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Video Scanning Hartmann 

Optical Testing of State-ofthe-Art Parabolic Trough 

Concentrators 

Preprint 

T. Wendelin 

National Renewable Energy Laboratory 

K. May 

Industrial Solar Technology

R. Gee 

Solargenix

To be presented at Solar 2006 Conference (ISEC ’06) 

Denver, Colorado 

July 8–13, 2006 

Conference Paper 

NREL/CP-550-39590 

June 2006 
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Solar 2006 Conference 

(ISEC’06) 

July 8-13, 2006 

Denver, Colorado USA 

ISEC2006-99172 

VIDEO SCANNING HARTMANN OPTICAL TESTING OF STATE-OF-THE-ART 

PARABOLIC TROUGH CONCENTRATORS 

Tim Wendelin 

National Renewable Energy Laboratory 

Golden, CO 

Ken May 

Industrial Solar Technology 

Golden, CO 

Randy Gee 

Solargenix 

Arvada, CO 

ABSTRACT 

Significant progress has been made recently in 

solar parabolic trough technology development and 

deployment. Part of this success is due to the changing 

world energy scenario, and the recognition that viable 

renewable energy technologies can play a role in 

supplying world energy needs. Part is also due to 

ongoing collaborative efforts by industry and the 

concentrating solar power subprogram (CSP) of the 

Department of Energy’s (DOE) Solar Energy 

Technologies Program to enhance the state of the 

technology in terms of both cost and performance. 

Currently, there are two trough concentrator projects that 

the DOE CSP subprogram is supporting. One company, 

Solargenix, is developing a design to be used in a 64MW 

plant outside of Boulder City, Nevada. This design is 

based on the original LUZ LS-2 trough concentrators 

employed at the Solar Electric Generating Systems 

(SEGS) plants in Southern California. Another company, 

Industrial Solar Technology (IST), is working on a scaleup of their design used historically for process heat 

applications. Very different from the LS-2 approach, this 

design is still in the research and development stages. 

One way in which the DOE CSP parabolic trough 

research assists industry is by providing optical testing 

and qualification of their concentrator designs. This paper 

describes the Video Scanning Hartmann Optical Test 

System (VSHOT) used to optically test both of these 

designs. This paper also presents the results of tests 

performed in the past year, and what impact the testing 

has had on the developmental direction of each design. 

NOMENCLATURE 

X = coordinate along parabolic trough length 

Y= coordinate across parabolic trough aperture 

Z= direction of parabolic trough optical axis 

k = number of VSHOT data points 

Bi,j = coefficients of mathematical equation used to 

describe surface contour 

fy = focal length of trough 

INTRODUCTION 

Over the past two years, significant progress has 

been made in the development and deployment of solar 

parabolic trough technology. This progress has been 

driven in part by the rapidly changing world energy 

scenario, and the recognition that viable renewable 

energy technologies can play a role in supplying global 

energy needs. It is also due to ongoing collaborative 

efforts by industry and the concentrating solar power 

subprogram (CSP) of the Department of Energy’s (DOE) 

Solar Energy Technologies Program to enhance the state 

of the technology by reducing cost and increasing 

performance. Currently, there are two industry trough 

concentrator development efforts that the DOE CSP 

subprogram is supporting. 

Solargenix is developing a concentrator to be used in a 

64MW plant outside of Boulder City, Nevada. This 

concentrator geometry is roughly patterned after the LUZ 

LS-2 trough concentrators employed at the Solar Electric 

Generating Systems (SEGS) plants in Southern 

 1 
California. Solargenix recently built a 1 MW Organic 

Rankine Cycle plant for Arizona Public Service that 

utilizes the first generation of their concentrator design. 

The 64 MW plant, scheduled for completion in early 2007, 

will utilize the next generation design. Solargenix has a 

subcontract with the National Renewable Energy 

Laboratory (NREL) as part of DOE’s CSP subprogram to 

assist them in the development and deployment of their 

technology. 

