Is 10 Watt direct pwm bad for the eyes  - Page 1
Forum section: electronics / beginners

author: dzarren
author_rank: Contributor
timestamp: May 27, 2020, 09:13:41 pm
content: I am making an LED light for my microscope which is basically a copy of this.https://www.ebay.com/itm/1LED-illuminator-retrofit-Kit-with-dimmer-control-for-older-LEITZ-microscopes-/264744992515?pageci=a5ec3ccb-e15c-44ce-b48f-174d9082057f&epid=8020072071It uses a 10 watt led and I think 20k hz pwm.I have also a 10 watt led, and a cheap pwm controller which does only 6hz.I have a question regarding the eye health of using pwm dimming at 10 watts, through a microscope.I wouldn't be running it anywhere near full power, I have a less than 1 watt light in there now, and it is already far too bright, the 10 watts will excel at dark field and non transmitted illumination.But if I dim it via pwm, especially at 6k hz, will it damage my eyes more than if I were able to reduce the intensity via NOT pwm?What I mean is, during the ON part of the pwm cycle, isn't the LED Iinstantaneous power at 10 watts? So for brief periods, my eyes arw subject to a full instantaneous 10 watts of power concentrated on my retina?It will appear dim, but the instantaneous power is 10 watts, and isn't that too insane?Is this safe, I'm sure it will feel safe, and the brightness will appear low, but the instantaneous brightness will still be at the full 10 watts right?

author: StillTrying
author_rank: Super Contributor
timestamp: May 27, 2020, 10:39:35 pm
content: At a few kHz as long as the average is not tooo bright it will be OK for the eyes.I've looked at a 100W COB flashed at 800W, on for only 0.04% of the time it looked quite dim.

author: chris_leyson
author_rank: Super Contributor
timestamp: May 27, 2020, 11:09:29 pm
content: I wouldn't drive a LED directly with PWM because if the PWM frequency is too low, say < 2kHz, it can lead to eye strain. Seehttps://e2e.ti.com/blogs_/b/powerhouse/archive/2016/08/26/led-brightness-adjustment-high-frequency-pwm-dimmingConstant current buck LED drivers are a good way to go because the LED current consists of a DC component and a small triangular ripple current at the switching frequency. I've used an LM3404 in the past for a machine vision application where the LED current was either fixed or PWM modulated and synchronized to the camera frame rate, 50Hz. One advantage of using a buck topology is that the LED can be bypassed with a small capacitor significantly reducing the LED ripple current, see fig. 20 in the LM3404 data sheet for example. I managed to get a 300mA white LED ripple current down to 5mA or so just with a single capacitor. LM3404 is fixed current but if you want adjustable current and high frequency PWM then look at LM3409 for example.

author: wizard69
author_rank: Super Contributor
timestamp: May 27, 2020, 11:27:04 pm
content: I fear this thread will turn into a long argument over what it takes to damage one eyes with high brightness light sources. Frankly I'm not sure if anybody really knows for certain at what point damage will result. Most people assume that you need to do thermal damage which makes sense.The problem is how do you determine how much thermal impact does an optical system put on your visions system. In a nut shell you need to determine how hot of a point the optical system can create at the back of your eye. Of course you also need to know at what point the structures in the eye can be impacted.I suspect that there are real risks and there has been some research on this but honestly not a lot or at least it is hard to find publicly. The key here seems to be your subconscious mind that will force you to pull away or jump back from a high brightness light source.If you need to have a high dynamic range of brightness then you really should design human eyes out of the equation. This especially when you have a huge variety of camera systems to choose from. The last thing you would want is for somebody to accidentally turn the intensity to 10 while looking through the microscope. Even if there is no long lasting damage to the eye the physical reaction might cause other injuries.Quote from: dzarren on May 27, 2020, 09:13:41 pm

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 28, 2020, 03:24:58 am
content: Many and cheap LED drivers exist which can turn a micro controller PWM output into an adjustable DC current source with an inductor and diode.These are common for backlight of TV's and such to prevent any interference between the raster scanning of the LCD with the backlight.Also for making pictures or video recordings you want a relatively constant light source.As such IC's start at < 10ct there is no good reason to not use them.

author: mikerj
author_rank: Super Contributor
timestamp: May 28, 2020, 07:35:33 am
content: Quote from: dzarren on May 27, 2020, 09:13:41 pmRegardless of the peak/average power issue 10Watts is the electrical power into the LED, not the optical power output, and only a fraction of that output power is going to be collected and passed through the optics to your eye i.e. you are not going to get 10watts into your eye (which would certainly be damaging).

author: Jwillis
author_rank: Super Contributor
timestamp: May 28, 2020, 08:11:48 am
content: Here's a study done with rats if you are interested.http://aes.amegroups.com/article/view/4933Typically you wouldn't be staring directly into the light anyway with a microscope. As long as there is sufficient illumination to clearly see whats under the objective you should be fine. The Duration of each pulse is far to short to cause damage to the retina and 10W I think is around 1000 LUX which is about the same as overcast daylight . Direct sunlight is around 32,000 to 100,000 Lux .

author: tooki
author_rank: Super Contributor
timestamp: May 28, 2020, 06:28:28 pm
content: Quote from: dzarren on May 27, 2020, 09:13:41 pmIMHO, go higher than 6KHz. While that’s enough to be mostly (but not entirely!) flicker free, there’s a good chance it’ll cause your power supply to whine. Not harmful, but super annoying. Try to go for a frequency that’s inaudible to you (for most adults, 16-17KHz is the limit of our hearing). I made a 20KHz dimmer and it works great, causing neither whine nor any trace of flicker, even at very, very low duty cycles.

author: Raj
author_rank: Frequent Contributor
timestamp: May 31, 2020, 02:23:59 pm
content: Quote from: dzarren on May 27, 2020, 09:13:41 pmcurrent limiters ain't hard to make...try it.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2020, 04:47:00 pm
content: 6kHz is just fine eye-wise. Don't go much lower, though; when you use high-power LEDs at very low duty cycles, you need surprisingly high modulation frequencies, because the peaks are short and bright. Ignore advice that assumes that while a fluorescent tube flickering at 100Hz "is not problematic", you could do the same when PWM modulating an LED.While a small indicator LED would look perfectly non-flickery at, say, 100Hz 50% PWM, a high-power 10W LED at 1kHz 5% PWM may feel really strange.At 6kHz, you are clearly safe, though.There are other issues of modulating high power at high frequencies, though, related to audible noise, EMI, and potentially efficiency. Consider adjustable current-mode switching supplies instead.