Why is twilight transmission lower than daylight? (1 Viewer)

Meopta is the only brand I've seen quoting separate transmission figures for daylight (presumably what everyone else states?) and twilight, which typically comes in several percent lower, e.g. 84% vs 88%. Why would that be the case? Are the coatings favoring red light over blue? (Sorry if this has been asked before, I searched and didn't see it.) Obviously it's unfortunate, just when you need brightness most.

Tenex I am on holiday and house-bound today due to bad weather so I am going to take a stab at this. Anyone is free to shoot me down if I get this wrong.

My understanding is that that the frequency of light to which the human eye / brain is most sensitive changes from daylight to twilight and since twilight is such a small part of the day, manufacturers optimise the coatings for the light that is most useful, for most observers, most of the time, and this means the daylight frequency, typically 550/555 Nm.

By the way, our own Gijs van Ginkel quotes daylight and twilight transmission figures in his bino reviews published on the House of Outdoor website.

Lee

tenex, post 1,
Our eyes have two different light detection systems (rods and cones): one for daylight (cones) color sensitive and one for night vision (rods) with optimal sensitivity in the blue spectral region. That is why night (rod vision) and daylight vision (cones) are published. You can find a description with illustrations in different textbooks, but if you prefer internet : I have composed an overview of literature data entitled "Color Viosion, brightness, resolution and contrast" in May 2013, it is published on the WEB-site of House of Outdoor in the chapter : Verrekijkers testen en vergelijken.
Gijs van Ginkel

It's probably a design choice to increase the perceived contrast under most viewing conditions (daylight). Zeiss seem to do that as well, but they use HT glass to increase transmission in the blue spectrum that will help in twilight viewing. Still the "Zeiss-curve" is a bit slanted with a clear maximum in the green part of the spectrum.

Svarowski usually go for a more flat curve and higher blue transmission, maximum transmission is on the other hand a bit lower. Having both might be difficult for technical and/or economic reasons as it will involve adding even more AR-layers. Swaro probably use "HT-glass" as well and the latest CL bin for example have a very flat curve indeed:

https://www.allbinos.com/index.php?test=lornetki&test_l=330

If you look at older Meopta bins, they had silver coatings on the prisms, instead of dielectric. That affected the transmission in the purple/blue spectrum in a negative way, see curve here:

https://www.allbinos.com/185-binoculars_review-Meopta_Meostar_B1_8x32.html

Current Meopta-units might use more modern dielectric mirror tech but I'm not 100% sure. Gijs can probably confirm if and when they switched or check out his measurements of later Meopta bins*.
I do know that Meopta did improve their AR-coatings a few years ago when they switched to a plasma depositing coating process. So their current AR-coatings should be up to date and fully comparable/close to the competition.

What glass used will also affect the final result of course. HT-glass for example is probably too expensive for Meopta to use considering the lower price you pay for their binoculars compared to the top tier models from Zeiss, Leica and Swaro.

* 2016 units seem still use silver mirrors:
https://www.houseofoutdoor.com/wp-c...an-de-Meopta-Meostar-B1-8x32-def-mei-2016.pdf

Troubador said:

My understanding is that that the frequency of light to which the human eye / brain is most sensitive changes from daylight to twilight and since twilight is such a small part of the day, manufacturers optimise the coatings for the light that is most useful, for most observers, most of the time, and this means the daylight frequency, typically 550/555 Hz.

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Hi,

mostly correct, although one usually uses wavelength in nanometers here instead of frequency... and 554 Hz is C5# which is audible only for most...

For day photopic aka day vision sensitivity maxima of the green and red cones (blue ones are less sensitive) are at 534 and 564 nanometers with 550 nm right in the middle. Blue is at 420 but those are quite a bit less sensitive.

