Can bright LEDs cause eye damage even if they are dimmed with PWM?

Question

I have an extremely bright LED, so bright that I wouldn't want to look at it when it's at full brightness. I am dimming it with PWM (pulse-width modulation) down to 1/256 of its original brightness. At a 1/256 duty cycle, the LED appears reasonably dim. (Still quite visible, but not blindingly bright.)

My question is this: since the LED is actually sending out super-bright pulses 1/256 of the time, can these bright pulses hurt the eye more than a hypothetical LED which was on constantly and was 1/256 as bright?

I'm using the TLC5947 LED driver, so if my calculations are correct, the frequency of the PWM is approximately 1 kHz. (The chip's internal clock is 4 MHz, and one PWM cycle is 4096 internal clock cycles long.)

The LED I'm driving is this RGB 7-segment display. The brightness of each segment is 244 mcd for red, 552 mcd for green, and 100 mcd for blue. So with all 7 segments illuminated, it would be 7 times that.

Answer 1

It is permissible within certain limits. The best place to look is probably the associated IEC standards (IEC 60285 Laser Safety and IEC 62471 Lamp Safety), which are generally internationally recognized as best practice. Unfortunately I can't post excerpts of them here since they're copyrighted.

Choosing which standard to apply depends on how the LED is used. OSRAM has a very comprehensive appnote describing how these standards apply to infrared LEDs and how to calculate permissible exposure.

Your particular case focuses on pulsed light. In general, PWM'd light is weighted against its averaged value, so long as the individual pulses do not exceed an irradiance limit (given by a chart in the standard of pulse length vs. irradiance). This is all outlined in the OSRAM appnote, although since you're in the visible range you'll have to refer back to the source standards to see what the particular limits are for your wavelengths.

Edit: Found another appnote which may be useful to you - OSRAM has an appnote on 62471 as a whole, not just IR.

The best place of course to look is the standard itself, but it costs about $250. If this is a product you're designing, that's probably worth it, but if this is just a hobby project I would scavenge information based on appnotes.

Answer 2

Firstly, a disclaimer: I am not a medical professional, nor do I have any professional expertise in the area of ophthalmology. I'll try to leverage my understanding of failure mechanisms in sensitive sensor systems and some outside sources to venture an educated guess:

According to this summary from an ophthalmology journal, mechanisms of damage to the eye can be categorized as photothermal, photomechanical, and photochemical. For each mechanism, we should ask what the relevant time constants are in order to understand whether the risk for eye damage would be correlated with the peak (on) brightness or the brightness as you see it, averaged over e.g. a PWM cycle.

Photothermal - this occurs when the temperature of the retina is raised by incident electromagnetic energy. The thermal time constant of the retena is likely to be on the order of seconds (my guess, based on scale and thermal conductivity of biological tissue), so that average and not peak radiance would correlate to damage. At any rate, photothermal damage is observed in exposure to very high irradiance level (e.g. lasers) and not a likely risk with even the brightest incoherent LED.

Photomechanical - this occurs when compressive or tensile forces generated by incident energy cause mechanical damage to sensitive optical structures. If these type of stresses can arise on a very small mechanical scale, there might be some concern that the relevant time constant could be below the PWM period of your LED. However, you can probably rest easy, since the article associates this damage mechanism with irradiance in the range of terrawatts per cm^2.

Photochemical - this is the most common type of retinal damage, associated with e.g. looking at the sun. The chemical mechanism is ultimately oxidative - electrons in chromophores get excited by incoming light energy and can occasionally generate free radicals which go on to damage a variety of sensitive tissues. In another summary article here, a discussion of retinopathy caused by viewing a microscope or opthalmoscope with irradiance of ~1W/cm^2 provides some relevant numbers and references. At this level, damage is indicated on time scales in minutes to hours. To me, this suggests that the relevant biochemical processes are much slower than a PWM cycle.

As a final thought exercise, consider that many humans routinely glance at the sun for probably hundreds of miliseconds without suffering solar retinopathy. It is only when people resist the biological impulse to look away and hold their gaze for seconds or more (because they are checking out an eclipse, for example) that damage occurs.

Answer 3

No. A 500 mcd LED with a typical 120° view angle is about 1 Lumen.
So max 7 lumens.

There is no chance 550mcd x 7 will damage the eyes.

1 lumen of 523nm green = 2mW/m²sr irradiance or 14mW for all 7 segments

If you look at the table on page 2 in the OSRAM PDF the minimum figure is 100 Watts. That is 7143 times greater than your 14mW.

On page 9 of the PDF it says (keeping in mind their high power LEDs go into hundreds of lumens):

A basic assessment of the high-power LEDs currently available
from OSRAM Opto Semiconductors in accordance with the IEC 62471 standard reveals that single LEDs as currently available in the colors green, yellow, orange, red and hyper-red always fall into Risk Group 0

There is consequently no need at the moment for individual, design-specific safety assessment of LEDs in this range of the spectrum (510nm ≤ wavelength ≤ 660nm) based on existing semiconductor technology.

I work with strips of LEDs that output 100-1000 lumens. The only danger is walking around with spots in my vision.

Answer 4

There's actually a lot of research into this. First, every bit as important as the luminance is the wavelength. Blue light is potentially very damaging to the eye, whereas red light isn't particularly dangerous at all. White light contains most of the visible spectrum, so it should be treated like blue light from a safety perspective. From there, you do need to know the luminous intensity, measured in candelas or millicandellas, and the luminance, measured in candelas per square meter. It's a really tricky subject and hard to find good info on. However, the reality is that it takes a lot to damage the eye from visible light. The sun has luminance of something like 10^9cd/m^2, which can cause retinal damage in less than a second, and the flash of an arc welder is an order of magnitude lower at least, with damage occurring in less than a second to several seconds.

it's highly doubtful the LED you're using can cause permanent eye damage, but the best you can do to be safe is to compare the datasheet numbers to other sources that are known to cause damage and extrapolate an allowable exposure time from there.

As far as your comment about PWM, you've got it backwards: 100% duty cycle is full-on (constant current) and therefore the maximum brightness. Anything less than 100% means that some of the time it will be off, and therefore the brightness will be a function of the on/off ratio. It's no different than the calculation of the DC average current, which is based on the ratio of the on/off time ratio. This all applies to the actual switching frequency though, because you have energy storage effects that may cause the LED to conduct longer than the on-time. Regardless, the total brightness must be something less than the brightness when the LED is on continuously.