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Light transmission question |
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scjason
Optics GrassHopper Joined: October/13/2004 Location: United States Status: Offline Points: 2 |
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Posted: October/20/2004 at 12:39 |
Question for the experts: What is the difference between TOTAL light transmission and light transmission PER LENS SURFACE? I have been doing a lot of research in manufacturer's websites recently and see this pop up a lot.
For example, Leupold claims a 99.65% light transmission per lens surface on the LPS, and up to 98% total light transmission on the VXIII. Burris claims their HiLume multi-coating gives 99.5% light transmission per lens surface and 95% total light transmission on the Fullfield II. Burris also 'calculates' the total light transmission of the VXI and VXII on their website to be in the mid to upper 80s.
I am guessing light transmission per lens surface is a factor of the lens (multi) coatings and total light transmission is a factor of the glass quality and scope design. Am I right, or do I have this backwards?
thanks in advance, Jason |
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Dale Clifford
Optics Jedi Knight Joined: July/04/2004 Location: United States Status: Offline Points: 5087 |
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Total light is the sum of surfaces. Some companies take an average. Some take as a percentage contribution which give a lower figure. Some companies measure the amount using spectrographs. (Schmidt and Bender, Zeiss, probably Leo). This property is much more important in microscopes than binoculars or rifle scopes and makes for good sales talk. Your are correct about total light transmission being a factor of the glass and quality, however nothing is said by any one that only certain wavelengths are measured by virtually any manufacturer because they change with wavelength (what makes prisms work). If one manu. says they have x amount on their lower line of scopes and x+1 in the next line, it probably has some comparison value. To compare accross the board is almost meaningless.
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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A couple of comments
To get total light transmission you multiply the transmission coefficient of one surface by itself as many times as there are surfaces in the scope . For example, 99.65% for one surface means 0.9965 of incoming light is passed through one surface. For two surfaces it is 0.9965x0.9965=0.993 (or 99.3%) I do not know how many surfaces a LPS scope has, but for 10 surfaces total light transmission is 0.9655 or 96.55%. Interestingly, if each surface only transmits 99.65% to get a total light transmission of 98% you can't have more than 6 optical surfaces, i.e. no more than three lenses. I do not know exactly how many lenses a scope typically has, but it is certainly more than three. Fixed scopes have fewer lenses than variable ones. That's why all other things being equal a fixed magnification scope will be brighter. Light transmission does depend heavily on the wavelength. For a narrow wavelength range it is quite feasible to get a multicoating with transmission of 99.99% or slightly better (I've worked with these in telecom industry and there are several companies that can readily supply superefficient narrow band coatings). Leupold's claim of 98% total light transmission has to be for a particular wavelength (color) of light.
Burris website has a cutaway picture of a scope that shows 5 lenses. For 10 optical surfaces at 99.5% each, total light transmission is 95.1% which matches their statement. We still do not know which wavelength range they are measuring.
Total light transmission is just a function of the light transmission of individual surfaces. The catch is that a scope can be very bright, but have poor resolution. Resolution depends heavily on coatings as well as on quality and polish of the lenses. The surface finish of the lens is typically measured in a test called "scratch and dig" which basically looks at scratches and nonuniformities of the surface. Another useful test is an interferometric measurement called "wavefront transmission" it looks at how long it took each wavelength to go through a lens (light travels at different speed through different materials). Nonuniformities in lens material, surface finish and coating quality can all result in poor wavefront transmission performance which correlates directly to poor resolution.
A good example here is with Burris Signature scopes vs. Fullfield II scopes. They have the same coatings, hence total light transmission is going to be roughly the same for the same magnification range. However, lenses in Signature line scopes are ground and polished to stricter tolerances resulting in better resolution than Fullfield II scopes.
I think I saw some tests somewhere that said that Burris Fullfield II fixed 6x scope is easily among the brightest in the world easily holding its own against expensive Euro scopes. However, that does not mean that it's the best scope since total light transmission does not garantee top resolution.
I think I'll stop here. This post has been a bit too long anyway.
Ilya Edited by koshkin |
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Roy Finn
MODERATOR Steiner Junkie Joined: April/05/2004 Location: United States Status: Offline Points: 4856 |
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Koshkin, that particular wavelength ( color ) would be the green spectrum which is the peak of what the human eye can capture. This relates to what is referred to as the cones or rods in the human eye that are the most sensitive to. This falls between 477-570 or so nM. It would only make sense for a scope/binocular manufacturer to use this measurement of color as it would be to their own benefit for advertising stats.