Scotopic aka night vision is provided by rods and those have their sensitivity maximum at 498nm in the green-blue and their sensitivity is quite a bit higher than cones. But they're mainly placed outside the fovea centralis, so for best results with very low intensity objects (like faint nebulae or star clusters) a technique called averted vison can be used where you look a bit beside the position where you suspect the object and see it in your peripherial vision.

See https://www.yorku.ca/eye/specsens.htm and https://www.yorku.ca/eye/lambdas.htm for some graphs on sensitivity of the human eye at different wavelengths and the following review of a Meopta pair with a transmission graph and a discussion of the day and night transmisstions...

https://www.allbinos.com/310-binoculars_review-Meopta_Meostar_B1_10x42_HD.html

I personally would not say that I am able to notice a difference in transmission of below 5%. In the lab, it certainly can be measured,

Joachim

Jo
Many thanks for pointing out my use of Herz instead of Nanometers, now corrected. My only excuse apart from advancing years is that we were in a hurry to go and watch otters (it is the last day of our holiday) and today we were successful and watched a mother and two of last years cubs, now well grown and very competent hunters.

Prosit
Lee

Vespo
Meopta have confirmed their B1 binos use silver prism coatings but protected by a clear dielectric coating. This is why, in the past, when asked do the prisms have a silver coating or do the prisms have a dielectric coating, they have said yes to both.

Lee

Troubador said:

Vespo
Meopta have confirmed their B1 binos use silver prism coatings but protected by a clear dielectric coating. This is why, in the past, when asked do the prisms have a silver coating or do the prisms have a dielectric coating, they have said yes to both.

Lee

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Ok, thanks for info! Very transparent of Meopta to provide that missing piece of information!
Seems like a reasonable solution in a "budget bin" like the Meopta. Fully dielectric coated prisms are probably close to twice the price.

Did you notice the new "face-lift" of the Meostar series btw?
B1.1 seems to be the version number.

https://www.meoptasportsoptics.com/en/produkt/meostar-b1-1-8x32-621/

Tenex,

It used to be common place for manufacturers to state minimum transmission values for "daylight" (either 550 or 555nm) and twilight (500nm or 505nm) though it has somewhat fallen out of fashion. The cause might be because most customers misunderstood what it meant in practical terms, but also, we now have a much better understanding of the underlying science than we did in the 1930s when some of there original studies were done.

Just a few points to bare in mind.

I haven't tried to locate the source of your Meopta data, but spectra I was sent for the Meostar HD 12x50 for a 2013 sample gave showed 87.5% and 90% as the average of both barrels at 505nm and 555nm respectively, and there is quite a lot of glass in that model.

The recent B1 Meostars have what is commonly called 'enhanced' silver prism coatings. They have a few layers of dielectric over a silver base, which not only serves to protect the metal layer, but enhances transmission, particularly at the shorter wavelengths. You will find that these models will have a higher overall transmission across the full visible spectrum than the full dielectric of the SF, EL or SLC for example, as a consequence.

The 'twilight' figure is largely meaningless for most of the birdwatchers I know who normally head home well before decreasing light levels make that value relevant. It is probably more important for the military and perhaps some owl watcher or hunters who use night, or scotopic vision, but probably a long way from the whole story.

We should also keep in mind that the original data which gave rise to these wavelengths was not only somewhat flawed in the experimental design, but it's purpose was to create an average result. As such, it represents almost no individual. To simplify the story, Isaac Newton described the visible spectum as 7 colours. Dismissing those who are officially colour blind, some will see the full seven colours, a good percentage only about five and a half, and a few would distinctly categorise several more than seven. Naturally, as a consequence opinions and preferences will vary the spectal choices made by different manufacturers.

Just a technical detail. The blue receptors (S-cones) are actually more sensitive than their green and red ( M and L-cone) counterparts but are present in much lower numbers and are absent from the fovea centralis. However, we also need to keep in mind that the available light after sunset often swings quite significantly into the blue, and the blue receptor contribution should not be undervalued.

