Zeiss ocular filters (1 Viewer)

[Ignatius]

Zeiss used to provide ocular filters for their binoculars. They were available in different colours at different times. Yellow, green, grey and Umbral*.

Originally only yellow filters were offered. The earliest catalog I have access to that lists them is a 1912 Zeiss London catalog that offers "Yellow Moderating Glasses at 5/- per pair.


The 1928 (T 380 D) catalog offers them for use in bright light providing better contrast, while also offering "Sonnengläser" - made of neutral dark glass. These were available in a light and a dark version, the latter for longer observations of the sun, while the first were intended for use during solar eclipses. Also offered were lenses to correct for myopia, hyperopia and astigmatism. All of these were simple clip on items.


Carl Zeiss Jena in 1952 offered both yellow and Umbral lenses as well as the two ND filters for solar observation, just like the other Zeiss across the border.
A Zeiss Oberkochen catalog from 1957 lists "Umbralgläser" in addition to the glare-reducing yellow filters and the grey filters for solar observations. All filters were DM 12.- per pair. Additionally there were corrective "Punktalgläser" at DM 15.50.
In a 1978 catalog from Carl Zeiss, Inc., NYC, yellow filters are listed at $13.50 and Umbral filters at $17.00.
In 1990 CZJ still listed yellow Umbral filters, but by then they were slip-ons for the objective end of their binoculars instead of clip-ons for oculars.
By the early 2000s at the latest both CZJ and Zeiss west had dropped all filters from their catalogs.
Only Zeiss Hensoldt continues to offer both yellow and green filters for their 7 and 10x50 military porros.
The green filters shift the filter effect more towards the violet part of the spectrum.

I have a pair of yellow filters which belong to my 1915 Zeiss Wien Feldstecher 6x. I recently sent them for a spectral analysis and the results were quite surprising.


But first a little background to yellow filters in optics. In a 2000 article entitled "Contrast Is Enhanced by Yellow Lenses Because of Selective Reduction of Short- Wavelength Light" in Optometry and Vision Science, Wolffsohn, Cochrane et al. tested the effects of clear control lenses (cut-off at 380 nm), yellow lenses (cut-off at 450 nm), dark yellow lenses (cut-off at 511 nm) and orange lenses (cut-off at 527 nm) on contrast sensitivity, colour vision, accommodative convergence and visual acuity.
The results in short were as follows: decreased colour vision caused by yellow lenses enhances contrast when viewing bright objects against a blue background, eg. the sky. Due to a selective reduction of short-wavelength light passing through yellow filters, the contrast of overlying objects is increased. This effect was greatest with the lenses with a cut-off at 450 nm, which roughly equates to a Wratten** 8 filter (B&W 022).

Here is the transmission curve for a Kodak-Wratten no. 8 filter:



Transmission curve for a B&W 022 filter, both with standard coating and multi-coated:


Transmission curve of the Zeiss 1915 yellow filter:


As can be seen, the Zeiss filter is nothing like the two other filters. A cut-off at 460 nm is perfectly ok, BUT transmission values of 8% and 27% at 500 and 550 nm respectively are very bad. A Wratten 8 filter has 63.5% and 88.4% respectively at those two wavelengths. A binocular with uncoated lenses from that time probably had a transmission of around 40-45 percent. Add in the also uncoated filters and things get quite dark. Strangely enough when looking through the filter-equipped binoculars the view is not all that bad, but I assume that the eye-strain is immense. What is also interesting, is that the transmission values stay more or less level from 560 nm to 660 nm (green to dark orange) and then take off quite sharply in the red part of the spectrum, reaching a maximum of 56% at 750 nm.


* Umbral: from 1924 to the 1980s Zeiss manufactured Umbral lenses for sunglasses (and binoculars). In 1924 they were revolutionary because Zeiss had a manufacturing process which ensured an even homogenous colouring of the lens. Until that time the lenses for sunglasses were darker in the middle and became lighter towards the edges.
** Wratten, Frederick (1840 - 1926) was an English inventor. Between 1906 and 1912, when he sold his company to George Eastman, he developed gelatin filters. Yellow first of all. He invented a numbering system for filters denoting their colour and filtering parameters.

