Introduction – Answering a Simple Question
In response to Hololens 2 Display Evaluation (Part 1: LBS Visual Sausage Being Made), someone asked, “How does the Hololens 2 (HL2) compare to the Hololens 1 (HL1) in terms of image quality.” I had read someone else comment that while he like many of the new features and ergonomics of the HL2, he though thought the image quality of the HL1 was much better.
I decided to fire up the Hololens 1 and see for myself. Sure enough, the Hololens 1 was dramatically sharper. I decided to shoot pictures of the HL1 display that were as close as possible to the conditions I shot the HL2. I used the exact same lens, camera, and test pattern.
The pictures below give a fair impression of how blurry/soft the HL2 is relative to the HL1. The difference is dramatic when you go directly from wearing an HL2 to an HL1, the HL1 is so much sharper. After looking through the HL1, the HL2 is like looking through a fogged-up window.
HL2 Uses Laser Beam Scanning (LBS) and the HL1 Uses LCOS
For those that don’t know, the HL2 uses laser beam scanning (LBS) and the HL1 uses LCOS microdisplays as the display device. The last article explained the HL2’s LBS process. There is plenty of information available on how LCOS (Liquid Crystal on Silicon) works (such as here).
On the HL2, the image was sized and the headset positioned such have one scan line per pixel in the test pattern in the center of the image (see part 1). On the HL1, the headset was positioned to have one pixel in the image equal to one pixel in the test pattern.
Below is a side by side comparison taken with the same camera and lens with very similar setups. If you click in the image below, it will open a larger but still scaled down by about 4X version. Even scaled-down, the HL1 is noticeably sharper. The HL2 FOV is slightly wider than the lens such that about 6% of the right side of the HL2’s FOV is cut off, but the lens roughly equals the HL2’s vertical FOV.
Below are some full camera resolution image crops from the center of the pictures above that show the sub-pixel-size detail. The leftmost image shows a single field taken at 1/125th of a second from the HL2 using the same focus as the middle HL2 image which averaged 15 fields over 1/8th of a second. The rightmost image is a crop from the HL2.
If you put the above image on a monitor and back away, you will get a good idea of how it looks in real life. If anything, this comparison benefits the HL2 as the HL2 also has temporal artifacts including flickering edges and rolling effects from the 4-way (plus) shifting.
Below are the HL2 and HL1 images in just green to better show the difference in resolution.
In the next set of image crops, the HL2 image has been resized to match the pixel sizes in the test pattern between to the HL1. A version filtering green has been included.
The HL2 Has Less Than 20 Pixels Per Degree
As reported last time, the Hololens 2 has about 854 scan lines in the middle of the screen and the resolution is less than the number of the scan line. The HL2’s vertical FOV is about 30 degrees so they only get 854scanlines/30_degrees = ~29/scan lines per degree. As I wrote, the resolution is more like 600 lines or 20 pixels per degree for the HL2. The HL1 has 720 pixels vertically over about 17.5 degrees or about to roughly 41 pixels per degree (41 ppd).
With 60 arcminutes in a degree, 20 ppd is 3 arcminutes-per-pixel (3 am/p). Human vision is a little better than 1 am/p (60 ppd) or roughly the same as what Apple called a “retinal display.” Most people will agree that 1.5 (40 pixels/degree) is good enough for most purposes. As the resolution drops to 2 arcminutes/pixel (30 ppd), people can notice problems with screen door effects or a blurry image.
Microsoft’s Alex Kipman statement on HL2: “47 pixels per degree of sight is an important number to remember“
In the Hololens 2 announcement in February 2019, Microsoft’s Alex Kipman made claims about the resolution that appear to be provably false. Quoting (with added highlights) starting at 29:55 in the video:
47 pixels per degree of sight is an important number to remember. Now, this is important because this is what allows us to read an 8-point font on a holographic website. This is what allows us to have precise interactions with holograms and ultimately, this allows us to create and be immersed in mixed reality.
Hololens 1 is the only headset in the industry capable of displaying 47 pixels per degree of sight. And today, I’m incredibly proud to announce that with Hololens 2, we more than doubled our field of view while maintaining 47 pixels per degree of sight on Hololens.