Because most of us have a peak daylight spectal sensitivity at the yellow end of the green spectrum, many seem to assume that transmission in this region was the key to perceived brightness. It's now understood that another non-visual receptor, the melanopsin sensitive retinal ganglion cells, have significant role. These have a broad absorption with a peak sensitivity in the 460 to 480 region of the blue. (They are aso involved in controlling pupil diameter and regulating the sleep cycle.)

I think we would have to say that the relevant science is in a state of flux. Molecular biology has revealed that we are visually much more diverse than was previously understood, and appears to go some way to explaining the contrasting opinions here. Though I suspect we have some way to go before we understand the full story.

There is no doubt individuals will differ, but as a rule of thumb you should probably reconsider the transmission in the region of the spectrum between 400nm and 500nm as a guide to levels of perceived brightness. The red end of the spectrum is somewhat harder to pin down, but there are large individual differences in the sensitivity to longer wavelengths, and by a quirk in the biochemistry it also affects the ability to see violet. I can't figure out quite how the dice need to roll exactly, but from the evidence of the forum individuals appear to react quite differently to transmission variation at both ends of the spectrum. The answer is probably in their genes.

I really don't know yet whether if the B1.1 if simply the addition of the locking dioptre or the step change in performance I've been eagerly anticipating for the last 6 years. Hopefully not long to wait.

David

New findings in mesopic vision I suspect is one reason for developing HT-glass and new coatings improving the transmission also in the blue part of the spectrum:

"There is not one single luminescence value where photopic vision and scotopic vision meet. To the contrary, there is a wide zone of transition between them. Because it is between photopic and scotopic vision, it is usually called the zone of mesopic vision. The reason that the zone of mesopic vision exists is because the activities of neither cones nor rods is simply switched 'on' or 'off'. There are reasons to believe that the cones and the rods both operate in all luminescence conditions.[3]"

https://en.wikipedia.org/wiki/Mesopic_vision

Not sure if Meopta have jumped on that train yet. Haven't seen anything in their marketing about HT-glass or something similar.
Leica did quite a lot of noise on using HT-glass in their Noctivids.

Vespobuteo said:

New findings in mesopic vision I suspect is one reason for developing HT-glass and new coatings improving the transmission also in the blue part of the spectrum:

"There is not one single luminescence value where photopic vision and scotopic vision meet. To the contrary, there is a wide zone of transition between them. Because it is between photopic and scotopic vision, it is usually called the zone of mesopic vision. The reason that the zone of mesopic vision exists is because the activities of neither cones nor rods is simply switched 'on' or 'off'. There are reasons to believe that the cones and the rods both operate in all luminescence conditions.[3]"

https://en.wikipedia.org/wiki/Mesopic_vision

Not sure if Meopta have jumped on that train yet. Haven't seen anything in their marketing about HT-glass or something similar.
Leica did quite a lot of noise on using HT-glass in their Noctivids.

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Mesopic vision seems to be one of those areas when the laboratory science and the real world have yet to fall into step. The science may pinpoint the standardised light level where the transition to scotopic should begin, but doesn't allow for relative 20 fold or more increase in the blue content that can occur after sunset. It seems to me that most birder head home when they struggle to distinguish the colours of the birds. That might be a better marker for the onset of mesopic vision than any light meter.

Some of the HT glasses can help make best use of the available light, particularly when it is blue shifted after sunset. However, these can be just the occasions when the red end of the spectrum is the first to become limiting, and where high red transmission seems increasingly important for distinguishing colour.

Obviously each manufacturer arrives at a design specification for their products, which will include transmission profile. I wasn't at all surprised to hear from Gerold Dobler of Zeiss,(ex-Swarovski, ex-Leica) that birdwatchers may not be the first consideration. They may not be at Meopta either, but I was rather encouraged to see their MTF analysis was conducted at four individual wavelengths as well as using polychromic light. Looks like we haven't been forgotten about.

