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Phokus Research Group Hoplite

10/28/13 - The Hoplite from Phokus Research Group is a Night Vision Focusing Cover that installs on night vision goggles. It utilizes interchangeable apertures which allow the user to leave the NVG focused at infinity then flip the cover closed for close-up work. It also provides dust and scratch protection for the lens.

Background - One of the issues that users of Night Vision Goggles (NVGs) or Night Vision Devices (NVDs) will be familiar with is transitioning back and forth between near-far focusing. NVGs like the PVS-14 and PVS-7 shown here can be focused from about 6" to infinity by rotating the objective lens at the front, but frequent refocusing is needed if you need to see both near and far as the depth of field is quite shallow. When going from outdoors into a building or structure, if the NVG is focused at infinity, anything under about 25-30 feet becomes out of focus. The closer the object, the more out of focus it will be unless the NVG is refocused. Everything inside a room is quite blurry if the NVG is focused at infinity. The Hoplite from Phokus Research Group is a device that installs onto the NVG that deepens the depth of field, allowing more to be in focus at different distances. This decreases the need to refocus as often, and enables the user to leave the NVG focused at infinity for most far and medium distance tasks. It replaces the dust cover and sacrificial lens.

Phokus Research Group was founded by a former military end-user and the company's primary mission is to partner with Soldier inventors for the purpose of protecting, developing and commercializing their ideas.

Description - The Hoplite works on the same principle that's used on any camera lens to deepen the depth of field: by manipulating the aperture to adjust the depth of field. With photography, a higher f-stop (smaller aperture) deepens the depth of field, which means that there's a wider range of distances from the point of optimum focus that objects will appear to be in focus. With a smaller f-stop (larger aperture), the depth of field is shallower, so that fewer object will be in focus as the distances vary from the point of optimum focus.

The Hoplite kit consists of the Hoplite body/cover assembly, two accessory apertures and a rubber adapter for the PVS-15 and 18. The body has a hinged dust cover top, which flips open and closed on a hinge. There is no detent; it relies on friction to keep it closed and open. The body has an integral lanyard loop for securing it to the NV device to prevent loss. The Hoplite body incorporates a rotating tension lock which decreases the inside diameter of the body so that it locks onto front of the NVG. Rotating the tension lock clockwise (when looking from the front) 'contracts' the inside of the body, which tightens its grip on the NVG objective lens. You can see the travel-limiting pin inside the body that moves in a slot in the photos below. The inside of the body has slits/cutouts. When the tension lock is tightened, the gaps close ever so slightly reducing the inner diameter of the body to secure it to the objective lens. The PVS-15 and 18 have smaller diameter objective lenses so the included rubber adapter is used with them. The Hoplite is made of polycarbonate and 18-8 stainless steel (for the retaining ring and small parts). It weighs 0.88 oz.

Kit contents

No apertures installed

Tension lock

The Hoplite has a clear sacrificial lens built into the body which is replaceable. Typically, the PVS-14 and 7 are supplied with a rubber dust cover and clear sacrificial lens; but they can't be installed at the same time. I usually just leave the clear sacrificial lens on the unit. With the Hoplite, you get both. The Hoplite dust cover lid has a hole in the middle that's larger than the pin hole found on the issue/standard dust cover, and also serves to protect the sacrificial lens from scratches and bumps. The aperture in the Hoplite dust cover top is 5/16" in diameter. The user can choose to use the Hoplite with the dust cover aperture only, or install either of the two additional accessory apertures inside it. The accessory apertures have 3/16" and 1/8" diameter holes, and snap into the inside of the dust cover top. In this writeup, I'm referring to the dust aperture (5/16") as 'large', the 3/16" as 'medium' and 1/8" as 'small'.

Sacrificial lens

3/16" aperture installed

1/8" aperture installed

Installed on PVS-14 - The Hoplite installs in seconds to the objective lens of the PVS-14. The recommended orientation is to have the dust cover top open away from the battery knob/variable gain knob where it doesn't interfere with the controls or obscure the IR illuminator when open or closed. As mentioned before, there's no detent keeping the cover open or closed; it relies on the friction in the hinge. It has just the right resistance; stiff enough to keep it open or closed securely without making it difficult to do either, or mess up the focus when opening it.

Installed on PVS-14


Installed on PVS-7B - The Hoplite installs on the PVS-7 in exactly the same way as the PVS-14 as they share the same objective lens size and take the same lens-mounted accessories.

Installed on PVS-7B

Replacing standard sacrificial lens with Hoplite

Notes/observations - A very important factor to consider is that the apertures reduce the amount of light entering the lens since they're so much smaller than the objective lens diameter. Depending on ambient lighting conditions, the use of the built-in IR illuminator on the goggles is recommended for indoor use in very dark rooms. I took the photos below through the PVS-14 to illustrate how the different apertures affect focus, and relative brightness. For the first picture on the left, the reflective target is at 6 feet and the PVS-14 was focused at infinity. The integral IR illuminator on the PVS-14 is turned on. I adjusted the gain as I swapped out apertures, turning it up as each aperture got smaller. You can see that with no aperture (Hoplite dust cover flipped open), the target is just a blur. As each aperture gets smaller, the target gets more defined. I did not adjust the focus of the objective lens for these photos.

In the second photo on the right, I wanted to compare the change in relative brightness when changing apertures. I kept the camera at the same manual setting as well as the gain on the PVS-14. The IR illuminator was also on. Since the target was reflective, it's brighter than it would be if the IR illuminator were turned off. The room was very dark, and required the use of the illuminator to see properly. If I had turned it off, the view with the apertures would have been too dark to compare properly. As you can see, the smaller apertures let in progressively less light and the target gets darker. I found that for walking through a house, the 3/16" (medium) aperture provided the best balance of brightness and focus. Also note that the field of view is reduced with the use of the apertures - I'd estimate that it's reduced about 10-15% around the perimeter of the circle depending on which aperture is used. Same with the PVS-7B.

Comparing focus at 5 ft by changing out apertures

Relative brightness between apertures

On both the PVS-7 and PVS-14, the objective lens focused at infinity is not pegged all the way to the left (when looking through the NVG); it's backed off a bit from the range of adjustment. During storage, or when mounting the NVG, it's easy to bump/rotate the objective lens so that it's no longer perfectly focused at infinity, so it's always a good idea to check it before use as it may not always be obvious, especially when it's very dark and there aren't any distant light sources to focus on. I wondered to myself whether a witness mark on the side of the lens would be of use, but probably not if you can't see it. I figured that with enough use, I'd figure out where infinity focus is, and remember how much to back off from the end of travel.

Without the Hoplite, walking inside a house requires refocusing if I want to see clearly down a 30 ft hallway as well as objects in a room. With the Hoplite, no refocusing is necessary. The IR illuminator isn't needed all the time as navigating through a dark house doesn't always require a very bright image, but if you want to take a good look inside a room, then the illuminator definitely helps light it up. With the medium aperture, I'm still able to see objects clearly down to about 4 feet away. Closer than 4 ft, or at arm's length, and the objects start getting out of focus, but I'm still able to see clearly enough to perform gross tasks at arm's length. For close up tasks where I'd have to see very clearly, like reading something, then a slight twist of the objective lens is needed to focus it.

The Hoplite was developed to reduce near-far focusing time and to allow nearby threat identification while the NVG's remain focused at infinity. It's most definitely faster than constantly refocusing, and using the aperture simply requires a flip of the dust cover. The end user just has to weigh that advantage against the slight reduction in field of view, and the subsequent reduction in brightness (which may be compensated for by using an IR illuminator).


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