Does ambient light/pupil dilation affect laser eye damage?

[julianthedragon]

Disclaimer: This question is not related to an incident that happened nor any activity I have planned, I'm just curious.

I'm sure everyone has noticed the influx of people coming to this forum to report some laser accident or asking for safety advice, and it got me mentally reviewing what I know about laser safety. I know that in the event of an incident, an opthomologist needs to check the eye(s) to see which parts were affected--cornea, lens, retina etc. And the strike location, angle, duration, laser power density and wavelength of laser will all contribute to the actual damage.

But a specific question I have is since the dilation of the pupil controls the amount of light let into the lens/retina, does that mean it focuses laser light differently in the dark vs bright sunlight? Assuming the wavelength of the laser in this scenario makes it past the cornea and into the pupil, is it "safer" to take a hit from the laser in broad daylight than in a pitch black room? Naturally I would think so since the aperture of the pupil would be smaller and thus let in less light, hence lasers appear much dimmer during daytime anyway. But I also think a smaller aperture could mean tighter focus, which increases power density. I'm sort of thinking with my photography background which involves similar optics

I'm really curious about this and please lmk if any threads already answer this. But if not hopefully someone with knowledge on lasers/optics/eyes can provide their take

 

[bostjan]

In general no.

The pupil blocks light by physically blocking that light. If the light is already focused into a point within the area of the pupil, the pupil will either contract and block the focused spot or not. If not, the same amount of light hits the retina as if the pupil were fully dilated.

 

[WizardG]

Rarely will the beam be, " already focused into a point within the area of the pupil". A far more likely scenario with our pointers is a strike from a beam (or reflection) that is infinity focused. If the beam is, for example, 6mm wide then a daylight strike when the pupil is contracted to a small size (let's call it 2mm) will deliver far less energy to the retina than a nighttime (pupil WFO) strike with the same beam.

 

[RA_pierce]

Here's a long-winded but hopefully somewhat thorough explanation:

Pupil dilation is a factor that can impact how severe laser damage can be.
Laser damage can occur due to direct and indirect exposure.
Pupil diameter is relevant when the light you are exposed to is distributed across an area that is larger than the pupil diameter (a beam that has diverged or from diffuse and specular reflections). Pupil dilation is relevant because it determines how much light can enter your eye.

I see why you are making the analogy with a camera lens. However, I think (I want to emphasize that this isn't something I've researched - I'm just applying my experience and education with optics and photographic equipment) the "tighter focus" of a narrow aperture lens is not going to increase damage potential.
When you stop down the aperture on a photographic lens, you do decrease the amount of light let through. However, what you describe as the focus getting "tighter" is that the depth of field increases. This means that images formed ahead of and beyond the precise focal point are less blurry. There is another optical phenomenon that photographers have to take into account, especially when narrow apertures are used - diffraction. Diffraction will result in a non-uniform pattern of light that results in "soft" images - essentially the intensity of the laser will vary across the beam cross-section.

Someone with more time and curiosity about this could model how the maximum intensity of a laser is affected by an aperture of varying diameters. My guess is that a narrower aperture will always result in lower power density (since the amount of light transmitted will always be lower) and that depth of field will have a negligible effect on power density at the focal point. This is assuming that the light source is spread over an area larger than the aperture.

Lastly, if you experience direct exposure to a laser where the beam is narrower than the pupil (e.g. at close range with a fully dilated pupil), the damage your eye sustains will likely be greater than with a contracted pupil because the exposure time will be greater. Imagine sweeping a laser across a surface - the larger that surface is, the more time that surface is exposed. The effect of depth of field is likely not important.

You can research depth of field outside the context of photography for a better understanding.

 

[julianthedragon]

Pierce thank you for sharing your knowledge and your intuition about the optics.

Someone with more time and curiosity about this could model how the maximum intensity of a laser is affected by an aperture of varying diameters.

I may or may not be that person with more time and curiosity...do you think an LPM, a variable aperture like this or this (would have to make sure it went up to a high f number), and a laser on a stand with beam diameter > the largest aperture I test would be enough to collect some meaningful results? This could be an interesting project to me at least lol

Also going back to the original question, when it comes to indirect damage like looking at the dot of a high powered laser on a white wall, is it true that a contracted pupil would be better here as well? As in, looking at this dot at the same distance for the same amount of time would have more potential to cause damage if it were at night than in broad daylight?

 

[WizardG]

Smaller pupil=less light entering the eye. So, yeah, observing the dot should be safer in daylight, all other factors being equal.