Industrial Solar Technology Corporation (IST), is working 

on a scale-up of their concentrator, which they designed 

originally for process heat applications. This design 

differs significantly from the LS-2 approach, and is still in 

the research and development stages. IST also has a 

subcontract with NREL to help in the development of their 

technology. 

Another way in which the DOE CSP parabolic trough 

research assists industry is by providing optical testing 

and qualification of their concentrator designs. 

Specifically over the last year, the Video Scanning Optical 

Test System (VSHOT) has been used to optically test 

multiple versions of the Solargenix concentrator as well 

as the IST baseline design. This paper describes the 

VSHOT system as it relates to trough concentrators and 

presents the Solargenix and IST test results for the past 

year and what impact this testing has had on the 

developmental direction of each design. 

VSHOT DESCRIPTION 

The VSHOT is a laser ray trace system designed 

to characterize the optical surfaces of solar 

concentrators. Originally designed to test point-focus 

concentrators1

 (sometimes referred to as dish 

concentrators), it has been modified to include optical 

characterization of line-focus (or trough) concentrators. 

Changes have also been made to increase the speed 

and improve the functionality of the system in the field. 

The VSHOT utilizes computer-controlled laser scanning 

and digital camera image acquisition to provide optical 

surface contour data and mathematical/statistical surface 

representation. 

Figure 1 illustrates schematically the VSHOT system in 2-

D. 

Figure 1. Schematic of the VSHOT system and test 

article with unknown curvature. 

The heart of the VSHOT system is a Class IIIa HeliumNeon laser that emits a visible beam at a wavelength of 

633 nanometers (red light). The Class IIIa laser is not 

focused and therefore does not require any significant 

hazard controls. 

The emitted laser beam passes through a computercontrolled shutter and proceeds into a turning mirror, 

which redirects the beam into a computer controlled X-Y 

scanning unit. This unit subsequently outputs the beam 

anywhere inside an approximately 80o

 full cone angle. 

The complete laser-scanning unit is mounted onto a 

movable, adjustable tripod for alignment purposes. This 

unit goes through a one-time calibration process in which 

the digital position (0 to 65,535) in both the x and y 

directions is mapped to the angles of the outgoing beam 

in both the x and y directions. The outgoing beam from 

the laser scanner passes through a hole in a flat square 

target attached to the front of the laser scanner assembly. 

The beam is directed to a user-defined point (via 

computer control) on the test article. The test article 

reflects the beam back towards the laser scanner. 

Ultimately the reflected beam hits the large square target 

mounted to the front of the laser scanner tripod assembly. 

Target 

FireWire 

Camera

Test 

Article 

He/Ne 

Laser 

Laser 

Scanner

Tripod 

Camera 

Support 

Arms 

Control 

Computer 

Shutter 

Laser 

“Spot”

 2 
When the reflected beam from the test article intersects 

the white front surface of the target, it results in a red 

“spot” on the target. The “spot” size and shape is a 

function of the microscopic surface quality of the 

reflecting test article. The location of the spot centroid on 

the target is a function of the macroscopic surface 

contour, position and orientation relative to the VSHOT 

optical axis and source position. A digital, computercontrolled FireWire camera mounted to the front of the 