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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You are probably correct, Roy. Green is right in the center of
the visible range, and I would expect them to maximize the transmission
there. It does not have to be that way though. A broadband
coating can maximize transmission anyywhere in its spectrum. That just
depends on the recipe. For example, if memory serves me
right, Zeiss claims that their German built scopes have a maximum
transmission at the blue end of the visible spectrum. Steiner
Predator binoculars (which you own, I think) maximize the transmission
of their broadband coatings on the red/brown end.
Ilya |
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Chris Farris
TEAM SWFA - Admin swfa.com Joined: October/01/2003 Location: Texas Status: Offline Points: 8024 |
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Don't know why Steve Optics PM'ed this to me instead of posting it...but here it is.
Hey Chris,
Could you please forward this message to:SCJason, Dale Clifford,Koshkin and Roy Finn
Hey scjason,
The answer to your question is actually quite complex, I do not have the time as I am a very slow typer to answer it correctly, so here is a simple version.
Dale, Kos and Roy, all make great points and bring up many important issues when trying to answer actual light transmission in bino's and scopes.
Actually though, when Burris, Leupold, Nikon ect. advertise "Light Transmission" they are only referring to "white Light !"
Fact, as light passes through an optical lens, coated or non coated, all shades of all colors in the spectrum bend. That is all except the "white light" These companies are advertising how much white light transmission the have or actually they are trying to tell you how little reflection and refraction they have.
As Koshkin points out, this has very little if anything to do with Resolution and Contrast.
Resolution and contrast have everything to do with how you see the object you are looking at through the instrument !!
To stop by the store and compare the Simmons $20.00 scope to the $1000.00 scopes you will quickly assume the Simmons to be brighter by looking across the store at a well lighted display. However, to compare them at dusk outside looking at a dark object with no artifical light shining on it you would quickly realize why some oprics sell for thousands.
Not only do all shades of colors bend, the real confusing is in the fact that the only way to get the colors directed into the same path is by using a series of lenses each with a specific coating formula to direct a particular color a certain way.
This is one reason their are 11 or more lenses in a quality variable scope.
Hope this helps a little
Steve Optics |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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I am not sure what Steve means by "white light". Typically white light means a combination of different wavelengths, from red to blue.
>> Fact, as light passes through an optical lens, coated or non coated, all shades of all colors in the spectrum bend. That is all except the "white light" >>
I am not sure how this makes sense. Any light bends when going through one media into another, i.e. from air to glass and vice versa. Fluorescent lighting actually produces a spectrum fairly close to sunlight, I think. Producing a coating optimized for white light means a broad band coating optimized for the whole product range. A lot of spectrometers and interferometers still use incandescent bulbs which output more radiation at the long wave end of the visible spectrum: red, yellow, orange and near IR. At sunset and sunrise, i.e. low light conditions, blue light transmission may be a little more improtant than in the middle of the day. That means that something optimized with a incandescent lighting would not be optimal for hunting.
Anyhow, I may be misunderstanding what Steve meant.
Ilya |
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Chris Farris
TEAM SWFA - Admin swfa.com Joined: October/01/2003 Location: Texas Status: Offline Points: 8024 |
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Steve,
- emailed to me with the request that it be posted on this thread. CF |
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M-100
Optics GrassHopper Joined: April/17/2011 Status: Offline Points: 10 |
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So why does not the industry set a standard of comparison based on an agreed standard set or sets of conditions. That way the consumer could evaluate based on objective data rather than subjective opinion?
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jonoMT
Optics Master Extraordinaire Joined: November/13/2008 Location: Montana Status: Offline Points: 4853 |
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I feel both baffled and illuminated by the transmission of all this info but I do see the light, white or otherwise. Dudes, my microwave's beeping at me. Time for 500 ccs of lunch. Pass the sauerkraut!
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Reaction time is a factor...
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SVT_Tactical
MODERATOR Chief Sackscratch Joined: December/17/2009 Location: NorthCackalacky Status: Offline Points: 31233 |
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FROM 2004 fellas.
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Kickboxer
MODERATOR Moderator Joined: February/13/2008 Status: Offline Points: 23679 |
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But still "worthy"...
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Opinion,untempered by fact,is ignorance.
