InViso eye tracker system
Inviso is latin for “Look into”
In viso means “In a perceived object”

InViso eye tracker specification

Content

Design Concept

Backlight requirement

Fresnel mirror

Design phases

Design verification

3D menu

Design Concept

· The eyeglasses are used as an interface between the user and a Android or iPhone application. The app connects through Bluetooth to the eyeglasses.

· 2 LCD displays on the glasses, one for each eye, is used to project a 3D image on a suitable distance in front of the eyes. The glasses in the front are used as mirror lenses to make this possible.

· 2 cameras, one for each eye, are used to measure viewing angle and calculate focus distance.

· The menu system is an application running on an Android or IOS phone that calculates the focus distance and viewing angle. The eyes are used as a “mouse” to navigate in the menus of the application.

· Some of the unique ideas in this design are:

o Eye sight is used as an input to the GUI

o Eye focal length is used as an input to the GUI

o Eyeglasses is a part of the lens-system (mirror lens)

o New type of Fresnel mirror, possible patent opportunity

o New eye scanning technique

Parts

1	Eye camera chip: OMV7690, 640x480. Size: 2.5 x 2.5 x 3 mm
2	Display: KCD-KDCF-AA, 800 x 600. Size: 12 x 9 mm
3	PCB: With Bluetooth transceiver and micro controller
4	Battery:, Lithium Ion, 3.7V, 800 mAh. Size:65 x 22 x 6 mm
5	Lens: Cemented Doublet from Qioptiq. Part.no:322223000
6	Mirror: custom Polycarbonate or Trivex mirror, 50% transparency. Radius = 50 mm
7	LED for Background light: VLMW322BACA5K8L
8	Flex Circuit board: 4 layers
9	Eyeglass frame: half rim Carbon fiber graphite (CFG) with fixed temples

Eye scanner

Below is a picture from the camera view of the right eye.
The red field represents the iris moving area and is used to track the iris leftmost position.
Cubic spline data interpolation can be used to speed up the readings and increase the precision of the measurement.

A threshold is calculated and everything above is consider to be white sclera
Iris X/Y position is measured and used in calculations of viewing angle
The camera is also used to calculate the ambient background illumination.
This is done between frames when the LCD-LED and sclera-LED is turned off.
The sclera white level is measured and used to control the LCD background and for the sclera/iris light source.

Viewing angle

Calculate the eye distance

During installation the iris size is measured (in mm)
During calibration the iris size (HVID) is measured (in pixels)
a = 51.2x640/HVID_pix. ECD=(HVID_mm/2)/tan(a/2) + eye_radius

Simplified example how to calculate the viewing x-angle

Eye radius = 12.2mm
Iris size (HVID, normally 11.8mm).
a = Eye center angle (calculated during setup & calibration).
ECD = Eye center distance (calculated during setup & calibration).
c = tan^-1(tan(b-a)(ECD/Eye radius - 1))
d = sin^-1(Iris size/2Eye radius)
x = 90-a-tan^-1(tan(b-a)(ECD/Eye radius - 1))-cos^-1(Iris size/2Eye radius)

Focal distance

Calculating Focus distance

Mean interpupillary distance (IPD) for adults is 63mm.
The users IPD distance is determent during setup & calibration.

Focus distance is calculated according to the formula below.
Focus distance is a vital part of the menu system.

Calculating Focus distance error

The camera, OMV7690, have an viewing angle of 51.2° and a resolution of 640x480
The effective measuring angle is only 20°, reducing the resolution to only 200 pixels.
The angular resolution = (20° x 640pix)/64° = 200pixels or 60°/200pix = 0.3°.
At 50cm the error is ±2.275cm.
At 100cm the error is ±22.0cm.
At 200cm the error is ±99.6cm.

Bluetooth

The graphics for the menu system is transferred to the eyeglasses using Bluetooth from an Android or IOS phone.

Bluetooth requirement

· Bluetooth v3.0 + EDR (Enhanced Data Rate) Class 3 (5m)

· Max transfer rate = 24Mbit/s thru 802.11 link

· Profile: Basic Imaging Profile (BIP) “Image Push” or “Remote Display”, if possible

Bluetooth Bandwidth Requirement

· Image resolution: 800 x 600 pix.

