"WOW! This is, like...*freaky-clear*!"

[Pete Hanlin]

Unfortunately, a 10% yellow filter does reduce contrast. By explanation, let's assume we have a background with very low luminance (e.g., 3 lumens) and are trying to see a child wearing a shirt that is reflecting 30 lumens from your headlights.

A 10% tint will reduce the background luminence by 0.3 lumens (now the ambient luminence is now 2.7 lumens). However, the luminence of the child's shirt will now be 27 lumens. The difference in luminence without the lens is 27 lumens, but the difference with the 10% yellow lens is 24.3... By reducing the difference in luminence between the background and the shirt, you are reducing contrast and therefore the patient's ability to see the child.

If its my child wearing the shirt, I'd rather you weren't wearing the 10% tint! :)

[Barry Santini]

That was a good example Pete. So if we accept the *contrast* trade-off with Yellow 10% lenses, perhaps that's a fair trade to reduce the *dazzle* which is what most people are complaining about with respect to headlight glare.

FWIW

Barry

[drk]

Pete, the formula you are applying to the situation is a simple ratio; a comparison between target and background luminance.

I think the more appropriate formula is Weber's Law where the DIFFERENCE IN TARGET/BACKGROUND LUMINANCE is compared to the background luminance:

C w = (I - I b)/ I b

C w is Weber Contrast
I is intensity of the object
I b is intensity of the background


Run this formula for I = 30, I b = 3 and see what the contrast is.

Then reduce each term by 10 or 20% and see that the contrast is unchanged.

For an intuitive feel on this, consider how large a difference between object and background is needed in bright surroundings vs. the minimal difference in luminance needed for visibility in low illumination. An airplane's running lights must be blazingly brighter than the sky in the day, but a star with minimal brightness difference over the dark sky's background is equally visible.

This better formula speaks to increase in sensitivity of the eye in low illumination.

It also speaks to the classic Weber-Fechner Law, which is used in perception, stating that a one-pound weight added to your 12 lb bowling ball triggers a perception of a bigger weight change than a one-pound addition to the 235 lb. barbell that I'm sure, Pete, you regularly bench.

This link is interesting:
http://www.aoa.org/x5352.xml

[Pete Hanlin]

drk,
Good catch- Weber's Law is used to determine the difference threshold required for an observer to tell the difference between two objects. So visually, if someone requires a difference of 5% in luminance to discern a difference between two lights, s/he will require the same 5% difference regardless of luminance.

However, Weber's Law has limitations- specifically if you have very small values. True, a 10% tint will not reduce the difference threshold in medium or bright lighting conditions. However, given very low levels of light (like those encountered by seniors at night), contrast perception will be impacted. You can demonstrate the same thing with two lines. Let's say an observer has a difference threshold of 10% to tell the difference between the length of two lines. Weber's Law states the subject would begin to tell a difference in line length at 10% regardless of line length (within reason). However, at 1mm and 1.1mm, the subject may be unable to tell a difference. Same holds true for levels of contrast.

Heck, if contrast remains constant regardless of tint, why not wear N-15 sunglasses for night driving (that should eliminate dazzle :^)? In the example above, the shirt will be harder to see with a tinted lens.

Next time you're in a CE course at a hotel, check out the wallpaper (many hotel meeting rooms have textured vinyl wallpaper). It's pretty easy to see the texture in the wallpaper when the room is brightly lit- but when the lighting is turned down for the presentation, it becomes harder if not impossible to see the texture. Try wearing tinted lenses and it becomes harder still...

As luck would have it, I'm traveling right now to do a quarterly visit with our independent clinical site. I'll make a point of running our debate by the doctors at the site to get their input (we've done some contrast testing between regular tinted lenses and polarized lenses- but that's a different problem... veiling polarized glare).

[Pete Hanlin]

The link you provided summarizes things really well (and reminds me that one of the best things you can do for a senior who drives at night is to recommend sunglass use during the day- while researching glare for a Transitions paper, I seem to remember that night blindness can be caused by "bleaching" of the retinal rhodopsin levels during the day- but I could be remembering incorrectly)...

