 # Development of a laser slit system in LabView

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Development of a laser slit system in LabView
Students: Paolo Bellandi Emanuele Ferrari Course: Optical Measurements 2007

Objectives Design of a laser slit system, using active triangulation
Implementation of the demonstrator Laser line projector CMOS camera Triangulation geometry Implementation of the measurement procedures in LabView Metrological validation of the system

System layout The camera sensor is positioned at the image plane. The reference systems are (xc, yc, zc), and (u,v). The former is centered at point C; the latter is centered at point O’; it is the central point of the image plane. P e C : exit and input pupils for the projector and the camera respectively H and V: sensor size along u and v coordinates; N and M: sensor resolution along u and v f: focal length od the camera lens Two geometrical parameters. Baseline d and standoff distance L. Their relation is: FW and FH represent the dimension of the Field of View (FOV). FW is the maximum length of the laser stripe. (x,y,z) is the global reference system.

Layout in the x,z plane Similar triangles:
To calculate the value of FH:

Triangulation Since the triangles are similar:
In the presence of the object, ray PO is deviated at Po and impinges at position A on the image plane. Point B is the corresponding point on the reference plane. The height of object point Po is evaluated with respect to the reference plane. The deformation induced by the object shape is evaluated as the difference with respect to the undeformed line on the reference plane. Consequently:

The demonstrator Laser line projector Camera d L f

Parameters evaluation 1
The geometry is: This means : d L f Laser CCD H FW d L f Laser CCD H FW z y x hence:

Parameters evaluation 2
Input values D = 16 cm L = 9.5 cm H = 768 pixels V = 576 pixels Pixel Size = 8,48 um FW = circa 6,5 cm Evaluation of parameter f We have used an objective with f=12mm and a 5mm ring. The corresponding value for FW is 7 cm.

Triangulation again Ray tracing in the presence of an object

Triangulation equations
O’P is evaluated from the measure in pixels multiplied by the pixel size. Then: The vallues of angles b and g: e Note that And

Triangulation equations
Applying the sine theorem to we have Sine triangles are similar and We get: then This is the measurement information we want

Detection of the light pattern
The centre of gravity of the signal acquired along direction v, for each value of the coordinate u must be calculated. The algorithm is: bc : position of the centre of gravity; lc : gray level of the c-element. The method must be iterated for all the columns in the image.

Labview subVi “Calcolo_Baricentri_Sub” (example)
Application of algorithm Acquire image and calculate number of rows and number of columns. Overlay of the centres on the image Image thresholding This subVi outputs the vector of the centres of gravity and the corresponding image Filtering of the input image (preprocessing)

Threshold to trim the detection algoritm
Front panel Live acquisition Freeze image Visualization window Threshold to trim the detection algoritm

Gray level visulatization along a profile line
Front panel Freezed image Gray level visulatization along a profile line

Front panel New acquisition Save image Save the centres of gravity
Evaluation of the measurement information. Triangulation is performed here. Error message if the results are not saved

Results Deformed profile Reference profile: Threshold equal to 229.
Value measured by the system: 4.72 mm Threshold equal to 229. Reference measurement (caliper used): 4.73 mm

Results Botton profiles threshold 3.13 3.23 229 4.29 4.40 9 5.76 5.77
Reference measure (mm) System measure (mm) threshold 3.13 3.23 229 4.29 4.40 9 5.76 5.77 39 3.26 3.91 4.08 21

Results botton Profiles threshold 2.88 2.80 21 3.10 3.12 3.26 3.23 51
Reference measure (mm) System measure (mm) threshold 2.88 2.80 21 3.10 3.12 3.26 3.23 51 3.18 72

Other results (influence of texture and of reflectivity
botton Profiles Reference measure (mm) System measure (mm) threshold 5.11 4.61 112 5.15 3.98 47 4.46 216 12.89 12.99 213

Conclusions For measurement ranges up to 10mm the measurement uncertainty is 0,1mm. The performances can be increased if absolute calibration is performed Camera model Intrinsic and extrinsic parameters must be estimated Projection model The projection plane must be estimated The front panel is user friendly