· Update frequency/speed: 5FPS (Frames Per Second) or 0.2s delay

· Image size 800 x 600 x 3(24bit) x 5FPS = 72Mbit/s! (uncompressed)
Maximum throughput with bluetooth is 24Mbit. (same data is transferred to both display, image + distance). This requires a compression of more than 2:1. In the table below there are three different standards to compress images.
Decompression needs a lot of processing and the microprocessor in the glasses needs to run slow with leaves PNG and JPG as an less attractive option.

Type	Pros.	Cons.
GIF	· Image is lossless (8-bit) · small amount of serial data flash space · decompr. fast so image displayed quickly · supports binary transparency · good for drawings	· not for full color photos · 256 different colors per image
JPG	· uses much less serial data flash space · full 24-bit color · good for photos	· decompression time is long · no transparency · compression can be lossy
PNG	· full 8-bit transparency · full 24-bit color · good for photos	· large serial data flash space · decompression takes time

Example Image 800 x 600	Format	Compressed	Ratio
	BMP	1.83MB (Uncompressed)	1:1
	GIF	75kB	1:24
	JPG	15kB	1:120
	PNG	170kB	1:10

GIF will reduce the colors to 1 byte 256 (colors) and compress the image up to 24 times
24Mbit/75kB = 32 FPS (worse case with all bits drawn)

Backlight requirement

Location	lumen	lux (lumen/m²)
100 watt bulb	1200	120 (10m²)
Living room		100
Bright Room		400
Daylight cloudy sky		5,000
Daylight clear sky		10,000
Daylight bright sunlight		20,000
Direct Sunlight		110,000
Full moon		0.01

· lumen = 1-cos α x 2 x π x candela (α = half light angle)

· lux = candela/distance²

LED	Intensity	Lumen	Lux @ 1cm² (LCD chip)
VLMW322ABBB5K8L	2850 mcd 120°	9	83 000
VLMW322BACA5K8L	3550 mcd 120°	11	100 000
BL-1210SMD-W120	7000 mcd 120°	22	220 000

· The above calculation doesn’t take into account the light attenuation when reflected through the mirror and the LCD backlight diffusion layer

Fresnel mirror

The mirror solution has a big disadvantage due to its unattractive appearance and bulky size.
A fresnel lens can be used to reduce the size and make the glasses more flexible.
The interlaced edges in the fresnel mirror has just a small impact of the visibility because the
edges are located out of focus and therefore becomes blurred and invisible.
The Fresnel lens consists of two parts, one with a reflective coating surface and the
other with an inverted shape with same refraction index. This design will provide a perfect
“see thru” combined with the reflective mirror lens from the unique fresnel design.
The reflective light from the opposite side comes from an IR light source used to
light up the iris for the eye camera.

Design verifications

WinLens 3D is a freeware program used to calculate lens and mirror sizes.
Below is a simplified drawing of the design. The lower left lens represents the eye.

A prototype with an similar design as above have been made
Two mirrors (eyeglasses) with two displays for each eye showing the text “Menu” floating approximately 40cm in front of the eyes. See below.

3D menu

Types of activation

Both eyes blink activation
Both eyes blink for “press down” same for release.
One eye blink activation

One eye for “press down” the other for release.
One eye for “press down” same for release.

· Eye focus-delay activation
Stare at the same location for short time activates “press down”.
Stare at the same location for longer time activates “release”.

· Head nod activation (g-sensor requirement)
Head nod activates “press down”.
Head nod the second time activates “release”.

· Head nod direction sensitive activation (g-sensor requirement)
Head nod forward activates “press down”.
Head nod backwards activates “release”.

· Chew activation (g-sensor requirement)

Chew/bite activates “press down”.
Chew/bite the second time activates “release”.

· Head Switch activation
Pressed switch activate “press down” release for “release”.

· Eye gesture activation

Drawing symbols activates different functions/programs.

· Nose poke activation (g-sensor and gyro)

A virtual stylus is moved with the head and used to activate menus.
(actually only the menu is moved for this illusion)