target via adjustable support arms looks back at the 

target and images the entire target including the laser 

“spot”. The camera goes through a calibration process 

similar to the laser scanner in which the camera’s digital 

pixel array is mapped to the Cartesian coordinate system 

of the flat target. This is done once for each initial setup 

of the VSHOT. Thereafter, the support arms lock the 

camera in place relative to the target. Camera calibration 

is not needed again unless the camera is repositioned 

relative to the target. This is a recent refinement, allowing 

more rapid data collection in the field. It should be noted 

that the system has been used when the ambient 

temperatures are comfortable enough for the user. To 

date, the system has been used successfully between 

50o

F and 85oF. VSHOT tests have been attempted at 

temperatures approaching 100o

F and beyond, but the 

laser scanner electronic control did not operate in a 

stable fashion at these temperatures. The system 

operators did not fair well at these temperatures either. In 

these cases, testing was performed later in the day after 

temperatures had fallen into the above range. Both the 

laser scanner and the video camera are very stable over 

this temperature range. A complete analysis of the 

measurement uncertainty has been performed and 

described in a previous paper.2

DATA COLLECTION PROCESS 

The laser scanning system is assumed to have 

already gone through the one-time calibration where 

outgoing beam angles are mapped to the digital positions 

of the X-Y scanning mirrors. As mentioned previously, 

the VSHOT was originally designed to test point-focus 

optics. The nature of a point-focus concentrator is such 

that the entire aperture (in X and Y) can be tested, 

because the return beams all come back to a practically 

sized target. However, it is not practical to test the entire 

aperture of a trough concentrator with this system, 

because the single curvature optics would require an 

extremely large target. The VSHOT can be used, 

however, to do profiles (vertical slices) across the 

aperture width at multiple locations along the 

concentrator length. The analysis is then restricted to the 

vertical or Y direction contour. Error in the horizontal or X 

direction is not considered, because errors in this 

direction result in rays that should still hit the receiver – 

just left or right of where they would have otherwise. 

Many such profiles can be averaged to yield a good 

characterization of the overall optical performance. 

Individual profiles provide detailed information concerning 

mirror facet contour and alignment. 

The trough concentrator is pointed toward the horizon. 

The VSHOT is positioned at a little greater than twice the 

expected trough focal length away from the trough vertex 

(2f). This distance is measured with a laser range finder. 

With the point source at this distance, the extent of the 

return “spot” pattern just fills the target (for an ideal 

parabola). The system is leveled, and the height is 

adjusted to match the height of the concentrator vertex. 

The ideal trough geometry (i.e. aperture width and focal 

length), the region of interest and the number of data 

points to scan inside that region are defined in the 

VSHOT control software. Via control software, the 

operator then directs the computer-controlled laser 

scanner to draw an outline of the region of interest 

(vertical strip) on the parabolic trough concentrator. The 

pattern of return “spots” is observed on the target to verify 

that the VSHOT system is properly aligned and centered 

on the test article, and that the pattern fills the target. If 

this is not the case, then appropriate adjustments are 

made to accomplish this (i.e. tilt and/or translation of the 

trough concentrator and/or the VSHOT). Because the 

process centers the return distribution on the target, the 

overall tilt of the concentrator is minimized. The data then 

provide the best assessment of the contour itself, which is 

the desired result. 

Once the VSHOT system is aligned with the optical axis 

of the trough, the position and zoom of the camera are 

physically adjusted to image the target. The camera is 

then locked into place using the support arms. A 

calibration using a target grid is performed to map the 

spatial X-Y coordinate system of the target plane field of 

view to the camera’s pixel array. The camera is now 

physically tied to the target eliminating the need to redo 

the camera calibration every time the complete system is 

moved. Various camera parameters, including aperture 

size and integration time, are then adjusted so that the 

camera “sees” only the return “spots” on the target and 

not other extraneous light sources. If tests are done in 

the field during the day, a He/Ne pass-band filter may be 

used to eliminate transient lighting conditions. 

Once the setup and pertinent calibrations have been 

accomplished, the operator begins the test. The 

computer tells the laser scanner to direct the beam to the 

first data point on the test article. The digital camera is 

then directed by the computer to take an image of the 

target with the reflected “spot”. A previously obtained 

background image is subtracted from the image with the 

laser “spot” and the imaged is further thresholded by a 

previously determined value. What remains is an image 

 3 
consisting of the laser “spot” on a black background. 

During the day, a He/Ne passband filter is used to 

eliminate transient background effects, so this 

background does not change significantly with time. 

However, if lighting conditions have changed significantly, 

or if the filter is not used, a new background image is 

taken periodically. The computer analyzes the image and 

calculates the spatial position of the ”spot” centroid in the 

target plane’s X-Y coordinate system. Once the 

computer is satisfied that the data point information has 

been properly recorded (i.e., outgoing laser direction 

angles, return “spot” location coordinates on the target 

and “spot” intensity), it directs the laser scanner to 

proceed to the next data point. This procedure is 

automatically repeated by the computer until the entire 

region of interest has been scanned. The number of 

datum points depends on the quality of the surface being 

tested. For glass trough mirrors, multiple tests have 

shown that a spacing of 0.5 inches is sufficient to capture 

the relevant features. In actuality, the region of interest is 

not a one-dimensional vertical strip, but rather a narrow 

rectangle the width of which equals the desired spacing. 