David

Very interesting thread. I've enjoyed reading.

Yes, my question was answered some time ago but it's always fascinating how much more information turns up here. I've been contemplating what David said about brightness perception being keyed to bluer wavelengths, and how that may figure into my impressions of different binos.

typo said:

Obviously each manufacturer arrives at a design specification for their products, which will include transmission profile. I wasn't at all surprised to hear from Gerold Dobler of Zeiss,(ex-Swarovski, ex-Leica) that birdwatchers may not be the first consideration. They may not be at Meopta either, but I was rather encouraged to see their MTF analysis was conducted at four individual wavelengths as well as using polychromic light. Looks like we haven't been forgotten about.

David

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Yes probably, but it's a bit puzzling to see that the new $300 Meopta Optika HD actually have higher twilight transmission (85% vs 82%) than the three times more expensive Meostar B1 models. Daylight transmission is the same.

Vespobuteo said:

Yes probably, but it's a bit puzzling to see that the new $300 Meopta Optika HD actually have higher twilight transmission (85% vs 82%) than the three times more expensive Meostar B1 models. Daylight transmission is the same.

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As I mentioned in #9 the 2013 sample of the 12x50 HD had about 97.5% transmission and had delectric enhance silver coated prisms, but just judging from the colour, I suspect it took a few more more years for the enhanced coatings to find their way into other models. I know the last 7x42 I tried was still pretty yellow, and I'm not sure all the others matched the blue level of 12x50 HD.

On Meopta's EU web site they give the twilight figure for most of the B1.1 models I've checked as 94% and 92% for the x32s and does seem lower than I might expect, but then we are hoping to see a B2 before too long. Could it be a future differentiator? As a pure guess, I suspect the B1.1 may have enhanced silver coated for a greater width of the spectrum, and the dielectric of the Meoptika HD might have a narrower band width, but I've not seen either yet.

David

tenex said:

Yes, my question was answered some time ago but it's always fascinating how much more information turns up here. I've been contemplating what David said about brightness perception being keyed to bluer wavelengths, and how that may figure into my impressions of different binos.

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I'm still somewhat waiting ! lol :-O
Some folks here have made some very good points.

Some of the differences in transmission levels across different wavelengths are indeed design choices (dealt with through AR, and Dielectric, and other coatings), some such as blocking UV light and (near) NUV light are safety features. Other causes are inherent in the different optical material types and how they handle the different wavelengths. How this all plays out for the individual is, well ...... very much an individual thing ! (physiological makeup, gender, age, yellowing of the lenses etc).

Between all of these factors, designers (and marketers ! dance around all of this voodoo to try and give the best view possible for the desired application. As David said, a lot more is known now than in the world war years. (See the 2005 data modification to the Standard for measuring brightness which attributes slightly more of the blue spectrum to the overall result). https://en.m.wikipedia.org/wiki/Photometry_(optics)

If you have a look at the Schott catalogues you can see that "HT" glass can be ~1.5% higher transmitting in the blue for a category type. Other manufacturers have their own secret sauce equivalents.
https://www.edmundoptics.com.au/resources/application-notes/optics/optical-glass/

As has been touched on, none of these peak sensitivity values are light switch on/off. There's a lot of interplay between the rods and cones (and the different types - S, M, L, or blue, green, red) , as well as environmental conditions, EP's etc. https://en.m.wikipedia.org/wiki/Photopic_vision
https://en.m.wikipedia.org/wiki/Cone_cell

I like Swarovski's 'flat' transmission curves, and the 'even' Zeiss HT ones too. I think a fairly flat transmission curve is fairly essential to allow unmodified perception of changing light in the landscape.

One of the most memorable and brilliant natural phenomena I have seen (well apart from ancient tree elders that uproot themselves, walk around, and shimmy and shake right in front of you !) was a Red Rainbow. It may have been caused by dust and/or smoke in the atmosphere, but it was all subtle different shades of reddish, peach, vermillion mauve, save for a tiny sliver of teal in the middle. Magnificent !

