Friday, November 28, 2014

Night Vision

Mk 1 Human Eyes

“Don’t get a light that’s so bright it blinds you, kid,”  the elder said to me one night over beers.  “You want a light that saves your night vision, just enough to let you move through a dark basement at night.”

We learn from experience, but nobody says we can’t learn from other people’s experience.  That is the definition of wisdom and the foundation of an education system.  But that doesn’t mean we should take everything on face value either.

How bright should a tactical flashlight be?  One school of thought says as bright as possible.  You want, they claim, eye-searing, sunburn-causing, uber-bright light that make the VCA (violent criminal actor) blind and cringing in fear.  Faced with that hype, other manufactures with less bright lights have retreat to claims about run time, size and functions.  It’s all marketing.

I haven’t heard one manufacturer advertise a tactical light that preserves your night vision. 

Let’s for simplicity define a lamp as anything that produces visible light, no matter if it’s a bulb, diode or carbon arc.

Lumens are a measure of the total light output from your lamp.  It doesn’t matter if the light falls on your subject or goes sideways into space.  It’s total output.  This might be the best, simplest way of comparing flashlights.

Many lights are advertised in watts, an expression of the rate of energy flow/transfer/conversion.  The power company speaks of kilowatt hours, or the energy consumption per hour they have to produce.  The problem with watts is efficiency.  A lamp rated at 3 watts at 80% efficiency produces 2.4 watts of light.  But a second lamp rated at 4 watts but 40% efficiency only produces 1.6 watts of light.

Lumens are a better comparison value.

While lumens define total light output, it doesn’t describe the amount of light shined on a surface.  That moves us to footcandles and lux.  These are measures of the amount or density of light that falls on a surface.

tactical light night vision
There was a time when specifically constructed candles were used as a standard of the amount of light thrown on a concave curved surface one foot away.
Imagine each of the fundamental particles of light, the photon, is a gumdrop.  If you put 1000 gumdrops on a the cover of a phonebook, people would say that’s a lot of gumdrops, but take the same 1000 gun drops and spread them out on a football field.  Not so many gumdrops.

That’s why measurements of light on surface usually have a value of some specific area buried in the definitions.  The light on a surface is luminance and is measured in footcandles or lux.  I prefer footcandles.

Are we there yet?  Someone in the back row has an inquiring mind.

No, not by 700 rows of cherry trees.

Imagine two walls in the same room, one is black and the other is white.  Which wall will reflect more light back at you?  The white wall has higher reflectivity and more light will enter your eyes so you can see it better.  The black wall doesn’t reflect so great.

So is my drinking buddy still sounding good to you?  Frankly, I’m sure you could design a light that would illuminate a specific area and not disturb your night vision, but outside of that specific location and conditions, the lamp would be useless.

The eye is our sensory organ for vision.  This complex biological structure converts photons of a specific range of colors to electrical signals the brains interprets as vision.
Of all the parts of the eye (cornea, iris, optic nerve, fovea, blind spot, rods, sclera, cones and all the rest), I’m only interested in rod, cones and fovea.

Night vision
This is an SEM image of rods and cones from an eye.

Each rod and cone is sensitive to a single photon of light.  Pretty amazing isn’t it?  The major difference between night vision and day vision is in inhibition, convergence and how the rods and cones are hardwired to the human brain.  The amount of photosensitive pigment available is also an important factor.

Combination of these four factors determines which mode the Mk 1human eyeball operates in. The modes are photopic, mesopic and scotopic.  Let’s deal with the first and last.

Photo receptor
Sensitivity to single photon
Color perception
No color perception

We refer to photopic as day vision and scotopic as night vision.

One can’t help wonder if the test of night vision would be to take a color chart with you and if all the colors look gray you are either fully color blind or are in night vision mode.

When there is a lot of light available for photopic vision, the cones and rods create lateral inhibition by decreasing the sensitivity to light.  That’s an edge sharpening effect.  Since our vision is highly dependent on contrast, edge sharpening does the same thing your camera app does.  It throws away light to make the contrast between objects sharper.  Since there’s plenty of light available, who cares about wasted light?

As the amount of light decreases the eye/brain decreases inhibition and begins convergence.  Convergence causes the rods and cones to sum their signal to the brain instead of reporting individual signals.  The immediate effect is loss of visual sharpness.

As the amount of light decreases, color perception changes as your cone contribute less to the image.  Cones are tightly packed at the center of the eye’s fovea, but both rods and cones have a distribution though out the eye.  Rods dominate the peripheral of the eye.  You’ve experienced that effect when an invisible object become noticeable seen at the edge of our field of vision.   When the cones no longer function the fovea becomes a blind spot and no image can be formed there.  A practical concern for laser safety officers is preventing laser damage to the fovea which would drastically reduce your daylight vision and could make you permanently and legally blind.

We often equate scotopic rod vision with night vision.  That isn’t true, especially in urban environments where there is enough light to prevent true scotopic vision.  Our eye’s work in these cases in a mixed mode of rod and cone vision called mesopic vision.  It’s complicated.  Too complicated for this walk in the woods. 

Table one shows levels of illumination expressed in candles per meter squared and typical sources as well as transition levels between visual modes.

Table one, in footcandles per square meter

So what’s a lumen in units of Cd/m2?  Beats me.  You need to know too many other variables but the thing to note is the transition between photopic and mesopic is around 3cd/m2 or a little past sunset.  Surely your lamp throws more light than that.  

Each lamp has a different angle in which it spills light.  That angle will affect how much light will fall on a square inch at any distance.  Smaller angle more light, larger angle, not so much light

True total night vision requires less light than a moonless night in the lampless woods.  Do you really want to use that level of illumination, and all the limitations of poor resolution, low contrast and no color perception when you need to determine if that’s a phone, wallet or semi-auto in his hand?