This is an artifact of the original point focus analysis. The 

vertical error analysis is not affected. Thus for every 

vertical position there are two data points side by side. 

For a trough with a 5-meter aperture width this translates 

into roughly 2000 data points. Upon test completion, the 

computer prompts the operator to save the data to a file 

for subsequent analysis. This completes the data 

acquisition process. The operator can then proceed with 

the data analysis part of the VSHOT software if desired or 

move down the trough concentrator length to the next 

location for a new data profile. 

DATA ANALYSIS 

The raw data is processed to provide useful 

information concerning the optical surface shape. 

The data file contains the geometry setup information 

including the measured distance of the target from the 

trough vertex and the desired focal length assuming an 

ideal parabola. The outgoing beam angles and the X-Y 

position of the return spot on the target are recorded for 

every data point. For each data point, the computer 

calculates where on the target the “spot” should be 

located, if the trough contour where ideal. For each data 

point, two surface slopes are calculated: one for the ideal 

shape and one from the actual position of the return 

beam on the target. The difference is calculated for each 

data point and included in a root-mean-square calculation 

to provide an overall value of the departure of the actual 

contour from the ideal. 

The actual surface is described mathematically by the 

following equation3

: 

jij

k

i

i

j

ji yxByxZ −

= =

= ∑∑

0 0

, ),(

An iterative least-squares process is used to minimize the 

overall R.M.S slope difference (error) resulting in the 

best-fit equation. Since the ideal trough concentrator is a 

parabola, only a 2nd order fit is used. Thus, the 

coefficients that are allowed to change include the terms 

B1,0, B1,1, B2,0 ,B2,1 and B2,2. In the case of a single-axis 

concentrator, only the B1,0 term = tilty and the B2,0 = 1/(4fy) 

terms are relevant. The user can choose to fix either or 

both of these, if desired, or allow all terms to adjust. The 

R.M.S. slope error is then a useful figure of merit for 

comparing the best-fit shape with the desired ideal 

parabola. Again, only the vertical component of the slope 

error (dZ/dY) is of interest, so the two directional 

contributions to the error are separated. In addition to the 

best-fit equation and the R.M.S. slope error, the 

difference in vertical slope between actual and ideal for 

each data point can be plotted as a function of Y. This 

provides immediate and informative feedback as to the 

departure of the actual contour from the ideal. 

Figure 2 shows the summary of a VSHOT fit analysis for 

a typical trough test: 

 4 
Figure 2. Example of VSHOT fit analysis for typical 

trough test.

TEST RESULTS FOR SOLARGENIX 1ST AND 2ND

GENERATION DESIGNS 

Solargenix has been developing a trough 

concentrator over the past several years with the goal of 

manufacturing and installing it in their first power plants. 

Solargenix began with the basic geometry of the LS-2 

concentrator, manufactured and installed by LUZ at the 

SEGS plants in Southern California. Solargenix has 

maintained this geometry throughout the evolution of their 

design. This choice was based largely on industry and 

operational experience with the LS-2 concentrator. The 

LS-2 has demonstrated good performance and 

maintainability since it’s installation in the 1980’s. 

However, Solargenix has made significant changes to the 

support structure and drive system with the goal of 

reducing cost while maintaining or improving 

performance. Also, Solargenix has doubled the overall 

length of its Solar Collector Assembly (SCA) compared to 

the LS-2. The Solargenix design incorporates the same 

glass mirrors used in the LS-2. Solargenix has looked at 

different mirror constructions including composite and 

metal substrates4

, but these approaches require more 

development and will not be pursued for use in their initial 

plants. 

