightbulbs…