[visionlist] Focal distance / accommodation distance HTC Vive Pro

Gislin Dagnelie gislin at lions.med.jhu.edu
Mon Oct 11 14:03:28 -04 2021

Maarten and Daniel,

You are both correct, in the sense that the focal plane of the lens is actually 
close to the OLED panel inside the HMD, but that the virtual image perceived by 
the wearer is at a distance somewhere in the 1 m range.  This is done in most 
HMDs, for 3 reasons:
1) Wearers are able to accommodate, so there is no reason to place the image at 
optical infinity (i.e., the screen in the focal plane)
2) Bringing the display screen inside the focal range (creating a virtual image 
closer than infinity) widens the wearer's field of view.  It also reduces the 
resolution, but no one seems to be bothered by HMDs having a resolution on the 
order of 20/70.
3) The optical quality of the Fresnel lenses is pretty poor, actually, and the 
focal plane isn't flat.  By reducing the central resolution the poor peripheral 
image quality becomes less noticeable.  I admit that this is speculation on my 
part, but it seems plausible.

If you want to actually measure the focal length you'll have to follow 
Maarten's example and take the system apart; then you can put the Fresnel lens 
in an optical bench (or a lensometer if you're in an eyecare setting) and 
you'll have your dioptric power = 1 / focal length.

The virtual image distance can be measured by placing a positive lens (4 D 
should work fine) up against the Fresnel lens and in a dark room projecting the 
HMD image onto a white wall.  Move the HMD until the image is sharp, measure 
the distance from wall to auxiliary lens, and apply the well-known formula:

1/f = 1/i + 1/s

where 1/f = 4 (or whatever dioptric power you picked for your auxiliary lens), 
and i is the image distance, i.e., from auxiliary lens to wall, in meters.

You'll then find s, the distance from the auxiliary lens to the virtual HMD 
image, again in meters.

Hope  this makes sense.

Best, Gislin

On 11 Oct 2021 at 15:31, Maarten Wijntjes - IO <M.W.A.Wijntjes at tudelft.nl> 

Dear Daniel,

Shouldn´t it be in the range of some cm´s as it projects the screen in front
of the eyes to far away/infinity? I thought a simple test for focal distance
measurement is to project something with it that is far away, e.g. a lamp on a
high ceiling or a window. The distance between the lens and the projection 
(e.g. table with white paper) is the focal distance. 

I hope I am right and did not make an optical fool of myself in front of the
vision list community ;)

Maarten Wijntjes

> On 11 Oct 2021, at 13:50, Oberfeld-Twistel, Daniel <oberfeld at uni-mainz.de>
> Dear list,
> we´d like to determine the focal distance in the HTC Vive Pro headset, in
order to use exactly this distance for tests of visual acuity before the actual
VR experiment. An internet search produced a variation of estimates, ranging
from 0.75 m to 2 m or even infinity. So we are wondering if someone here made
optical measurements of the Fresnel lenses inside the Vive, to determine the
effective focal distance of the display.
> Best
> Daniel
> ---------------------------------
> Prof. Dr. Daniel Oberfeld-Twistel
> Johannes Gutenberg - Universitaet Mainz, Experimental Psychology &
> Laboratoire ICube UMR7357 Université de Strasbourg
> Wallstrasse 3
> 55122 Mainz
> Germany
> _______________________________________________
> visionlist mailing list
> visionlist at visionscience.com <mailto:visionlist at visionscience.com>

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