A prototype of their first design iteration was built and 

tested with the VSHOT in 2004. This was the first such 

VSHOT test performed outdoors and demonstrated the 

usability of the instrument under these conditions. These 

tests demonstrated that the basic design was capable of 

meeting their optical specifications, but also that 

improvements could be made that would reduce part 

count, installation time and cost, as well as improve 

performance. Specifically, individual panel 

tilt/misalignment was observed, which could be minimized 

through adjustments. The source of this tilt needed to be 

understood in order to minimize or eliminate it in 

subsequent designs. 

A full SCA test string utilizing this design was installed In 

Eldorado Valley near Boulder City, Nevada. This site 

has proved invaluable to Solargenix in the development 

and testing of their concentrator. In February of 2005, 

VSHOT tests were performed on the 1st generation 

design. The goal of the VSHOT tests was to further 

evaluate the optical quality of the design and determine 

whether additional field adjustments might be warranted 

to achieve additional optical accuracy. 

Testing occurred over the course of a late 

afternoon/evening session and a daytime session. At this 

time, only a limited number of mirrors had been installed 

on the concentrator. The Solargenix trough module 

aperture width spans four separate glass panels, two 

identical inner panels and two identical outer panels 

installed symmetrically on both sides of the vertex. 

Vertical VSHOT data profiles spanning the aperture width 

were collected on three separate columns of glass mirror 

panels. The VSHOT then analyzed this data to determine 

overall optical shape relative to the ideal parabolic 

contour and also the shape and tilt of each individual 

panel relative to the ideal. Results indicate that, while 

there were modest variations in the three sets of panels, 

as installed, panel alignment/tilt was now within 

Solargenix’s specification. Based on this result, field 

adjustments were not expected to be necessary, however 

more testing would be required to understand the source 

of the mirror panel tilt and continue to improve on it. 

Figure 3 shows the original VSHOT test setup at the 

Eldorado Valley site with the camera independent of the 

target. The camera calibration grid can be seen on the 

target to the right. Since this time, a new target/camera 

mounting system has been implemented that integrates 

the camera to the target via support arms necessitating 

only one camera calibration per session. 

 5 
Figure 3. The VSHOT test setup at the Solargenix Test 

Loop in Boulder City, Nevada. 

By July 2005, the entire test string had been fitted with 

mirrors providing the opportunity to VSHOT test an entire 

SCA. Testing occurred over the course of a late 

afternoon to early morning session between 5:00pm and 

3:30am. High temperatures (>100o

F) and evening winds 

(in excess of 30 miles per hour) limited the test windows 

to this one time slot. The system can operate in 

moderate winds of 10-20 miles per hour 

As mentioned previously, the Solargenix trough module 5-

meter (~200 inches) aperture width spans four separate 

glass panels, two inner panels and two outer panels 

installed symmetrically on both sides of the vertex. There 

are five such mirror columns per module. An SCA 

consists of twelve modules, six on each side of the drive. 

Vertical VSHOT data profiles spanning the aperture width 

were collected on two randomly selected columns per 

module. Time did not allow testing each mirror column of 

the SCA. In all, twenty-six scans were performed 

including some repeat scans taken after module 

alignment adjustments had been made. The VSHOT 

software was used to analyze the data to determine 

overall optical R.M.S. slope error relative to the ideal 

parabolic contour and also provided additional 

information on the shape and tilt of each individual panel 

relative to the ideal. A summary of the results is 

presented in Figures 4-6. 

R.M.S. Slope Error (mrads)

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Average

Figure 4. R.M.S. Slope Error for each of the 26 scans 

and the average of all scans. 

Figure 4 is a bar chart of the R.M.S. Slope Error for each 

of the 26 scans taken along the full length of the SCA. 

The focal length term in the calculation was allowed to 

vary in this analysis. Although there is considerable 

variation, the overall average of 4.4 milliradians is less 

than the 5.0 milliradian Solargenix specification. Figure 

5 presents the best-fit focal length for each scan. Again, 

some variation exists between a minimum of roughly 58.2 

inches and a maximum of 59.2 inches, but the overall 

average is right on the design value of 58.7 inches. 