In the night vision section, this paragraph is under "contrast sensitivity."
Objects can be seen at night only if they are either lighter or darker than their background and can be discriminated by subtle differences in contrast. Because VA at night is a function of small differences in the brightness (luminance contrast) between objects and their background, any transparent medium through which the flyer must look should be kept spotlessly clean. Contrast discrimination may be reduced by light reflected from windshields, visors, spectacles, fog, or haze.
I would interpret the final sentence as a recommendation against tinted lenses during night vision (if reflection from spectacles- which lowers transmitted light reduces contrast, I imagine a tint would as well).

[Barry Santini]

But aren't we really to reduce contrast for glare-complaining seniors at night? Their persception is that the dazzle from the headlight glare overwhelms them. So aren't we really seeking, at this for this goal, to reduce the *difference*, i.e., contrast between the headlight and the background?

Sure, using tints at nite may further impact low-level CSF. But this may be a small trade off to help those whose are really debilitated by on-coming heaadlight glare.

Barry

[Pete Hanlin]

Well, I suppose it comes down to a decision, then. Obviously, my take is a tint solution reduces vision constantly in an effort to provide intermittent relief during glare. Seems to me a safer solution is to look at the white line at the edge of the road during oncoming glare conditions (this is the recommendation of the NHSTA as well, I believe).

At the very least the claim (i.e. that yellow tints increase contrast at night) is incorrect.

Good discussion.

[bob_f_aboc]

Am I missing something here? If we are talking about seniors complaining of 'dazzle' from headlights, wouldn't the majority of them have some form of cataract in place? With a cataract, contrast is reduced, glare is increased, and overall light entering the eye is reduced.

Why would anyone want to further reduce the amount of light entering the eye of a person whose vision is already impaired?

I'm coming into this one late. My 2 cents worth.

[DoubleD]

I am very interested in this conversation, as it directly involves most of my patients. I am NOT an advocate of cutting any type of transmission for nite driving, but if glasses must be worn at nite, I really push AR for better quality vision. Most Docs I beleive do not follow this train of thought.
Yes there are many cataracts forming out there, nut my experience shows majority of issues related more to macular degeneration, and yes daytime bleaching of pigments. School of thought suggests elimination of blue- highly scattered light, but this really does not help much in low light for contrast.
It's not just the oncoming lights, but as much the newer designs in highway/road signs with increased reflectivity.
I beleive the scattered light is more the issue.
Denny

[drk]

[Useful definitions:
Discomfort glare: what you get when you stare into the sun or a super bright light. It hurts. (Biggest source of patient complaints night driving IMO.) Contrast is not an issue as discussed above.

Disability glare: veiling luminance that occurs from reflections, scatter, etc. (Glasses issue, ocular media issue, dirty windshield issue). Contrast sensitivity is an issue here.]



What we really have, Barry, is a different task than what you're solving.

You are limiting the discussion to a white headlight on a black background (not unlike astronomy), and YES, technically, you can reduce the discomfort glare with a tint. BUT...

...an analysis of the rest of the situation reminds us that a motorist with discomfort glare ALSO IS USING THEIR OWN HEADLIGHTS to illuminate the road surface and objects. By wearing a tint to reduce the comfort glare, they reduce their visibility to see everything BUT the glare source.

You might as well have them reduce the efficiency of their headlights 10%, you know?

[Robert Martellaro]

I have 3 to 4 patients year (mostly hyperopes) who say they are blinded or bothered by oncoming headlights. One woman who wanted tint for night driving told me she was closing her eyes with on coming cars.

I found one study that discusses this issue.

http://www.pubmedcentral.nih.gov/art...?artid=1771460

The best advice I've seen is to look to the white line on the side of the road when faced with oncoming headlights.

That's what I recommend. You would be surprised how many people look right into the light. Kind of like a moth flying into the flame. Another trick is to close one eye. This assures at least one functioning eye after the exposure.

[drk]

Most people complain about "blinding" headlights (which conjures practitioner mental images of transient adaptation effects from photoreceptor bleaching) but what they are really moaning about is discomfort glare. I guess closing one eye could work for transient adaptation problems.

Their dark adapted eyes hurt when a light blasts them, really. This can only be helped by having them avert their eyes, as Pete & Robert mentioned.

[Barry Santini]

[Useful definitions:
Discomfort glare: what you get when you stare into the sun or a super bright light. It hurts. (Biggest source of patient complaints night driving IMO.) Contrast is not an issue as discussed above.