 

[Swedpat]

It's strange that different filters are not offered by manufacturers today!

 

[Binastro]

Solar observing filters for binocular and telescope eyepieces should be disposed of in the trash.

Besides the fact that they can crack in splinters in use they are a real danger to ones eyes.

In Europe in the early 1900s large numbers of people damaged their eyes during solar eclipses.

There were also the idiotic radium treatments.

There was also a doctor who recommended staring at the Sun, maybe mid 1900s.
A real dangerous charlatan.

Zeiss is not alone in stupidity.
The fact that they even had a lighter version is madness.

I had to deal with a rogue maker and dealer who sold, through official astronomy channels, a solar filter made of orange plastic.
It transmitted up to 27% in the infra red.
It heated observer's eyes.
It was put into filter holders of many sizes, and was expensive.
It took a top university lab, a barrister and several years to stop this dangerous individual.
Even when we eventually stopped him, he turned up later under another name.
Maybe 1980s.

B.

 

[Ignatius]

It's strange that different filters are not offered by manufacturers today!

Maven binos have filter threads, so people can purchase filters and use them. But Maven themselves only offer clear 'protective' filters.
I plan to get a B.3 6x30 and use old Agfa G1 or G2 filters. And maybe get some B&W022 filters for my B.5 12x56.

B.1 - 49 mm
B1.2 - 49 mm
B.2 - 49 mm
B.3 - 35.5 mm
B.4 - 58 mm
B.5 - 58 mm
B.6 - 52 mm
B.7 - 27 mm

 

[Scott98]

It's just a guess but I think color filters were more popular when binoculars & telescopes had more uncorrected false color. When you reached perfect focus in one color (probably green, since our eyes are most sensitve there), the other colors were slightly out of focus. Leading to less sharpness.

In astronomy, color filters used to be standard operating procedure for viewing planets. Now, at least with refractor telescopes, they are apochromatic, so all the colors are in focus simultaneously, so adding a monochrome filter doesn't increase contrast.

 

[Ignatius]

A little comparison between the 1915 Zeiss yellow filters and ca. 1980s Zeiss/Hensoldt filters.

Zeiss/Hensoldt on the left, 1915 Zeiss on the right.




I plan to send the newer Zeiss/Hensoldt ones for spectral analysis soon.

 

[Ignatius]

Why do military optics have a yellow-tinged image?

Here is an article (Gelbstich) by Albrecht Köhler, who worked as a developer at VEB Carl Zeiss Jena from 1977, at Docter-Optic from 1991 and at Analytik Jena since 1997. As he has not dated the article, I have no idea when it was written, but references in the content lead me to believe that it was before 1995, the year Docter-Optic declared bankruptcy.

Why do military optics have a yellow-tinged image?
This question shall be discussed using the EDF 7x40, issued to the army of the former GDR from the late 1970s, as an example.

Within the framework of trials in the field and subsequent assessment in a larger circle of testers and departments the present technological and design solution was selected and put into production. One of the decisive requirements for the specific deployment in secutity relevant areas is hightened contrast, even in inclement conditions, which cannot be ruled out in professional use. Think foggy or misty weather or strong reflections caused by ice and snow in winter.

This requirement was fulfilled by choosing special glass in combination with appropriate coatings which partially suppress the extreme blue parts of light, and absorb the detrimental UV part of the spectrum completely. This causes a noticeable shift of the spectral segments in the image as evidenced by an emphasis of the complementary colour segments. Sadly this is deprecatingly called a yellow tinge, tint or hue.

These experiences were not only implemented in the piece of equipment currently under scrutiny, but also are also applied in the NAUTIK binocular developed for maritime use, which is a special version of the NOBILEM DF 7x50. There, the effect was not only implemented because of heightened contrast in fog, but also because of the increasingly important argument of protection against UV radiation due to diffused reflections on water surfaces. The advantages can be confirmed by users in that arena.

