Idea for temporary optical coupling liquid.

[ulf]

I have an idea for temporary optical coupling liquid, simulating a fixed glued filter stack.

As discussed elsewhere there is a transmission loss for each air - glass transition.

You lose roughly 4-5% for each passed surface.

 

That is why a glued filter stack can have a better transmission as it only has two air - glass transitions.

 

I was playing with an idea, to test if one can make a very temporary working glass to glass bond, with something else than UV-glue.

 

If I remember correctly pure glycerol has a rather good UV-transmission and a refractive index in the same region as most filter glass types.

 

When I get enough time and get some glycerine, I will do some experiments, measuring a pair of uncoated cheap "UV"-filter glasses, bonded and not bonded.

 

Normally I would not consider the 8-9% of exposure time an improvement worth the trouble, but it is interesting to know if any gain is possible with this.

As glycerol is mixing well with water any minor mess will be reasonably easy to fix and I hope that the surface tension will keep the glycerol in place between the glasses.

 

Comments?

[JMC]

My UV Leitz microscope lenses are designed for using Glycerine as the coupling fluid, which would suggest it is good for UV. So it's worth a go.

 

Glycerine is very hygroscopic and will pick up water from the air. Not sure how much that will impact its usefulness (if at all).

[Stefano]

In theory the glycerine “filling” should stay trapped between the filters, as it will “stick”?

Regarding water, the refractive index of water is 1.33 if I am not mistaken, so a bit lower that the typical ~1.5 of most glasses, so it shouldn’t alter the results too much. I don’t know if water can significantly change the surface tension of glycerine, making this layer less stable.

[ulf]

https://refractiveindex.info/?shelf=organic&book=glycerol&page=Rheims

Closer to filter glass than water.

 

It has also higher viscosity making it mechanically more stable as a thin film between the glass discs

[Andy Perrin]

This sounds like a very gooey filter sandwich. I wish you luck and towels, and I look forward to the results.

[JMC]

Glycerine is extremely sticky, so keep that in mind if you are attached two filters together with it (just thinking about safely getting them apart again).

[Stefano]

(just thinking about safely getting them apart again).

Is it that sticky? Look like Ulf will have some fun with it.

[JMC]

Is it that sticky? Look like Ulf will have some fun with it.

Yeah, think of it like 'runny honey'. For a couple of filters 50mm diameter, safest thing would be to slide them along their flat axis, rather than pull them apart.

[dabateman]

It might be a great idea.

You will need pure glycerol, as many are crude.

The melting point is 18C or 64F and the refractive index is 1.47. So maybe liquid or solid depending on your temperature.

 

I commonly see it as suppositories. As a solid that melts internally and pulls out some fluid to elevate constipation.

 

Pure glycerol isn't that sticky.

[SteveE]

Just last night, I was reading an article in the January 1997 issue of Micscape magazine discussing refractive index.

From that article:

 

cover glass: 1.518

glycerine jelly: 1.44

 

Google says water is 1.333 at 589.29nm

[Andy Perrin]

Yep, which makes the calculated single-bounce reflection losses:

 

cover glass / glycerine jelly = (1.518-1.44)^2/(1.518+1.44)^2 = 0.07%

 

cover glass / water = (1.518-1.333)^2/(1.518+1.333)^2 = 0.4%

 

cover glass / air = (1.518-1)^2/(1.518+1)^2 = 4.2%

[enricosavazzi]

Immersion oil for microscopes comes foremost to my mind. It is designed with a refraction index closer than glycerine and other oils to standard microscope coverglass (which may or may not match the refraction index of UV glass filters) and often for good UV transmission. Many different types with slightly different optical properties are available.

 

Methyl salicylate is often mentioned as a substitute for immersion oil, e.g. see https://www.tandfonl...rnalCode=ibih19 .

 

See this thread on microbehunter.com for more substitutes for immersion oil: http://www.microbehunter.com/microscopy-forum/viewtopic.php?f=14&t=10225

[ulf]

Immersion oil for microscopes was the seed of the idea, but I did not know how good those oils for UV-transmission and wanted a more commonly accessible type of liquid.

As glycerol mixes with water it will also be easier to clean off afterwards.

If the goal was to be a bit more permanent bonding and some edge seal was possible immersion oils would be a better alternative.

I found a bottle of 99% food grade glycerol that I can start to try with. If that works OK a more pure variant will be even better.

 

The goal for my test is to see if it works down to 300nm or so, where normal full spectrum-converted cameras still can see something.