Disability glare: veiling luminance that occurs from reflections, scatter, etc. (Glasses issue, ocular media issue, dirty windshield issue). Contrast sensitivity is an issue here.]



What we really have, Barry, is a different task than what you're solving.

You are limiting the discussion to a white headlight on a black background (not unlike astronomy), and YES, technically, you can reduce the discomfort glare with a tint. BUT...

...an analysis of the rest of the situation reminds us that a motorist with discomfort glare ALSO IS USING THEIR OWN HEADLIGHTS to illuminate the road surface and objects. By wearing a tint to reduce the comfort glare, they reduce their visibility to see everything BUT the glare source.

You might as well have them reduce the efficiency of their headlights 10%, you know?

Good points as always, drk! And I understand the distinctions you're making.

For me, allowing acuity thresholds of 20/40 *binocularly* (less monocularly) for most state DMV's means many are alerady losing contrast on the roadway at night. Yet, we accept this today as OK. So, when complaints of headlight glare cause people to squint or avert their eyes from the roadway, I'm concerned and think that this deserves a priority addressment in this context.

I see your points that I was unaware of till now.

Barry

[drk]

Your threads are the best, Barry.

[Uilleann]

One of the biggest things I believe we're trying to do from the dispensing side is to maintain a clean focus all the way to the fovea. We , unfortunately, are any able to control one aspect of the light path - the glasses (or contacts in some cases, but we'll stick with glasses for the context of this particular discussion.)

So what is the best way to maintain as close to a perfect focus as possible? Digital/free form lenses? A/R lenses? Tints?

As we work to minimize every possible form of light scatter and lens inefficiency, how are we best able to achieve the end goal? Of course, there never will be a single "magic bullet" cure for every eye out there, and every lighting condition, but it does become increasingly difficult with our geriatric patient bases.

The docs I never get are the one's who want both a tint, and A/R. Kindergarten logic suggests (to me at least) that these two cancel the other's benefit and end up a large waste of money, and a very likely source of frustration for the patient.

In the case of night driving, if a bright headlight is kept as focused and small on the retina, the chance for dazzle or temporary saturation and blindness due to glare would seem to be dramatically reduced - as opposed to an out of focus "fuzzball" of bright dazzling light, if you'll pardon the non-technical description.

Working at the local planetarium here for some years, I remember in designing our laser shows that even though the points of laser light were often very bright, they were also very focused. This allowed us a very high contrast ratio against the dark areas of the dome, and an incredible amount of detail in our imaging. We could control the final brightness of the beams of course, but found that below a certain intensity threshold, we lost detail - rapidly. Very unscientific in this case I understand, but I would think it may be somewhat similar to effects we commonly see in night driving.

As to dark adaption, When driving at night, it really is not possible - or certainly at least highly unlikely that the human eye would ever become dark adapted. At least, not in the way that you truly would in an astronomical/caving situation. Your own headlights prevent it, to say nothing of the car's interior illumination, and other exterior light sources. That being said, oncoming headlights can be uncomfortable at best - even to young eyes. And I would second the suggestion to follow the white shoulder line on the side of the road in those instances. Of course, you'll always get that one Jackhole who loves to flash his brights at you just to be a jerk!

This is a great thread, would love to hear more thoughts on the subject of final resolution and focus, and how we can best achieve them.

Cheers! :cheers:
Brian~

[Barry Santini]

Thanks drk. All I wish to do is to provoke a constructive, intelligent discussion in a global fashion about information we may already take for granted.

Change (in thinking) is good, IMHO.

Barry

[Pete Hanlin]

I spoke with the doctors at our clinical site today (who have done numerous studies on contrast). They were pretty adamant that a clear lens is optimal for night vision.

I propose we start another thread on the subject of night driving- it probably deserves its own thread with a title referencing night driving. Also, there are a few studies I'm going to look up (one performed in Sweden is supposed to have investigated into this at some depth, and I'm sure I can find a couple at the NHTSA site).

Great thread!

[Barry Santini]

I spoke with the doctors at our clinical site today (who have done numerous studies on contrast). They were pretty adamant that a clear lens is optimal for night vision!

For the reasons stated above, I respectfully disagree. "optimal for night driving" may not be inclusive or properly weighted with regard to Seniors complaints of headlight glare.