To put it in perspective, and to highlight the generational leap, this is the equivalent of moving from 720p television to 2K television for each of your eyes. Now, no such technology exists in the world. So, in the same way, we had to invent time of flight sensors for connect, waveguides for Hololens, and holographic processing for the edge, with Hololens 2 we invented industry-defining mems display. Now, these are the smallest and most powerful efficient 2K displays in existence.Alex Kipman, Microsoft at MWC19 Barcelona
And these were not some off-the-cuff remarks or misquotes. There were prepared visuals to go along with each claim as shown with clips from the video below.
Let’s start with the easy one. The Hololens 1 has a vertical FOV of about 17.5 and has a display with 720 pixels. Dividing 720p by 17.5° gives 41 ppd so he is fudging to say the HL1 had 47 pixels-per-degree. One could say this is within the “marketing margin of error.”
But as the pictures viewed through the HL2 prove, the HL2 has less than half the claimed resolution. They could try to play some word games as they did with the 2X FOV (more on that in a bit), only he also stated that an 8-point font would be readable. There is absolutely no way that when displaying 47 pixels per degree than an 8 point font, as commonly defined, would be readable by anyone. BTW, the default font on the Microsoft Edge browser is the size of the 16-point font in this image.
The images below were taken with the same test pattern. The top image shows the HL2 viewed at 29 pixels/degree (setting the test pattern height to equal the number of scan lines). The second image below shows the HL2 with the test pattern set up to display at 47 pixels/degree. And the final picture shows the first generation (HL1) viewed at 41 pixels/degree (pixels in the text pattern equal to pixels on the LCOS display). The HL1 image gives an idea of what 8 point text should look like and note it seems sharper and more readable than the HL2 at 29 pixels/degree.
The test pattern above is the first I have shown with some color content. The pictures give a hint that there are problems with color as well with the HL2’s LBS display (a subject for a later article).
Kipman’s saying, “this is the equivalent of moving from 720p television to 2K television for each of your eyes” is just absurd. Commonly, 2K would mean 1920 by 1080p. The HL1 does have about 720p resolution (but the color uniformity is very poor). But with the HL2 the resolution is lower than the HL1. The HL2 has about 2.4X less effective horizontally and 1.8X less vertically than “2K.” Or since Kipman likes to use area calculations (see next paragraph), less pixels buy a more than a factor of 4X.
The “more than 2X field of view” fib was covered by many websites back in Feb. 2019. Kipman later “clarified” that he was talking about “area” when everyone else measures FOV linearly (see my Hololens 2 First Impressions: Good Ergonomics, But The LBS Resolution Math Fails!, RoadtoVR, and UploadVR among others).
Conclusion & Comments: Why the Unforced Error By Microsoft?
Hopefully, pictures speak for themselves and reinforce what was written in Part 1, namely that the effective resolution of the HL1 is something less than 800 by 600 pixels. Others were so focused on the Kipman’s misleading (being generous) 2X FOV improvement over the HL1 statement that they never considered that the statements before and after it were categorically false.
In February 2020, most people that took the Hololens 2 to task on the “2X FOV,” assumed that the did it to make it sound like the HL2 FOV was significantly better than the Magic Leap One when it was about the same (see image on the right from UploadVR)
It is less obvious why they out and out lied about the resolution and I can only speculate. On the surface, it seems like an unforced error. Maybe they were trying to sound better than Magic Leap again. Maybe it was just seen as “good” marketing. Maybe they were trying to convince people, perhaps upper management at Microsoft, that they made the right choice to go with laser beam scanning (LBS) displays and spend hundreds of millions of dollars on development and manufacturing setup when they could have simply bought much higher resolution displays from multiple companies. One thing for sure, there were many people at Microsoft that know they were lying about the resolution.
Also, please don’t buy into the Emperor’s New Clothes argument that was Tweeted By Kipman that the HL2 does something magical that only works when directly seen by your own eyes. Yes, a camera works differently that the human visual system. With careful picture taking, you can get representative images, as shown above.
Appendix: The Photography Setup
The same camera (Olympus E-M1 Mark III) and lens (Olympus 25mm F1.8 prime lens) were used. The camera was located relative to the respective headset to give the best image possible. The HL2 was shot at ISO100, F11, and 1/8th of a second to average out as much of the pixel-shifting/wobulation (see part 1) for the “Interlaced” Test Pattern. The color pictures the text and “Elf” with the HL2 were shot at 1/15th of a second at F8 and ISO100. The HL1 was shot at ISO200, F8, and 1/30th a second to average out the color field sequential effects. The pictures were all shot with manual focus. All images were shot in the Olympus RAW (.ORF) format and were converted to JPEG just before uploading.