70° D 
reference sensor 
er second. This 
by an order of 
le of the sensor 
ines the field of 
that in order to 
| on one side we 
m. Firstly when 
from four sides, 
i-sensor system. 
yhotogrammetric 
s only 75 mm, 
in be measured 
ight ratio cannot 
established on a 
f thumb that the 
1:16 (refer for 
uhmann, 2000). 
ore not exceed a 
t a larger stand- 
op of each other 
n). 
Figure 2 depicts the two different situations of either a longer 
stand-off distance or a shorter one. The shorter stand-off 
distance requires at least two sensors to be stacked on top of 
each other to achieve full coverage. However it gives the 
advantage of better triangulation accuracy and a more compact 
setup. 
4. SENSOR TESTS 
Since the manufacturer does not specify sensor repeatability and 
accuracy in depth, these quantities have to be established in 
suitable tests. Such tests have been carried out by different 
research labs, for example by (Menna et al, 2011). We have 
designed our own test strategy which separates repeatability (or 
precision) and accuracy. In addition we do not perform the tests 
on a single unit of one sensor model only, but we test several 
units in order to establish variations due to manufacturing 
tolerances. We also consider interference generated from 
additional sensors which overlap the field of view of the sensor 
under test. 
As the rough photogrammetric estimates described above have 
shown, the sensor cannot be expected to provide reliable depth 
measurements at long distances. Thus we keep the distances 
reasonable for all tests following. We aim at measuring objects 
at approximately 1 m distance. 
4.1 Repeatability 
We test repeatability by observing two spheres in the field of 
view of a sensor over time. The sequence of depth 
measurements is recorded and later single frames of the 
recording are extracted and evaluated. The quantity we measure 
is the distance of the two spheres which of course is kept 
constant over the duration of the measurements. Since we are 
only interested in repeatability there is no need for a reference 
value of the distance. 
Figure 3 shows the setup for this test. The sensor tested is the 
leftmost sensor. The distance of the sensor to the spheres is 
approximately 1 m. The distance of the spheres is 
approximately 0.5 m. 
Two further sensors are added to test interference. Since the 
  
  
  
  
two spheres, both with and without interference 
from other sensors. 
sensor uses static pattern projection, two sensors potentially 
generate some interference, when their field of view overlaps. 
This interference can occur in two forms. For one when two 
projectors illuminate a common area the brightness, roughly 
speaking, doubles. This can create sensor saturation and as a 
consequence creates a blind spot on the sensor or a gap in the 
depth measurement. This occurs most often on highly reflective 
surfaces. The API to the PrimeSense NUI sensor allows 
adapting sensor gain to compensate for this. However this is not 
a trivial procedure and is highly dependent on the scene. The 
second form of interference which we are interested in occurs 
when the projected dot patterns overlap and the sensor actually 
miss-matches the sensed pattern with the stored pattern. This 
situation occurs less frequently and it is almost unpredictable if 
it occurs at all or how strong the effect is. 
In order to quantify this effect we place a second sensor at a 
distance of 0.5 m to the right of the sensor under test and 
  
514 
Un 
I 
N 
510 
508 
506 
Distance (mm) 
504 
502 
500 
22 58 71 
Frame # 
  
  
»no interference 
  
interference 0° 
m interference 45° 
103 107 
  
  
Figure 4. Repeatability of the measurement of the distance of two spheres. Three different scenarios are tested: no interference, i.e. 
only one sensor is switched on, interference from a sensor at 0 degree tilt angle and interference from a sensor at 45 
degree tilt angle.