3 Methods to Test Lens of Google Cardboard

No doubt Google cardboard is a magic, simply several pieces of cardboard, plus 2 lenses, boom~ open the gate to virtual reality. Now it’s already mass produced and widely used, no more just a DIY gadget it born to be. Like most of us in this group, more and more people use it for work, sell it to customers, then of course, we concern what’s bad and good.

The main parts of a cardboard viewer include cardboard paper and lens. What impresses us first when getting a cardboard is the paper, the making, the printing, while to some of people, I believe quite many, these are their only concerns, and basis to judge if it’s good or bad. They complain the paper is too weak, layers of cardboard not aligned, unglued or glue runs everywhere, button not work functionally, bad printing … yes, these are all problems, but not real problems, if the manufacturer put his heart into the work and has quality criteria in mind, or just be more careful. Some manufacturers, especially some Chinese, they like to make stupid mistakes, and have no intention at all to make it good, but that’s not the point.

In this post I mainly discuss about the lens. It’s not a job you can do well just be enough careful. Lens is the most “technical” parts of Google cardboard. Though Google provides detailed specifications and drawing, because of its optical characteristics, there’s very strict requirements on molding and injection. Every single deviation during manufacturing process like incorrect temperature, time, will cause a disaster. That’s why you can easily find dozens of manufacturers of cardboard or lens in China, but need good luck to find a good quality one.

I collected 2 pieces of samples from 2 main lens suppliers in China (marked C & D), and do some comparison with 2 of ours (A &B), so total 4 different pieces of lens, all 34mm. First, I checked by the method given by Google in WWGC guidelines: put them before a straight light. Here I got:

IMG_2174

Figure 1. Left 1 – type A, good (line is pretty straight); Left 2 – type B, fairly good; Right 2 – type C, bad ( notice the wavy distortions of the straight line); Right 1- type D, worst.

Second, I drew a black grid on computer, then place the lens before screen (distance around 13cm), then I took the photo.

IMG_2182

Figure 2. Left 1 – type A, good (line straight and sharp); Left 2 – type B, fairly good; Right 2 – type C, bad ( notice the wavy distortions of the straight line, and blur of edge); Right 1- type D, worst.

Besides the distortions showed above, there’s another common issue – incorrect focal distance, see Figure 3.

defocus_01

I mentioned it ever in my earlier post. To do this test, you just put your phone (shut off the screen) or other similar board into the cardboard viewer, as you use it normally. Then put it under a liner or circular light (avoid too much other light sources, a dark room is best, else you cannot get a clear image). Obviously, the right (black) ones defocused. According to Google’s specifications, the lens focal length is 40mm, object distance is 36mm, so there’re 3 possible reasons: one is the incorrect focal length; another is incorrect object distance; or if your viewer uses custom lenses, it didn’t coordinate the 2 parameters properly.

Whew~ finally come to the finish. Thank you for bearing my long and poor text! This is only my personal knowledge. Welcome to discuss with me: gary@adcardboard.com.

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