So no, I think my drinking buddy has had too much to drink on that subject.  I suspect you want to operate in low levels of photopic or upper levels of mesopic vision mode.  But it isn’t all about light levels; it’s also about transitioning between levels.

Mythbusters did a program on why pirates wore a patch over one eye.  Their answer was that one eye would be dark adapted when they go below deck into the darkness of the ship’s hull.  I think the loss of stereoscopic vision on a moving deck would be a poor trade for one dark adapted eye.

Dark adapting.
Rods and cones contain light sensitive chemicals called photopigments.  There are three basic types which give us the range of color sensitivity we enjoy.  Light changes or degrades these molecules which produces the electrical signal the brain needs.  It’s the speed that these pigments regenerate which effects how fast we dark adapt.

The rate at which rods and cones regenerate is different.  Cones (color vision) regenerate faster than rods.  In absolute darkness, cones take about 5-7 minutes where as rods take 30-45 minutes to reach in normal eyes, full dark adaption.  Cones however do not have the same level of sensitivity as rods.  Rod will take longer, but reach a higher level of sensitivity of dark adaption.

Roughly speaking, depending on pre-dark adaption, it could take 30 minutes to reach 80% of full and complete dark adaption.  Of course the eye is expanding the size of the pupil to accept more light and the cones and rods are converging to produce a stronger signal.  80% isn’t bad.

Let’s look at the other side, going from dark to light.  All that adaption can be lost in a second of exposure to bright light.  Exposure to the illumination level of a neon lamp is sufficient to lose total dark adaption and suffer flash blindness. 

Flash blindness is visual impairment during and following exposure to a light flash of extremely high intensity.

Want to weaponize your flashlight?  You just need a light as bright as white paper in a good reading light. When you shine it into someone’s eyes your target will be blinded.  Turn it off and if the general illumination level would support lower mescopic or the upper part of true night vision or scotopic, your target will remain night blinded.  

But you’ve got to get it in their eyes, in both eyes and remember the level they are exposed to will influence how long the effect will last.  Even so, gaining 2 seconds on a VCA is a tremendous tactical advantage.  So use a little more light and get more flash blindness and time.  Flash blindness, however, isn’t a paralytic condition that immobilizes the exposed person
Also note the light is being reflected back at you, so the more you use, the more your dark adapted mesopic or scotoptic vision will suffer. 

Did you catch that part about pre-dark adaption?  Simply put, spend a day in full sun on a sandy white beach and it will take longer for your eyes to dark adapt when you walk out of the sun and into a dark basement as if you spent the day inside with the shades drawn.  

night vision
This officer is wearing both a brimed cap and very dark glasses.  Her back ground suggest she works in a very bright area.
You can improve the process by wearing, neutral gray sunglasses with side shields that decrease all wavelengths to a 15% transmission level.  Wearing a cap with a brim also helps.

Tactical considerations
Pre-dark adapt whenever possible. 

Accept that in the city and many rural areas you will never be fully dark adapted, so going from yard lit with a security light to an unlit basement will be a problem.  If possible, get out of the doorway so you’re not backlit and move to an area of relative safety to allow your eyes to reach a better level of adaption.

A weapon light with two levels has some advantages in different situations, very bright for distances in the open or for causing flash blindness and a lower level for maneuvering indoors and identifying people and objects.  You must master these controls at the unconscious competency level.

Remember flash blindness is a two way street.  Light decreases by the inverse square of the distance (double the distance and an object receives only a quarter of the light it would receive if it was at the original distance).  So the light in the VCA eyes will seem 4 times as bright as it does to your eyes.  In most cases it will still be bright enough to alter your level of dark adaption.  Pick you light with at in mind.

At night or in a darkened room a very bright, focused light illuminating a 4 foot circle on wall will create very dark, concealing shadows two feet past the edge.  Your vision, no longer dark adapted, will be photopic from the ultra bright light reflecting back into your eyes.  You will not be able to see well into the inky dark shadows further down the wall or in corners of the room.  Think of this as a form of tunnel vision and correct for it by:

  • Expanding (refocusing) the area illuminated to see more,
  • Selecting a less brilliant light,
  • Working with a partner who has the responsibility to check the shadow.

The best way to select your flashlight is to try one or three.  Get a buddy and find out what the light does at to him at difference distances.  Let him flash blind you with it and find out what you can see and for how long.  Run the experiment on him and find out what happens to your vision.

We buy flashlights to see in the darkness.  One can only imagine the influence nightfall had on early man as familiar objects disappeared outside of the limited area of safety a campfire produces.  Learn to control your flashlight and you can rule the night.

Personal note:  I’ve been running a few experiments about light and vision.  When I’m truly dark adapted I can’t see colors.  Some fade into the ones next to them and are indistinct.  Others appear to be different levels of gray.  My visual acuity under these conditions stinks.   I’m not sure I could identify a threat hiding from me with scotopic vision only.  I need more light.

When I use my 150 lumen light and I’m truly seeing only grays,a flash of illumination to check a tight corner by a wall in my basement creates enough reflected light to blind me and makes my eyes spasm in pain as the dilated pupils slam closed to reduce light.  

I’m momentary blinded while my eyes rapidly move from scotopic to photopic vision.  During this instant I am truly blinded.  While this occurs and is actually worse for the VCA, he only has to unleash a barrage of un-aimed fire at the light and hope one of the rounds connects.  As the law abiding citizen (cops, too!) we don’t have that option. We must identify the threat first.

I suggest you don’t waste your time waiting for scotopic or true night vision.  It’s too easy to lose and takes too long to recover.  Try to stay in mescopic vision. It’s a wide range of vision, fast to recover and gives combination of color and resolution you need.  

No comments:

Post a Comment