These findings in the physiology of vision are not new and are not only applicable to the area of imaging optics but are also used in lighting engineering. For example the yellow main beam headlights as prescribed in France, which allow better seeing in foggy conditions.

However, we must not hide the fact that in areas where at least temporarily the colour of objects must be assessed, there can be no general application of such glass because colour purity in the image is of special importance. Binoculars for use in ornithology and astronomy come to mind.

The transmission values for daylight are hardly impacted because the values in the blue segment of the spectrum only have low weighting and because transmission in the threshold to the green segment increases quickly. Increased contrast may be achieved in two ways:

Absorption of the UV/blue portion, or
Reflection of the UV/blue portion.

With absorption the effect is achieved through the use of yellow filters (a concept known from photography) or by the use of special glass which has spectral characteristics akin to those of the required filter, and which are then deployed as optical parts (lenses or prisms). This is the case with the EDF 7x40. The lenses in the original military issued binoculars are radiation resistant, ie. they will not be destroyed by nuclear radiation. A bonus characteristic is that they show a spectral curve similar to the desired yellow filter which means that suitably constructed binoculars will not need an additional part in the form of a yellow filter.

The reflective method depends on the choice of an anti-reflective coating whose edge in the shortwave segment reflects the blue and UV portions of the light. In practical terms this means that the transmission curve of this coating is shifted into the longer wave zone of the spectrum. For the DOCTER NOBILEM DF 7x50 Nautic this coating, which appears to be bright blue, is applied directly to the front of the objective lens. This ensures that the UV portion of the incident light never enters the device and the extreme blue portion of the light is reduced in its intensity.

Both variants are comparable in their result. The reflection method is easier to work with modern coating systems. Eye protection is better achieved using the absorption method since a coating on the outside may be damaged in use, thus losing its effectiveness.

Finally, some notes on the apparent colours of anti-reflective coatings. Today, it is usual for manufacturers of high quality optical devices to make use of multi-layer anti-reflective coatings. Because of the many varieties of combinations in layer composition (choice of substrate and layer thickness), coatings can be adapted to all sorts of requirements; for example in the military field the protection against laser light. The transmission curve of a conventional anti-reflective coating is never even for the whole visible spectrum, but shows minimal inhomogeneities. These then show themselves as iridescent colours, visible when looking at the lens at certain angles. Without any loss of quality this may be greenish, bluish or violet. The noticeable differences in transmission for any given plane are always well below 1%.

Things are very different, when extreme reflections are desired. Examples are the "Firebird" coating or the green coating offered by Steiner. The green coating reduces the transmission of the whole optical train by about 10% at 550 nm, a frequency the eye is very susceptible to. Such endeavours should therefore always have sound optical reasons and not just be some gimmick.

DOCTER products only have wideband anti-reflective coatings, optimised for maximum transmission over a large spectral range, unless UV reduction or contrast enhancement were the specific brief. Different colour hues of the coatings, even within a batch, have no technical relevance. Normally though, binocular objectives are selected to be colour matched.

Often flint glass is the culprit when it comes to a yellow(ish) image. With the EDF the yellow hue is caused by the choice of SF3R (R = radiation resistant) glass in the negative lens of the objectives and in the first lens of the oculars. It is produced by the Jenaer Glasswerk. The radiation resistance was tested with a dose of 0.8 to 1.2 x10⁴ R/h over a period of 10 hours. The direction was set parallel to the optical axis. Transmission was expected to be at least half of the original value (≥ 35%) two hours after irradiation.

The EDF 7x40 is still being produced by DOCTER in a civilian version called DF 7x40 BGA. It is a robust, waterproof binocular for general use, which has colour-neutral lenses from Ohara or Hoya, as requested by the market.

 

[Maljunulo]

I believe that yellow filters increase apparent contrast, and help a bit with haze penetration.