Some subtlety: Are most of the roadway that the complainant drive well-illuminated, or not? In both cases, the *dazzle* will remain. In the case of good roadway lights, the use of a a tint may have less of an effective trade-off on decreased CSF that unlit roads.

Something to think about. And again, while you're thinking, ask yourself why, especially for night driving, 20/40 (or worse, since DMV's don't screen acuity in low luminance levels) snellen acuity is "OK". I'd like to see studies about the functional vision and CSF of individuals driving at "night" with 20/40 vision.

Disclosure: This topic of driving and what we traditionally accept as OK acuity thresholds is a pet project of mine.

Barry

[drk]

Barry, you need a few more disclaimers than that.

First of all, this is a legal matter, not a matter of professional opinion. There are rules and regulations for state DMVs, and any optician should be well-versed in them before making any recommendations.

What's more, there is no reason that I know of that an elderly patient would do better with a tint night driving. I challenge you to think of one.

Respectfully!

[Barry Santini]

drk, I really wasn't trying to make any recommendations in my last post, but rather speculating on the more complex mix of factors that "glare, night driving and contrast" could include.

And in my mind, legal never defines best or optimal. But, from a legal perspective, if society grants those who have borderline 20/40 acuity, tested under well lit conditions, to drive at night, it begs us to question whether broad brushstroke statements like "all tints decrease vision at night" are inconsistent with what we societally endorse as acceptable sight through the eyes of a legally-sufficient 20/40-corrected eyeglass-wearing driver wearing "untinted" (and perhaps scratched and non-AR coated) lenses.

Barry

[Barry Santini]

One possible reason to wear a mild tint to offset headlight glare: If it prevents a senior citizen from averting their vision from the glare (and therefore the roadway!), then I might be inclined to suggest an clinical investigation in this context.

Barry

[drk]

Barry, you are without a doubt the most genteel poster on this board, and the most creative thinker. Please don't misconstrue my back-and-forth as any sign of disrespect.

We must have perspective on vision standards for drivers' licenses. First of all we must come at it correctly philosophically: the state should restrain rights to drive (non-commercially) only for the protection of others.

Yes, there is no constitutional right to operate a motor vehicle, but in the spirit of liberty laws should be as liberal as possible. I'm sure you have no argument there.

Secondly, I want to restate that we have to follow the regulations as they are written, whether we wish to do an exploratory surgery on them or not. That is critical for your and my profession to have in the forefront of our minds: follow the laws whether we agree with them or not.

The vision standard in my state is 20/40 for unrestricted for binocular patients. It is tested relatively uniformly with a specific vision screener (Titmus, I believe).

There is no glare standard. It is conceivable that they could have a glare acuity test incorporated, but I do think that glare testing is less repeatable/reliable and that's why it's not done. However seniors are among the most responsible of citizens and tend to self limit. It's a good thing teenagers don't get media opacities.

Bottom line: practicality is a factor.



Our role should be to:

1. emphasize vision loss prevention
2. maximize (recognition and contrast) visual acuity with best possible optics.

As to "rehabilitative vision" exercises such as fiddling with tints for night driving, I humbly think we are on shaky scientific and legal grounds.

[Barry Santini]

Gosh, I'm always glad to get my daily Optiboard *fix*, especially when I read your contributions, drk.:cheers:

I learn. I think. I change my mind. I hope others will be open minded as well.

Barry

[drk]

I learn. I think. I change my mind. I hope others will be open minded as well.

Barry

We should all be as humble and honest as this.

[uberoptiker]

One thing that everyone is missing (and is unfortunately beyond our control as eye car professionals) is the offender, the headlights. I invite all of you to do a search for Daniel Stern Lighting. He has written some wonderful articles on the absurdity of the design standards DOT headlights must conform to. E-code headlights are far superior not just for proper illumination, but reducing the glare oncomming drivers experiance. Also, one of the worst things France did as the E.U. sought to standardize regulations was get rid of the yellow (iodine) headlights and streetlights. That is explained on Mr Stern's website as well. As far as yellow lenses at night, they give me a headache, but they seemed to work O.K. at LeMans. Then again, Peugeot (the team that wore them) was beaten by Audi (again).
Cheers!