The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
178 
point of view. In real-life practice, a user’s top priorities, 
typically data quality and project cost-efficiency, may or may 
not be directly fulfilled by the “better” numbers presented by 
the instrument manufacturer. 
Moreover, owing to a lack of generally accepted guidelines for 
lidar performance characterization, lidar system manufacturers 
may choose different methodologies to characterize and to 
present system performance capabilities. That is why it is very 
important for the user of a commercial lidar system to 
understand the underlying technical premises behind values on 
a specification sheet and to make informed decisions to fulfil 
project requirements (Figure 1). This paper will help lidar 
system users to understand the underlying relationships among 
various numbers on an airborne lidar specification sheet and to 
bridge the gap between the “bare” numbers and the expected 
real-life performance capabilities of a lidar system. 
2. DATA COLLECTION EFFICIENCY VERSUS 
PERFORMANCE SPECIFICATIONS 
How fast can the system collect data? How quickly can the 
project be completed? In other words, how cost-efficient is the 
lidar system? Contrary to a mistaken assumption, the most cost- 
efficient approach is not simply to set every operating 
parameter of a lidar system to its maximum capacity. In fact, 
the operating parameter that contributes most pertinently to 
maintaining high density of points and achieving maximum 
area coverage rate is laser pulse repetition frequency (PRF). 
Because of its direct connection with data collection rate for 
achieving survey time cost-effectiveness, PRF has become a 
prime differentiating factor in the marketing of both lidar 
sensors and data collection services (Flood, 2001). However, 
considering PRF as a sole figure of merit without its connection 
to other lidar parameters can be misleading. We will describe 
how different mechanisms used for laser beam deflection and 
scan pattern may affect point density and area coverage rate and, 
hence, the operating parameters and cost-efficiency of a survey. 
2.1 Link: PRF and Scan Patterns 
Several scanning techniques, each with advantages and 
disadvantages, are employed in airborne lidar systems. The 
most common are (a) constant-velocity rotating polygon mirror 
and (b) oscillating mirror (Figure 2). The advantage of a 
rotating polygon mirror is its scan pattern, which appears as 
linear unidirectional and parallel scan lines on the target. 
However, its primary disadvantage is that for a certain period of 
time during each rotation cycle, range measurement is either not 
taken or, if taken, should then be discarded. As a result, with 
this type of scanning mechanism, laser PRF does not equate 
with data collection rate; hence, in most cases, manufacturers 
specify the PRF and data measurement rate separately. 
The oscillating mirror scan mechanism seems to be more 
popular for airborne lidar systems. The mirror is always 
pointing to the ground, and the system’s laser PRF is equivalent 
to its data collection rate. However, laser PRF does not 
immediately translate to area coverage rate for a given point 
spacing because two distinct oscillating scan patterns— 
sawtooth and sinusoidal—manifest two different laser point 
distribution outcomes (Figure 3). In a sawtooth scan pattern, 
scanner velocity is kept constant for most of the swath. This 
gives an almost uniform point distribution across the swath with 
slightly increasing point density towards the scan edges. In a 
sinusoidal scan, point density is the lowest at the centre of the 
swath and grows toward the edges of the scan line. That is why 
a lidar with a sinusoidal scan pattern has to operate at a much 
higher laser PRF to maintain the same nadir point density as a 
lidar with a sawtooth scan pattern. It was shown (Ussyshkin et 
al., 2008b) that at a 1-km flying altitude, 30-Hz scan frequency, 
and ±25° scan angle, a lidar with a sinusoidal scan pattern has to 
operate at a 158-kHz PRF to achieve the same point density at 
nadir as a sawtooth scanner operating at 100 kHz. 
Figure 2. Scan patterns: (a) constant velocity rotating polygon 
mirror and (b) oscillating mirror 
Sinusoidal Pattern, Non-Uniform Sawtooth Pattern, Mostly Uniform 
Figure 3. Sawtooth and sinusoidal scan patterns from an 
oscillating mirror scanner 
Thus, from the user’s point of view, laser PRF cannot be the 
only figure of merit for data collection efficiency since scan 
pattern significantly changes cross-track point distribution and 
affects one of the most important project requirements—ground 
point density. To meet project-required point density on the 
ground and to maximize area coverage rate, the lidar system 
user must carefully consider the choice of scan pattern along 
with laser PRF. 
2.2 Link: Scan Frequency and Scan Angle 
Maximum scan frequency, as specified on a lidar system 
specification sheet, is another very important instrument 
parameter that affects data collection efficiency achievable in 
the field. Again, comparing “bare” numbers of the maximum 
scan frequency may be quite misleading. 
Scan rate for rotating polygon mirror versus oscillating 
mirror: With a rotating polygon mirror, the scan rate is the 
number of scan lines per second (Figure 2a). For example, a 
100 Hz scan rate means that the scanner can provide 100 
parallel scan lines every second. With an oscillating mirror, a 
scan frequency of 100 Hz means that the mirror completes 100