XXIV ICTAM, 21-26 August 2016, Montreal, Canada
IS COMMERCIAL CCD/CMOS CAMERAS TRUSTABLE FOR PHOTOMECHANICS?
Chao Wang, Qinwei Ma, Hao Gu, Shaopeng Maa),
School of Aerospace Engineering, Beijing Institute of Technology, Beijing, China
Summary The parasitic deformation of the images from a digital camera in long-time measurements is investigated, and the heat-induced image
expansion and pixel motion are observed. The start-up time differences of multiple high-speed cameras with synchronous triggering are measured,
and the possibility of occurrence of unneglectable start-up time differences is calculated. The measuring results show that commercial CCD or
CMOS cameras cannot faithfully record the image data in photomechanics measurements with high-precision requirements. The systematic errors
from image recording should be compensated.
INTRODUCTION
Commercial CCD or CMOS cameras are very popular equipment for image acquisition in daily life and industrial
applications. After compensation of the lens distortion, cameras are thought to be capable to record the real scenes and then
are trustable in these circumstances. In the previous applications of photomechanics, this is also thought to be true. The raw
image data consisting of the pixel grey matrix series and corresponding time series for imaging are directly used for
deformation field evaluation and further mechanical analysis. Recently, some studies [1-3] reported that the image captured by
commercial CCD or CMOS cameras might contain slight systematic errors. For long-time measurements, it is found that there
is a parasitic expansion to the image because of the self-heating or/and the environment temperature increasing [4-6]. Although
it is slight enough and could be neglect in most other applications, it may produce apparently systematic errors on
photomechanics methods. For high speed measurements, it is observed that there may exist start-up difference among multiple
high-speed cameras even though when they are triggered simultaneously. This may produce apparent errors on time for the
high-speed images series when multi-cameras are used, for example the stereo-vision applications. This paper introduces the
two kinds of errors firstly and then confirms that the commercial CCD/CMOS cameras are not trustable for photomechanics
if the compensation techniques are not considered.
HEAT INDUCED IMAGE ERRORS OF CAMERAS IN LONG-TIME MEASUREMENTS
In the long-time measurements, temperature of the camera may vary because of the self-heating and/or the variation of
the environment temperatures (as shown in Fig. 1a). Fig.1b shows that the temperature variation could induce the structural
deformation of the camera, leading to the change of the equivalent optical path (as shown in Fig. 1c). The image deformation
caused by the optical path changing is investigated as follows: 1) Capturing the speckle images of an unloaded board using
IMPERX 12M-3L digital camera, and recording the temperatures of the camera simultaneously. 2) Analysing parasitic
displacement and strain fields from the images. Fig. 2a shows that the self-heating could induce a strain error of 200 με with
about 12 °C temperature increment (about 15 με/°C). The displacement errors (see Fig. 2b) is up to 0.5 pixels, which is
partially caused by the image expansion expressed by the parasitic strain and also result from the heat induced rigid motion
of the CCD chip (see Fig. 2c).
Fig.1 Heat induced image error of CCD/CMOS camera. (a) the temperature variation of the camera case, mount and lens within 5 hours,
(b) the motion of the different parts of the camera, (c) the changes of the equivalent optical path, (d) the parasitic strain, (e) the typical
parasitic displacement field, and (f) the schematic of the rigid motion of the CCD chip.
a)
Corresponding author. Email: [email protected].
START-UP TIME DIFFERENCE ERROR FOR MULTIPLE HIGH-SPEED CAMERA SYSTEM
In a high-speed stereo-vision system, dual high-speed cameras are used. The two cameras are triggered simultaneously
and then is regarded starting recording images at the same time. A system as shown in Fig. 3a is used to check if the two
cameras are exactly synchronized. It is found that sometimes there exists start-up time differences (STD), which means that
the two cameras do not start recording the images simultaneously with a certain probability. For a typical high-speed cameras
pairs (two Photron SA1.1 cameras manufactured in different years), systematic investigation shows that the possibility of
occurrence of apparent start-up time difference is about 13%, and the STD is up to several hundreds microseconds (as shown
in Fig. 4a). This error is not very critical for other applications, but it is unacceptable for photomechanics. As shown in Fig.
4b, the possibility of the occurrences of STD of different dual-camera imaging system is counted according to the similarity
of the two cameras. The lower similarity, the greater of possibility of occurrence of STD. Synchronizing cable is helpful for
reducing the percentage of occurrence of STD but cannot eliminate STD completely.
Fig.2 STD measurement and statistic for dual high-speed camera system: (a) the schematic of the experimental setup triggering the two
cameras simultaneously and capturing the laser point with the sinusoidal intensity changing, (b-c) two typical intensity variation analysed
from image series recorded by the two cameras, (d) distribution of STD for one dual high-speed camera system, and (e) percentage of
STD for different assemblies of Photron high-speed cameras (I: SA1.1-SA2; II: SA1.1-SA2 with synchronizing cable; III: SA5-SA5; IV:
SA5-SA5 with synchronizing cable; V: SA1.1 (made in 2009) - SA1.1 (made in 2013) with synchronizing cable).
CONCLUSIONS
For standard digital camera used in long-time measurements, there exists the heat-induced image expansion as well as the pixel
motion. The strain error induced by the image expansion is up to tens με per °C, and the displacement error could be up to pixel
level. For dual high-speed cameras used in stereo vision system, there exists start-up time difference and thus error on the time
history of the image series from the two cameras, even though when they are triggered simultaneously. Images are the raw
information of photomechanics measurements, thus the errors consisting of the parasitic deformation and the mismatching of time
history of the image series could inevitably produce measurement errors with photomechanics methods. Therefore, commercial
CCD or CMOS camera are not trustable enough for photomechanics. To perform reliable photomechanics measurements, the
errors should be evaluated and the image data should be corrected. For the heat-induced image error, an unloaded specimen placed
in the same field of view with the measured specimen could be used to eliminate the errors. For the high-speed measurement using
multi-cameras, a specially designed independent, high-frequency timer may be used to align the different image series.
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