Oct 31 2007
In a pair of binoculars, is there a way to get more light through the same sized exit pupil?
jondalar469 asked:
The exit pupil diameter is normally found by dividing the diameter of the objective lens by the power of the instrument. But there ought to be a way to get more light through the instrument without changing the exit pupil diameter. Maybe a preamplifier optical instrument that has a larger objective, which feeds the binocular objective from its own exit pupil?
The exit pupil diameter is normally found by dividing the diameter of the objective lens by the power of the instrument. But there ought to be a way to get more light through the instrument without changing the exit pupil diameter. Maybe a preamplifier optical instrument that has a larger objective, which feeds the binocular objective from its own exit pupil?
Is there such an instrument? Would it be possible to make such an instrument?
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3 responses so far
The answer to your question is currently “No!”
Your idea of a “preamplifier” is interesting, but it too is doomed to the reality of optics.
Say you had a 100 inch objective, with a 2 inch exit pupil. That would mean the “preamplifier” was working at 50x. If you put a 10x50mm telescope (binocular) behind the preamplifier, you will indeed get an exit pupil of only 5mm, but you have pushed the magnification of the system up to 500x!
However photo-multiplier tubes have been used for years to increase the light amplification, with some degree of added noise. A better solution would be for a wide-band (visible spectrum) LASER (Light Amplification by the Stimulated Emission of Radiation) to actually amplify incoming photons at their individual wavelengths. Unfortunately, no such creature exists at the moment.
The I3 eyepiece is used in astronomy in amateur telescopes. It’s basically a real-time ccd imager that is vastly more sensitive than the eye, sells for around 2 grand. You can’t use it during the day because it is too sensitive. Perhaps a reworked version could be used for daytime use. Anyhow these eyepieces (see link) dropped into astronomical binoculars would greatly enhance faint object detecion. It would be a pricey combination but interesting to try. Technically this isn’t “more light” but it is a vastly increased efficiency in using the light that’s already there and amounts to the same thing.
As far as conventional optics goes the exit pupil is a simple ratio. 10×40 binocs give an exit pupil of 4mm, or 40 divided by 10. So 20×80 binoculars would also give an exit pupil of 4mm. Now you’ve got more than twice the light gathering (due to pi-r-squared on the objective lens area) and twice the power, but the exit pupil stays the same. That satisfies your requirement of more light and the same sized exit pupil.
Looked at another way, 10x40s are also twice the aperture of 5x20s which are also available typically as opera glasses or pocket binoculars.
So my answer is
1. Yes, optical image enhancing is already on the market in specialize applications.
2. Without electronics, you can get more light through the same sized exit pupil by increasing magnification along with the diameter of the objective, and keeping the exit pupil ratio the same. 5x20s, 10x40s, and 20x80s are all on the market and all have the same exit pupil of 4mm. Try binoculars.com to see what’s for sale.
As you change aperture and keep exit pupil constant your magnification increases (as in the three examples above). So the binoculars get harder to hold steady and to use without a tripod. The difficulty of holding the binocular steady is one reason why manufacturers tend to mess around with exit pupil (magnification).
Hope that helps,
GN
Not without changing the magnification.
If you posit a ‘pre-amp’ with a larger objective you
are also changing the magnification of the
whole system aren’t you?
Better minds than our have been trying to do
this for a lot of years, and the best solution they’ve
come up with is electrical image amplification.