Best Fit Focal Length (inches)

57.6

57.8

58

58.2

58.4

58.6

58.8

59

59.2

59.4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Average

Figure 5. Best-fit focal length for each of the 26 

scans and the average of all scans. 

Figure 6 presents the same analysis, however the focal 

length has been fixed to the design value. As expected, 

 6 
the overall average R.M.S. slope error increases, but only 

slightly to an average value of 4.8 milliradians. 

R.M.S. Slope Error (mrads)

Fixed F.L. of 58.6inches

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Average

Figure 6. R.M.S. slope error for each of 26 scans 

fixing the focal length to the desired focal length and 

the average of all scans. 

Figure 7 is a plot of the slope error as a function of 

vertical position across the aperture for Scan 1. In this 

case the R.M.S. slope error for the scan was 3.4 

milliradians. The features seen in this plot are typical of 

all scans and contain both contour errors in the glass, 

mirror panel tilt, and discontinuities at the edges of each 

glass panel. 

DZ/DY ERROR RELATIVE TO BEST FIT PARABOLA

(MILLIRADIANS)

-20

-15

-10

-5

0

5

10

15

20

25

-100 -80 -60 -40 -20 0 20 40 60 80 100

Y (INCHES)

Figure 7. Typical plot of the slope error relative to 

best-fit parabola vs. position across the aperture. 

All four panels can be distinguished beginning with the 

outermost lower panel on the left. The inner panels 

extend from 0 to + 56 inches. A relatively flat, but nonzero slope error would be an indication of panel tilt 

relative to ideal. The higher frequency features are 

features present in the glass. One commonality observed 

among all glass panels was the localized higher curvature 

at each edge resulting in spikes in the error relative to the 

ideal shape. Although very evident in the plots, the 

amount of actual aperture area involved is so small that it 

doesn’t affect the overall R.M.S. slope error that 

significantly. 

Based partly on these results and on the desire to further 

reduce part count and assembly time/labor, Solargenix 

developed a next generation concentrator design. Under 

an Advanced Component subcontract with NREL, 

Solargenix developed an alternative, low-cost trough 

design for parabolic trough concentrating collectors. The 

new design is an advanced hub-based, field-assembled 

space frame. The new design resulted in more than a 

20% weight reduction compared to the prior-generation 

Solargenix space frame and a significantly reduced part 

count, which Solargenix expects to translate into 

significant installed cost reductions. 

Several prototypes, constituting two generations of the 

new space frame were fabricated, and one of them 

installed at the Eldorado Valley test site. This module 

was tested on two separate occasions with the VSHOT in 

October 2005. The test results show that it has excellent 

optical accuracy approaching 3.0 milliradians while also 

exhibiting less variability from mirror column to mirror 

column. This design has the potential to significantly 

reduce assembly time and may, in fact, require little or no 

mirror adjustment. These results led Solargenix to use 

this design in the 64MW plant, which they will begin 

building early in 2006. 

TEST RESULTS FOR INDUSTRIAL SOLAR 

TECHNOLOGY TROUGH CONCENTRATOR 

 Industrial Solar Technology Corporation (IST) has 

a subcontract with the DOE CSP subprogram through 

NREL to develop a larger-scale version of their already 

successful trough concentrator used for process heat 

applications. This new design would compete in the 

electric power generation market. Phase I of the 

subcontract involves conceptual design and analysis. If 

successful, Phase II would result in a prototype for 

testing. As part of Phase I, a performance baseline of 

IST’s current standard trough concentrator design was 

required. VSHOT tests were performed on two separate 

occasions. The first set of tests was done on a module 

utilizing optical reflector film. This is the standard material 

used by IST. Because IST is interested in using glass 

 7 
mirrors for the larger module design, the second set of 

tests was done on the same module using thin glass tiles. 

The IST approach is considerably different than the 

Solargenix design in that the concentrator is a single 

surface metal substrate/mirror laminate, which itself 

becomes the support structure when combined with 

structural cross members across the aperture. Potential 

advantages include highly efficient use of materials and 

elimination of individual mirror misalignment, effectively 

lowering cost and maintaining or increasing performance. 