You just wouldn't be able to fully appreciate the beauty of that and the light changes leading up to and from it without a neutral colour rendition bin. The 'clarity' of 100% internally reflecting prisms also add to the experience.

I'm sure manufacturers could do better if they put all the bits of the pie together, but it's getting to the point of stacking BB's ..... whether you'd notice a huge change or indeed any change at all is debatable.

Not for me though, the Martian red, Sepia, or P** brown shades of some makers ....... let alone green ham ! :-O





Chosun :gh:

typo said:

Mesopic vision seems to be one of those areas when the laboratory science and the real world have yet to fall into step. The science may pinpoint the standardised light level where the transition to scotopic should begin, but doesn't allow for relative 20 fold or more increase in the blue content that can occur after sunset. It seems to me that most birder head home when they struggle to distinguish the colours of the birds. That might be a better marker for the onset of mesopic vision than any light meter.

Some of the HT glasses can help make best use of the available light, particularly when it is blue shifted after sunset. However, these can be just the occasions when the red end of the spectrum is the first to become limiting, and where high red transmission seems increasingly important for distinguishing colour.

Obviously each manufacturer arrives at a design specification for their products, which will include transmission profile. I wasn't at all surprised to hear from Gerold Dobler of Zeiss,(ex-Swarovski, ex-Leica) that birdwatchers may not be the first consideration. They may not be at Meopta either, but I was rather encouraged to see their MTF analysis was conducted at four individual wavelengths as well as using polychromic light. Looks like we haven't been forgotten about.

David

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Hi David,

As you have presented data for us before, there is also the large drop off in acuity as light levels drop.

Fascinating insight into the MTF analysis - can you elaborate any more on this? Were there differences between the four wavelengths ? (what were they - did they correspond to the 3 peak cone sensitivities or something else?) , and were they appreciable? Same goes for the polychromic light - ? Anything else you can say? Were these measured or theoretical values? Thanks.

Could you also clarify on the enhanced (silver/dielectric) mirror coatings on the S-P prisms ...... is that 97.5% reflectance ? transmission through the whole prism assembly ? or the overall binocular ! ...... ?
And do you think they could ever be combined with FL and HT glass and appropriately 'tuned' coatings to ever give anything resembling a neutral colour rendition high transmission view ? Thanks again.





Chosun :gh:

Chosun,

Meopta asked me not to post the actual numerical values and did not elaborate on their experimental methods or any underlying rationale, so this is my reading of what they did. The wavelengths appear to correspond approximately to a mid point in the blue, the sensitivity maxima of the retinal ganglion cells, close to the photopic maximim and possibly the mid point in the red. That might seem a reasonable thing to do, but on the other hand it might be nothing more than the wavelengths of a bunch of dichroic filters they had in the draw. The MTF values they sent me were for a spatial frequency regarded as important for perceived sharpness in good light, but would also correspond to a typical acuity at the onset of twilight. Both sound pretty useful parameters to consider.

I'm sure Meopta have a lot more data than they were prepared to share with me. What they did send seems to indicate that they are not just considering a transmission profile but using some relatively recent advances in the understanding of visual interpretation to evaluate their technology. It's quite possible other companies do something similar but I suspect it's a very small minority.

The 87.5% figure for 505nm was the mean I took from two separated transmissions profiles for 12x50HD samples.

I'm pretty sure Meopta are quite capable of flattening their transmission profile further, but I'm guessing that they might not see that at the first priority for delivering best performance. Hopefully we will get to see the B1.1 and Meoptika in the next month or two, but I suspect they still have more up their sleeves.

David

Thanks for saying so much David - I suspect any more and you'll be bundled off in a dark windowless van ! :eek!:





Chosun :gh:

marcsantacurz said:

Very interesting thread. I've enjoyed reading.

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Wholeheartedly agree!