There are some who do not fear death... for they are more afraid of not really living |
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tahqua
MODERATOR Have You Driven A Ford Lately? Joined: March/27/2006 Location: Michigan, USA Status: Offline Points: 9042 |
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Yes, another long dead thread risen from the dead. And, a good question. Why no standard.
Part of it is, beyond wave lengths, reflection, refraction, etc, etc, much is subjective. Edited by tahqua - June/07/2011 at 20:18 |
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Doug
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M-100
Optics GrassHopper Joined: April/17/2011 Status: Offline Points: 10 |
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Although I obviously wasn't a member here in 2004 I had the same question then and for that matter many years prior. Why doesn't some optics geek ( complementary) set up a lab test protocol
and become the consumers guide of optics? Unless of course it costs millions to do so.
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Dyelynn
Optics Apprentice Joined: March/07/2011 Location: Washington Status: Offline Points: 231 |
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because even if someone sets a standard, and all the manufacturers conform to that standard, which is a stretch in itself, individual buyers will still have differences of opinion because much of why we like certain optics has more to do with brand recognition and how it looks to us, than how it conforms to some measured number. pretty sure it would amount to a whole lot of work for nothing. marketing ploys, brand recognition and personal preference would still win out. |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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OK. Since this thread came from the dead...
Which standard do you mean? What are you looking to standardize? ILya
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M-100
Optics GrassHopper Joined: April/17/2011 Status: Offline Points: 10 |
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I'm no engineer ( optical or otherwise) so don't attack my numbers. This is just an example to answer ILyas question.
I was thinking of a numerical performance scale.
Say an optic could resolve the difference between a 1mm hexagon and a 1mm circle under a specified
light condition at a standardized distance, it would be rated a 99 for resolution for that condition. Any practical optical component that is objectively measureable could be rated in this manner for the use of the consumer. I hope that makes sense.
Possible or not?
Practical or not?
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Kickboxer
MODERATOR Moderator Joined: February/13/2008 Status: Offline Points: 23679 |
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Except everyone's optical perspective is different...
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Opinion,untempered by fact,is ignorance.
There are some who do not fear death... for they are more afraid of not really living |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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As a matter of background, at the moment I am a Technology and Business Development Manager for a company that makes the equipment for all manner of optical measurements including lens testing, camera testing, etc. I deal with this stuff every day of my life. There are perfectly quantifiable ways of measuring light transmission, contrast, resolution, etc. How are you planning to transfer that knowledge into what your eye actually perceives? Under what lighting conditions? With what targets in mind? More importantly, how are you planning to pay for all that? Scope manufacturers, for obvious reasons, have zero interest in releasing such data (and they have most of the necessary equipment). Let's say, I decide to go ahead and set up a lab for such measurements. That will require an investment of somewhere between $75k and $200k for a truly comprehensive set up, depending on the extent of the testing and on the variety of the measurements I would need to make. How do you propose I recoup the costs of the equipment and the time invested? ILya
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Plange
Optics GrassHopper Joined: July/20/2010 Location: Winchester, VA Status: Offline Points: 25 |
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Hi Guys,
Thought I would throw some more wood on the fire.
To go back to the OP and the question and some of the answers......with what intrument are you testing light transmission with?
Usually you use something called a spectrometer. Am I correct on this Ilya and other optics guys?
Now this intrument can be made by many companies but the idea is to compare the light traveling through either a lens, group of lenses or the whole scope to a known light source within the tester. You can test at certain wavelengths or test the overall performance at say....the visible spectrum of light which is what we would be talking about here lets just say a little below 400nm to a little above 700nm (bear with me guys as I am just giving aproximations).
I have had 2 scopes manufactured by the same company tested on 2 spectrometers made by the same company, however one tester was "modified" in some way and the other was bone stock. The readings were 3-4% different. When I contacted the tester company and started to delve into this I found that numerous things can be done to alter the readings of these units.
So I guess is what I am saying is using just a spectrometer to measure light transmission will only be accurate if the same unit is used and the scopes are set up the same way ie: the reticle moved out of the light path as this can make a huge difference in readings.
OR if the unit is calibrated and in stock condition and has not been fooled with by a non factory person.
I do tests here on a calibrated Perkins Elmer spectrometer that is factory stock and I use the findings for internal use only unless someone asks me to perform a comparision and then I will run one. The calculation of light transmission and the reality of what you get through a completed scope can be vastly different.
Sorry for ranting, but in this instance the equipment used can play a large factor in the end result.
Thanks,
Paul
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