Figure 8 is a picture of the VSHOT/IST trough module 

setup at IST’s facility in Golden, Colorado and provides a 

good view of the IST concentrator. Ten profiles were 

taken along the length of the module in the first round of 

testing (optical reflective film). A summary of the results 

is shown in Figures 9-11. Figure 9 is a summary of the 

R.M.S. slope error relative to the best-fit parabola 

allowing the focal length to change. The average of all 

the scans is 4.5 milliradians, which is considered 

acceptable for solar process heat applications. The bestfit focal length is shown in Figure 10. The focal lengths 

for each profile vary between 29.8 and 30.1 inches with 

the average being 29.9 inches. This is less than the 

design focal length of 30.35”. Figure 11 is a plot of the 

slope error as a function of vertical position across the 

aperture. All of the scans exhibited the same basic profile 

demonstrating the consistency inherent in this continuous 

surface design (no individual mirror panels). The slope 

error increases near both edges indicating a flattening of 

the surface relative to the ideal parabola. IST 

acknowledges this feature and has ideas to improve on it 

in the next design iteration. Overall, IST is very pleased 

with these results for their baseline concentrator. 

Figure 8. The VSHOT test setup at Industrial Solar 

Technology’s facility in Golden, Colorado.

R.M.S. Slope Error (mrads)

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10 11 12

Average

.

Figure 9. R.M.S. Slope Error for each of the 12 scans 

and the average of all scans. 

 8 
Best Fit Focal Length (inches)

29.6

29.7

29.8

29.9

30

30.1

30.2

1 2 3 4 5 6 7 8 9 10 11 12

Average

Figure 10. Best-fit focal length for each of the 12 

scans and the average of all scans. 

dZ/dY Error Relative to Best Fit Parabola

(milliradians)

-50

-40

-30

-20

-10

0

10

20

30

40

50

-50 -40 -30 -20 -10 0 10 20 30 40 50

Y (inches)

Figure 11. Typical plot of the slope error relative to 

best-fit parabola vs. position across the aperture. 

SUMMARY 

The VSHOT has proven to be a very useful 

instrument in the continued development and 

improvement of parabolic trough concentrator technology. 

Modifications and changes to the system have made it a 

valuable instrument for those designs already 

implemented in the field as well as those designs still in 

the research and development stages. Test results have 

enabled Solargenix to confidently move forward with the 

2nd generation concentrator in their next power plant and 

have provided IST with an understanding of their baseline 

design, which they hope can be scaled up to a 

concentrator suitable for electric power generation. In the 

future, the VSHOT will continue to be used in support of 

both of these efforts as well as to benchmark new optical 

testing techniques being developed in 2006. 

REFERENCES 

1. Jones, S.A., “VSHOT Measurements of DISTAL II 

Dish Concentrators”, Proceedings of the 

Renewable and Advanced Energy Systems for 

the 21st Century, a joint ASME/JSME/JSES/KSME 

International Conference, April 11-15, 1999, Maui, 

Hawaii. 

2. Jones, S.A. Gruetzner, J.K., Houser, R.M., Edgar, 

R.M., Wendelin, T.J., 1997, “VSHOT 

Measurement Uncertainty and Experimental 

Sensitivity Study”, Proceedings of the 32nd 

Annual Intersociety Energy Conversion 

Engineering Conference, Vol. 3, pp. 1877-1882. 

3. Malacara, D., 1978, Optical Shop Testing, John 

Wiley & Sons, Inc. 

4. Wendelin, T. and Gee, R., 2004, “Optical 

Evaluation of Composite-Based Reflector Facets 

For Parabolic Trough Concentrators”, 

Proceedings of the ASME International Solar 

Energy Conference 2004, Paper # ISEC2004-

65151.

 9 
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Video Scanning Hartmann Optical Testing of State-of-the-Art 

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This paper describes the Video Scanning Hartmann Optical Test System (VSHOT) used to optically test parabolic 

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Concentrating solar power; parabolic solar trough; parabolic trough 

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