Are these lenses safe for DIY HMD?

[oogawagga]

Hi Everybody!

Awesome forum! I've been working on a DIY HMD and would like to contribute to the community after ensuring these lenses are safe to use...

I purchased the following lenses because the diameter reduced vignetting... They seem to work fine, however I noticed the structure is different than say the Oculus (entirely flat on one side and convex on the other).

http://www.aliexpress.com/snapshot/6008909234.html

1. How can you test to see if these lenses are safe to use?
2. The current design has the lenses approximately 40mm away from the display (and ~15mm away from eyes), whereas the specifications say the focal length is 63mm. What numbers should be used to calculate FOV, etc?

Thanks for your help! Enjoy the rest of your weekend!

[Savaro]

Hi Everybody!

Awesome forum! I've been working on a DIY HMD and would like to contribute to the community after ensuring these lenses are safe to use...

I purchased the following lenses because the diameter reduced vignetting... They seem to work fine, however I noticed the structure is different than say the Oculus (entirely flat on one side and convex on the other).

http://www.aliexpress.com/snapshot/6008909234.html

1. How can you test to see if these lenses are safe to use?
2. The current design has the lenses approximately 40mm away from the display (and ~15mm away from eyes), whereas the specifications say the focal length is 63mm. What numbers should be used to calculate FOV, etc?

i dont think that is good idea: diameter is too small, focal length is too big, they are safe to use but u dont get rift like view angle.
to be safe for eye - all points of image must be within the accommodation of the eye. here is highly dependent on the location of the display and the lens.

imho - optimal for 7 inch screen - 50mm diametr \ focal length or clouse, or similar values.

Lenses theory:
http://www.mtbs3d.com/phpBB/viewtopic.php?f=138&t=16180

2. The current design has the lenses approximately 40mm away from the display (and ~15mm away from eyes), whereas the specifications say the focal length is 63mm.

if I'm not mistaken, if you put the display at a distance of 40mm from the lens, focal length is little more that 40mm, cos important the maximum distance to the edge of the visible region.

[Fredz]

If you're making a HMD with a similar display than the one in the Oculus Rift you should use similar lenses. The UltraOptix 7X are probably the lenses that have been used for it, I took a photo of both and they look identical.

To know at which distance from the display you should place the lenses, you need to know the front focal length. Generally it's not advertised and only the effective focal length is given (EFL), which is a bit higher (distance from the principal plane instead of distance from the front surface).

You can calculate the FFL for a given lens using the formulas at Wikipedia, but that's for spherical lenses and I don't know if it's applicable to aspheric lenses.

To calculate the FOV I don't see any other solution than implementing a ray-tracer using the appropriate equations for the Snell–Descartes law and aspheric surfaces, but that's not simple.

The FOV will be dependent on the distance from the eyes to the lenses, and there doesn't seem to be a consensus about the location of the center of projection in the eye considering the various discussions I've read about it on the Web, although I'm pretty sure it's the nodal point as defined in various eye models, 7.33 mm from front cornea in "Handbook of Optical Systems: Vol. 4".

Your best bet is probably to use the UltraOptix lenses and to try to find the correct distance experimentally with a laser diode and a graph paper.